Instruction Manual 207
Solution Delivery Series
two channeltube
mic And Instrument
Instruction Manual
P/N 999-4160
Revision 1
Released 01/01/2002
Manufactured by
Aphex Systems Ltd.
11068 Randall St.
Sun Valley, CA 91352
Copyright 2002 Aphex Systems Ltd. All rights reserved.
Produced by: Donn Werrbach. Creation tool: Adobe InDesign 1.5. Printed by: Stuart F. Cooper Co., Los Angeles.
instruction Manual
Dear Aphex Customer,
We are pleased to present the Model 207 Tube Microphone and Instrument Preamplifier as part of our Solution Delivery Series. Extensive and unique features combined with high performance and wrapped in an attractive package make this preamplifier an essential piece of gear for anyone serious about creating high quality
As with all our products, we are extremely proud of the ingenuity of design and the manufacturing quality of
the Model 207. We love to hear from you, our customers, about your experiences with any of our products.
Our customer support is unmatched in the industry, so please do not hesitate to contact us.
Marvin Caesar
Safety Declarations
CAUTION: For protection against electric shock, do not remove the cover. No user serviceable parts inside.
WARNING: This equipment has been tested and found to comply with the limits for a Class A digital device pursuant to
Part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference when
the equipment is operated in a commercial environment. This equipment generates, uses, and can radiate radio frequency
energy and, if not installed and used in accordance with the operating guide, may cause interference to radio communications. Operation of this equipment in a residential area is likely to cause interference in which case the user will be required
to correct the interference at his own expense.
The user is cautioned that changes and modifications made to the equipment without approval of the manufacturer could
void the user’s authority to operate this equipment.
It is suggested that the user use only shielded and grounded cables to ensure compliance with FCC Rules.
Page 2
Conforms to standards
UL60950 and EN60950.
Aphex Systems Ltd. Model 207
Copyright 2001 by Aphex Systems, LTD. All rights reserved. All Aphex products are trademarks or registered trademarks of Aphex Systems,
LTD. Other brand and product names are trademarks or registered trademarks of their respective holders.
two channel tube mic preamplifier
Table of Contents
2.1 What Distinguishes the Model 207 ......................................................................................
2.2 What Does the Tube Add? ..................................................................................................
2.3 What Else Is So Special? ......................................................................................................
2.4 Unpacking and Inspection ...................................................................................................
2.5 Block Diagram .....................................................................................................................
3.1 Installation .......................................................................................................................... 9
3.2 Rear Panel View ..................................................................................................................
3.3 AC Line Connection ............................................................................................................
3.4 Mic Input Connections ......................................................................................................... 9
3.5 Instrument Inputs ...............................................................................................................
3.6 Insert jack ........................................................................................................................... 9
3.7 Output Connectors .............................................................................................................. 10
3.8 Operating Level Switch ....................................................................................................... 10
3.9 Output Match .................................................................................................................... 10
4.0 USING THE 207
4.1 Split Independent Preamp Sections .....................................................................................
4.2 Using the Instrument Input .................................................................................................
4.3 Using the Microphone Input ................................................................................................
4.3.1 Using Phantom Power ........................................................................................
4.3.2 Using the Polarity Switch ...................................................................................
4.3.3 Using the Pad ....................................................................................................
4.3.4 Using Low Cut ...................................................................................................
4.3.5 Using MicLim™ .................................................................................................
4.4 Replacing Your Console’s Mic Preamps .................................................................................
4.5 Bypassing the Console Altogether ........................................................................................
4.6 Using the 207 for Stereo Recordings ...................................................................................
4.6.1 Spaced Pair Technique .......................................................................................
4.6.2 X-Y Technique ....................................................................................................
4.6.3 M-S Technique ...................................................................................................
4.6.4 Binaural Technique ............................................................................................
4.6.5 Further Reading .................................................................................................
5.1 Limited Warranty ................................................................................................................ 17
5.2 Service Information ............................................................................................................. 17
6.1 General Specifications ......................................................................................................... 18
6.2 Architectural Specifications ................................................................................................. 19
Appendix A - Introduction to Balanced and Unbalanced Wiring .................................................
Appendix B - Dealing With Ground and Hum ............................................................................
Appendix C - Proper Wiring Techniques ......................................................................................
Appendix D - Standard Cable Wiring
Aphex Systems Ltd. Model 207
Page 3
instruction Manual
1.0 Controls and Indicators with Quick Setup
When the unit is turned on, audio is not passed
through to the output for a few moments while the
tube warms up. During the warm-up time, the power
indicator is yellow. When the power indicator turns
green, the audio will pass through to the output. This
protects you from heavy turn-on thumps.
You can use any type of balanced, low impedance
microphone from dynamics to ribbons to condensers. The 207 supplies a very clean and stable 48V
phantom voltage source suitable for even the most
expensive microphones. Phantom voltage is separately selectable on or off for each channel at the
front panel.
The two outputs can be used at the same time. They
are not fully isolated from each other, however. To
unbalance the XLR output, simply use pin 2 (high)
and pin 1 (ground) leaving pin 3 either grounded or
floating. You may ground the ring of the phone jack,
as will happen when you plug in a mono style phone
plug. The XLR output will still remain balanced. The
output impedance of the XLR jack is low enough to
optimally drive loads of 600 ohms and higher. It is
designed to interface with fully professional balanced
gear. The phone jack output impedance is significantly higher but meets the requirements for driving
typical semi-pro (“prosumer”) balanced and unbal-
Glows yellow during the standby
start delay to allow for the tube’s
warmup time. Turns green when unit
is ready to operate. Light is off when
the unit is switched off.
Page 4
Switches the +48V
phantom power to
the microphone jack
on or off.
anced equipment.
This allows you to insert any kind of line level equipment into the signal path between the 207’s frontend preamps (mic and instrument) and tube output
stage. The operating level at this jack is in the vicinity
of IHF level (-10dBV). You should set up your inserted
outboard gear for -10dBV I/O sensitivity. All Aphex
processors can be readily set to operate at this level.
As the level approaches 0dB, you run out of headroom and the output will go into clipping unless
MicLim is turned on. There is a span of 27dB indicated, so you can watch your signal level for safety
and convenience. When MicLim is on, the peak level
will be held to about 3dB under clip for over-level
inputs rising as much as 20dB beyond the normal
overload point.
This input is available at the front panel through a
quarter inch phone jack. The input impedance is 1
megohms to match all coil type pickups and all powered pickups. Fully passive piezo pickups will work,
but may not deliver full tone. High impedance unbalanced microphones will usually work extremely well
with this input. When a plug is inserted, the mic
input is inoperative.
Inverts the relative
polarity (phase) of
the mic and instrument inputs.
Inserts 20dB of loss
at mic and instrument inputs, allowing for excessively
high levels.
Switches on the
70Hz Low Cut filter.
Affects mic and
instrument inputs.
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
Quick Setup
Standard Hookup to Rear Panel
This adapts the model 207’s internal dynamic range
to match either professional balanced equipment
or semi-professional unbalanced equipment at the
output. It controls both output jacks. Both jacks
supply the same output level. Set this switch to match
the requirements of your system.
Mic Source
Passive or
Phantom Powered
Use high quality balanced mic cables.
If you are unfamiliar with the use of microphone preamplifiers, then we recommend reading this manual
for detailed instructions. Even experts will find some
of the unique features of the 207 well worth a little
extra study!
See Appendix D
Indicates the available headroom in decibels
below clipping. At 0dB the output level reaches
the maximum level available at all outputs
depending upon the Output Trim and Operating Level settings.
Select the correct OPERATING LEVEL
to most closely match your receiving
equipment’s input sensitivity.
Line Level
Compressor, EQ
To Inputs of
Line Level Devices.
DO NOT Connect to
Mic Level Inputs!*
*Why You Shouldn’t Connect a 207’s Output to a Mic Level Input. The bottom line: you will get a lot of noise and possibly a lot
of distortion. The Model 207 is designed to generate a line level output from a mic level input. It optimizes the signal to noise ratio by
giving you a strong signal far above the preamplifier’s basic self noise level. This strong output signal can be up to 65dB higher than the
mic signal. It is perfect in level for a standard line input of a mixer or console. However, it will overload a mic input. If you turn down the
207’s gain to reduce its output low enough to stop overloading the mic input of your mixer, then your large console gain amplifies the
basic noise floor of the 207. That will result in a very poor signal to noise ratio. Always use the most gain possible in the 207 and adjust
your mixer’s fader or line input gain to accept it, and never feed a mic input from a 207.
Input impedance is 1 megohm to
properly accept 99% of all music
instrument pickups. It is not designed
to receive extremely high impedance
non powered piezo pickups, however.
Switches on the
MicLim operating
mode. Use is always
Switch off mainly for
Aphex Systems Ltd. Model 207
Use balanced or unbalanced cables as
needed. Consult Appendix D for wiring
Shows when MicLim is
Adjusts the microphone preamplifier and instrument gain.
Screwdriver adjustable, works
with the rear panel OPERATING
LEVEL switch to calibrate the
207’s output level to precisely
match external equipment.
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instruction Manual
2.0 Introduction
There are certainly a lot of mic preamps available
in the world. It seems that every audio equipment
company has at least one model to offer. A study of
currently available products shows that a large percentage use similar circuits based on one or two popular commercial i.c. chips. The basic design has been
done by the chip maker and the chips do a competent job. We could have brought out such a “me
too” design, but it is important to Aphex that we
offer products that are more than just “competent”.
They must also be different and better. That is why
we did not introduce our first mic preamp until we
invented something that would make all the difference.
That invention turned out to be a new and unique
way to use a vacuum tube in a circuit that yields the
clear and detailed tube sound yet has none of the
limitations of conventional tube circuitry like short
life, noise, heat, and low bandwidth. That invention
is called the Reflected Plate Amplifier (RPA), and was
granted U.S. Patent #5,450,038. We introduced our
first microphone preamplifier incorporating the RPA,
the Model 107, in 1996 and it soon became one of
the most popular mic preamps in the world. People
appreciated the difference it made in the quality of
their work. Tens of thousands of 2-channel units were
More recently, we came up with another invention
pertaining to mic preamps. We had the idea that if
we could build a mic preamp that not only sounded
great, but also had a way to automatically prevent
input overloading over a significant dynamic range
above normal clipping, then we would have another
innovative hit on our hands. Many months of research
went into inventing a method to achieve this goal.
When we succeeded, we called it MicLim™. Later,
we introduced the Aphex Model 1788 Remote Controlled Mic Preamp including the MicLim™ feature.
Months of trials proved it to be extremely useful in live
sound reinforcement applications. Additional testing
showed it is also useful in recording. When we next
introduced our very high-end Aphex Thermionics™
Model 1100 Thermionic Mic Preamp, we included
the MicLim™ feature and it has received high acclaim
among some of the world’s top sound engineers.
Meanwhile, our Model 107 remained a solid product
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in the marketplace because of its low cost, high performance and value. However, after enjoying many
years of sales, we thought we owed it to the world to
update the Model 107 with the new things we have
learned and developed since its inception. Hence, the
207 was born. Production of the Model 107 was discontinued.
In addition to improving on the Model 107, the
Model 207 is a whole new product. We transported
the front-end and RPA components that characterize
the sound of the 107 but we completely revised
our thinking about every other detail. We added the
MicLim™ feature and a professional quality instrument input. We even added a processing insert jack
so you can insert EQ, gating, or compression after
preamplification but before the tube output stage.
In this way you always get the sonic benefits of the
tube. We created a totally new and attractive style
for the chassis and panel that makes it look as great
as it performs.
With the Model 207 you are getting exclusive Aphex
design advantages and legendary sonic quality at a
very modest price. We hope you enjoy using it for
many years to come.
The Model 207 uses a discrete solid-state transformerless front end carefully crafted for extremely low
noise and high common mode rejection. It runs without feedback, making it nearly ideal as an input stage,
never reflecting any feedback current back out to the
microphone. Certainly, the input stage has a measurable distortion level, but it is very low and does
not impart any appreciable tonality. The tonality for
which our tube preamplifiers are famous is due exclusively to the tube.
Our unique and stable RPA tube circuit retains the
tube’s transconductance and grid characteristics while
at the same time creating a much more efficient
amplifier stage that runs on low power supply voltages. We designed the RPA in the Model 207 to
deliver a specific tube distortion curve that subtly
“voices” the preamp. If you have been using the
Model 207 or its predecessor, the Model 107, then
you know what that means. The preamp is characterized by most users as very transparent and detailed,
but not colored. The beauty of tubes is that their
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
distortion is so natural. The very slight tube distortion
of the Model 207 comprises typically less than 0.02%
of second order harmonics at average levels. What
is even more interesting is that this curve follows
the tube’s law as regards level, and furthermore,
it responds to certain very subtle dynamic spacecharge effects known only in tube amplifiers. Thus,
the sound with an RPA can have a large and open
feeling that is very natural and transparent.
It may sound trite, but everything else about the
Model 207 is special. When we designed its circuitry
and decided upon its features, we took into consideration a very wide range of factors. We “blocked in”
the things that our marketing experts wanted, but
then made sure that every circuit and feature could
work in harmony. Some things were discarded by
engineering because they would have compromised
the sound. Once we had the Model 207 very well
defined, we went to work at embodying the circuits
cleanly and cleverly, using some great ideas from
the Model 107, but redesigning most of it from
scratch through computer modeling and simulation
and finally through actual physical prototypes and listening sessions. There are no databook or textbook
preamplifier circuits in the 207. It’s all Aphex.
That being said, let us point out just a few of the
things that are special about the Model 207. Then, as
you read more of this manual, you will gain appreciation for many other special aspects of this product.
First, let’s look at the front end. Other preamps, even
those costing many times more, pass the mic signal
through switch contacts to enact the pad and polarity features. This eventually leads to contact distortion
creeping into the sound because switch contacts will
eventually oxidize and, in the very least, need to be
operated frequently to keep themselves clean. That is
a big problem, especially for fixed installations where
it is impossible to exercise all the switches regularly
We used expensive d.c. operated, sealed, bifurcated
gold contact relays to switch these functions. These
relays never oxidize and can remain stationary for
many years without contact distortion.
not just rumble cutting. We very slightly peaked the
response at 120Hz by about 1dB so there will be a
natural phase compensation effect in the important
low-mid bass region helping regain a greater sense
of bass presence. That benefit is immediately apparent to experienced mic preamp users. We received so
much praise for this type of filter in the Model 107,
that we kept it for the 207.
Just one more mention. Let’s look at the RPA tube
output stage. Here, we merge a real operating
vacuum tube into the world of semiconductors. The
tube serves as the main gain element and brings
us all of its warmth and clarity while remaining
directly linked to its solid-state support unit. There
are no transformers or high voltage supplies, yet the
strength of the output signal is more than powerful
enough to drive any practical load to full professional
levels with ease. Because of the unique capabilities
of the RPA circuit, we were able to mix the mic and
instrument signals directly to the tube without any
interaction. The result is exceptionally high performance.
Your Aphex product was carefully inspected and packaged at the factory prior to shipment. The carton and
its internal packing materials are designed to protect
the unit from most rough handling than can occur
during transport and handling. However, you should
thoroughly inspect the carton and its contents for
signs of physical damage before attempting to use
this device. It is your responsibility to immediately
report any damage to your dealer or the freight company so that a damage claim can be appropriately
filed. Shipping claims are always the responsibility of
the consignee (that’s you).
We also encourage you to save all of the original
packing materials in the event that this unit should
ever have to be returned to the dealer or factory for
Next, let’s look at the low cut filter. It is a second
order active filter that is designed for musicality,
Aphex Systems Ltd. Model 207
Page 7
Page 8
+4dBu | -10dBV
1/4” PHONE
instruction Manual
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
3.0 Installation & Interfacing
The Model 207 occupies a single rack space (45mm or 1-3/4 inches) of a standard EIA equipment rack.
When rack mounting, use appropriate cushioned rack screws. Never restrict air flow through the device’s fan
or vents. When installing the units into a rack, distribute the units evenly. Otherwise, hazardous conditions
may be created by an uneven weight distribution. Connect the unit only to a properly rated supply circuit.
Reliable earthing (grounding) of rack mounted equipment should be maintained. Try not to position the 204
directly above devices that generate excessive heat such as power amplifiers (unless adequately ventilated) or
near equipment with heavy transformer hum fields.
Use only a power cord that carries approvals for use in your location. The 207’s internal power supply is
designed to operate from all nominal power sources from 100 to 240 volts a.c. at 50/60Hz without requiring
the user to change any settings. In case of failure, do not attempt to change the internal fuse because it will
never blow unless the power supply fails catasrophically. The power supply will need to be serviced by a competent service technician in such a case.
The MICROPHONE input connectors are located on
the rear panel. They are the standard XLR-3F type.
Use only properly wired balanced mic cables.
The INSTRUMENT inputs are located on the front
panel providing a high input impedance of 1 megohms. The instrument jack accepts standard 1/4”
phone plug music instrument cords (i.e., guitar cords).
This input is also suitable for high impedance microphones having cables with 1/4” phone plugs. The doit-yourselfer can refer to appendix D for numerous
examples of proper cable and plug wiring.
Female XLR
Male XLR
CAUTION: Beware that 48 volt PHANTOM POWER
may be applied to either or both of the microphone inputs, creating a potential shock hazard.
Standard practice dictates shutting off the phantom power before plugging or unplugging microphones. Wait at least 10 seconds for the voltage to
fall sufficiently. This is not only for safety, but for
protection of sensitive microphones against power
Aphex Systems Ltd. Model 207
The Model 207 allows you to insert additional signal
processing between the mic preamp’s front end stage
and the tube output stage. This is clearly shown in
the 207’s block diagram at left. The interconnect is
unbalanced by way of a stereo (TRS) I/O jack on the
rear panel. You will need a conventional “Y” cable
to interface with external equipment. Such cables are
available commercially. Make sure you have the correct cable: tip is send, ring is receive, and sleeve is
ground. Check the following chart.
Page 9
instruction Manual
Installation & Interfacing
Between the input and output jack is located the
Operating Level switch. This sets the nominal operating level for both output jacks simultaneously to
either -10dBV or +4dBu. You should select the position that most closely matches your receiving equipment. In the event that you are not familiar with the
operating levels of the equipment in your system, you
should consult your manuals. In general, most entry
level “home studio” equipment operates at -10dBV
as it is assumed that the user will likely intermix home
hi-fi equipment, such as cassette decks and CD players. Most professional recording gear and live sound
equipment operates at +4dBu.
Sleeve - GND
The output connectors are located on the rear panel.
There are two output connectors per channel, one
1/4” TRS phone type and one XLR-3M type. They
may both be used at the same time to feed separate
equipment. Both jacks run at the same output level
but are not fully isolated from each other. Shorting
out the XLR will take down the 1/4 phone. However,
shorting the 1/4 phone will not affect the XLR. You
may safely plug a mono style (TS) 1/4” phone plug
into the phone jack and take an unbalanced output.
The diagram shown below explains the electrical relationship of the two output jacks.
In addition to the OPERATING LEVEL switch on the
back panel, there is a front panel trim for precise
output level matching. The OUTPUT TRIM is recessed
to stay out of your way and can be adjusted with a
small screwdriver blade. The range of adjustment is
0 to -12dB. First, set the OPERATING LEVEL switch to
the appropriate position. Next, using a loud input or
an oscillator, drive the OUTPUT HEADROOM meter
to ZERO. Set the OUTPUT TRIM to just barely drive
your equipment to its maximum input level. If you
are in doubt about how to do this, simply set the
OUTPUT TRIM fully clockwise and rely on the OPERATING LEVEL switch to get you close enough.
If you intend to take an unbalanced output from
the XLR jack, you must not ground either pin 2
or 3. Simply take from pin 2 and use pin 1 for
ground. Leave pin 3 unconnected.
Refer to Appendix C & D for more information on the
proper wiring of balanced and unbalanced lines.
2 (+)
Page 10
3 (-)
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
4.0 Using the 207
The Model 207 is a two channel unit. It contains two
separate, independent, preamplifier channels. Each
channel operates as a mic preamp or an instrument
preamp. Plugging into the instrument jack automatically disables the mic input and substitutes the instrument input.
This input is designed to work with most standard
instrument pickups, both active and passive. Active
pickups have an amplifier built into or attached to
the instrument or instrument strap. These are identified by running on a battery. Active pickups generally have a higher output level than passives, but not
always. Passive pickups couple directly through the
instrument cord to the music amplifier or preamp.
Magnetic pickups are exclusively associated with steel
stringed instruments like basses and guitars. These
pickups are wound with coils of wire that generate
an output voltage when the strings interfere with
a magnetic field set up by an internal permanent
Acoustic instruments that don’t use steel strings are
usually amplified from piezoelectric pickups placed
on the instrument where vibrations will twist or bend
the piezo crystal to generate a voltage output.
90% of all piezos in use are amplified, but passive
pickups are available. Passive piezos cannot usually
be connected directly to a music amplifier with good
results. The sound will be very thin and weak. They
require a special input stage that provides extra gain
and an extremely high impedance like 10 megohms
or more. Magnetic passive pickups and most active
pickups (piezo and otherwise) are most suited to a
1 megohm impedance at the amplifier input. Since
Aphex Systems Ltd. Model 207
we wanted to design the Model 207 to meet the
widest requirements, we gave it a 1 megohm input
impedance. Therefore, we cannot recommend it for
any type of passive piezoelectric pickup.
That said, let’s look at this input a little further. The
INSTRUMENT jack is located on the front panel where
it can be accessed by musicians most easily. The front
end of this preamp is a low noise FET amplifier stage
so you should notice this preamp is unusually quiet.
However, because of the high impedance nature of
an instrument pickup, the noise level of the INSTRUMENT preamp will be higher than the mic preamp
for the same amounts of gain taken. This is a law of
physics and not a design fault. To help mitigate the
noise problem when the INSTRUMENT is not going
to be used, we designed a way to defeat the INSTRUMENT input and its associated noise.
TIP: You will reduce the INSTRUMENT preamp noise
by running the volume on the instrument higher and
running the 207’s GAIN lower.
The Model 207 is perfect for all types of microphones,
either powered or not. We encourage you to try every
mic you own with the 207.
Many features of the 207 are standard with all professional preamps - polarity - pad - etc. Some features are unique and we hope you will fully exploit
All controls for the MIC preamp are on the front
panel. You won’t have to reach around back for the
phantom power switch or pad. Please read the rest of
this section to pick up any tips we may have for you.
Active microphones that take power through the
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instruction Manual
Using the 207
standard mic cable fall into a class called “phantom
powered” mics. The power is called “phantom”
because it rides the mic cable invisibly, without interfering with the audio signal carried on the same
48 V.D.C.
Figure 4-1 Phantom Powering
The industry standard phantom power source is positive 48 volts d.c. supplied to pins 2 and 3 through
precision low noise 6.81KΩ resistors. You may note
from spec sheets that many mics rated for phantom
power actually run at something less than 48 volts.
For example, the Audio Technica AT3031 small diaphragm condenser mic is rated for 11 to 50 volts at 3
milliamps. On the other hand, the Neumann TLM193
spec sheet states only that the supply voltage should
be 48 +/- 4 volts and the current consumption is
not stated. Don’t let these specifications confuse you.
They all run perfectly well off the standard phantom
power source described above. For the technically
interested, here’s why.
The 6.81KΩ resistors mentioned are shown in figure
4-1 below. Since the power to the microphone is carried equally (a requirement carefully observed by all
mic manufacturers) on pins 2 and 3, it is like powering the mic through a single series resistance equal to
the two resistors in parallel, which is a resistance of
half, or 3.405KΩ. A voltage drop will occur across the
series resistance equal to the microphone’s current
consumption times the series resistance. In the case
of the AT3031, consuming 3mA, the drop is .003 X
3405 = 10.22 volts. That means the voltage actually appearing on pins 2 and 3 is 48 - 10.22 = 37.78
volts. That is why, if you use a voltmeter to check
phantom power while the mic is plugged in, you will
always see something less than 48 volts. This is not a
Page 12
fault with the mic preamp.
Plugging and Unplugging a microphone when phantom power is switched on can sometimes be dangerous. Some microphones can be damaged by power
inrush. Good practice calls for switching off the phantom power before changing or inserting a microphone. Wait ling enough to hear the mic go silent
before unplugging.
You should be aware of the shock hazard with the
phantom power system. Long, open mic cables that
are disconnected from the preamp while phantom is
on can hold a d.c. charge for long periods of time,
sometimes days. They will act as a storage capacitor
and you can get shocked most rudely by holding
the XLR plug and touching the pins inside. Also
beware of microphone patch bays that may carry
phantom voltage. Don’t hold the patch cord by the
metal parts, only the plastic shell.
There will be times when you need to reverse the
polarity (phase) of a mic signal. Vocalists monitoring
themselves on headphones will hear a different sound
when the phase is reversed, for example. The reversed
phase may sound fuller and more truthful. That is
because there is a cancellation of frequencies within
the ear when the external sound from the headphone
mixes with the sound directly conducted to the ear.
“Flipping the phase” can make the effect less noticeable.
to Mic2
to Mic1
Σ = Cancelled
Figure 4-2 Phase Cancellation Effect
Another time when phase reversing can be helpful is
in mixing multiple microphones that are within each
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
Using the 207
An input pad is nothing more than a resistive
attenuator that drops the level coming from the
input jack. Its purpose is to give you a way of preventing overload of the preamp when incoming signals
become excessive. Another purpose is to change the
preamp’s mic input into a line input.
In the 207, we provided a pad of 20dB. That means
when the pad is on, the net gain of the preamp is
20dB lower than normal. You will almost never need
the pad unless you are working a live concert or other
very loud venue. If you have brought the GAIN all
the way down and you still have an excessive output
level then switch on the pad. You can then readjust
the gain as desired.
In the practical world, mics pick up all sorts of
unwanted low frequencies such as hand noise, wind
rumble, or lectern thumps. We designed into the 207
a very effective way of cutting out these low frequencies while maintaining a sense of normal low
end response. Switching on the LOW CUT FILTER rolls
off all frequencies below 70Hz at 12dB per octave
but places a slight compensation around 120Hz to
improve the low end phase distortion and perception of remaining bass. We first offered this cutoff
shape with our Model 107 tube mic preamp and
it received tremendous acclaim from vocalists and
recordists alike. So, not wanting to waste a good
thing, we carried it forward to the Model 207.
Use the LOW CUT filter whenever you need it. However, please try going without if whenever possible.
Even though it is outstanding as a rumble fixer, you
will notice it’s presence. We suggest, as a matter
of good practice, that you always try isolating the
mic from unwanted vibrations instead of summarily
switching in the LOW CUT filter.
Aphex Systems Ltd. Model 207
Figure 4-3 LO CUT Response
Response, dB
other’s coverage areas. The “crosstalk” between mics
may come through different delays due to the differing distances from the sound. These sounds may
tend to cancel out in the mix. You may get a “nasal”
or hollow effect when all mics are on. Changing the
polarity of one or more mics will often clear up the
problem. It is always worth the time to experiment
with mic polarity.
Frequency, Hz
4.3.5 USING MicLim™
MicLim™ works best on microphones having an
impedance of 50 ohms or higher. That includes just
about every powered and passive microphone in existence. Additionally, the use of phantom power in no
way inhibits the performance of MicLim™.
All you have to do is turn MicLim™ on and it will
instantly become armed. Nothing will happen, however, until an audio peak approaches clipping: then
the limiter will act on the microphone signal to prevent the peak from reaching the clip point. Whenever the limiter hits a peak, the red LIMITING light will
There are no special rules about making MicLim™
work for you. The concept is simple: run the preamp
gain at a point where the peaks indicated on the
Headroom Meter only reach the MicLim™ threshold
infrequently or unexpectedly. The great advantage of
operating in this manner is to keep the audio signal
far above the noise but fearing no distortion should
the sound level suddenly increase. Recording this way
can give you an uncanny sense of dimension because
the background will have no audible noise, even
at high monitoring levels. For those who will send
this 207’s output to a digital audio workstation, the
greatest benefit is maximizing the use of the quantum dynamic range. The closer to the “top” you digitize a signal, the more digital resolution you get. You
lose 1 bit of digital for every 6dB under maximum,
so keeping the signal “up to the top” guarantees the
best digital audio.
By now it may be apparent that if you deliberately
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instruction Manual
Using the 207
If you’re not sure about whether to use MicLim™, just
use it. If you never need it then it won’t have any
effect at all. But when that big ass peak comes along,
Bam! You’re saved.
The most basic application for the 207 is to provide
an upgrade over the microphone preamplifiers in
your mixing console. We are not insinuating that your
console’s preamps are no good, just that the Model
207 is better. If you did not know that, you probably
would not have bought your 207.
Consoles are built for functionality at a target price
point. Lower priced equipment may have a huge
array of facilities rivaling more expensive equipment.
However, everything inside is designed as cheaply as
humanly possible, even to the extent of compromising sound quality. Even the inordinately expensive
world class studio desks place a compromise on the
mic preamps they build in. Good preamps require
a lot of circuitry that just won’t fit inside. Let’s not
forget, too, that a really great mic preamp requires its
own ingenious circuitry and design. It is not surprising that of all the parts of a mixing desk, it is mainly
the mic preamps that ever get replaced.
No matter how noble the intent, there is one dilemma
that can thwart you in successfully bypassing the mic
pre of some consoles. You must become aware of
this problem in order to deal with it. We have been
instructing you to connect the Model 207 to the
LINE inputs of your console or other equipment, and
NEVER connect it to a mic input. The problem is
that when connecting your 207 to your console’s
line inputs, you may actually be connecting to mic
Page 14
inputs and not knowing it. That is because many
consoles simply put a heavy pad in front of the mic
pre input to make a line input out of it. The whole
microphone preamplifier circuit is still in line and carrying your signal! You have to suffer the sonic limitations of the console’s mic preamps and that may
mask out the benefits of using the 207.
There is a solution if your console has pre-fader
inserts. You can plug the output of your 207 into the
console’s insert jack, bypassing the channel’s whole
front end. That may eliminate the equalizer and any
other processing provided there, but it may nevertheless be an excellent solution. You will be able to have
tracks that sound incredibly alive and dimensional
not needing much additional processing anyway.
increase the preamp gain until peaks frequently drive
MicLim™ to limiting, you could obtain a very dense
output level by bringing the average signal so much
closer to the peak ceiling. Although it is certainly possible to operate MicLim™ in that fashion, and often
the effect will be desirable, we recommend the use
of restraint. MicLim™ was designed to be fast and
unobtrusive for protection against accidental overloading, especially with voices. You may find the fast
speed of this limiter too aggressive for general purpose effect limiting. Your favorite studio compressor/
limiters can still be put to good use.
When driving a console’s insert jack, you may need
to build or obtain a special insert cable, usually in the
form of a “Y” cable. That is because the insert jacks
of low cost consoles are usually 1/4” stereo phone
jacks that carry send and return as unbalanced lines
using tip and ring, similar to the Model 207’s insert
jack. You should consult your console’s manual for
instructions on connecting outboard equipment to
the insert jack. The 207’s output can easily adapt to
driving an unbalanced insert. You simply set the operating level switch to whichever setting is closer to the
insert’s operating level. It will usually be -10dBV for
home studio mixers and +4dBu for fully professional
studio desks.
When you are laying down tracks, why even involve
your mixing console? Maybe you had to do that to
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
Using the 207
be able to use console’s the mic preamps, but with
the 207 you can go direct to tape or disk and eliminate an entire layer of electronics. Usually, effects and
equalization are best added later during mixing.
Simply driving your recording electronics directly from
the 207 gives you the best initial sound tracks possible. Most studio recording sessions lay down 1 to 4
tracks at a time, session by session, until the needed
tracks have all been created. If you have one or two
Model 207’s, you would have enough outboard preamps to handle most recording sessions.
The main trick is to set the preamp gains low enough
to prevent overload, then record straight through
without riding gain. This gives you natural levels to
work with later. In mixdown you can compress tracks
and ride gain all you want. With MicLim™, you can
run all of the recording levels higher, using higher mic
gains, and not worry about overload. When MicLim™
acts, it is fast and smooth. Most of the time you
won’t notice it work. You will truly appreciate having
hot, clean and quiet tracks at mixdown. You will be
amazed at the quality you can achieve and how easily
it comes to you with great initial tracks. You will find
yourself messing around with EQ and compression
far less because the sound won’t need all that fixing.
Whatever artistic or production effects you want to
use will work better and be easier to accomplish.
Stereo recordists will find the Model 207 perfect for
their work. The low noise and highly detailed, open
sound of the 207 delivers breathtaking results. All
expert recordists know the techniques we’re about to
show you. However, others may find them interesting
to learn about.
About the most obvious technique is using two
microphones spaced apart some distance and pointed
directly at the sound source. The spacing between
mics and distance from the source depends on the
width of the sound platform you want to include.
With spaced-pairs there will be a phase difference
between the two mic outputs. This can cause cancellation of certain frequencies. An accepted compromise is called the 3:1 Principle. According to this
Aphex Systems Ltd. Model 207
principle, you should place the microphones 3 times
as far apart as their distance to the front of the sound
source. With such a wide separation of the mics,
omnidirectional types are called for. Otherwise you
might exclude the whole center of the sound stage
from adequate coverage.
Also called a “COINCIDENTAL PAIR”, this technique
employs two cardioid pattern microphones. The
grills of each microphone should be placed as close
together as possible without touching. Each microphone should be positioned at right angles to each
other. Use the intersection of this 90’ angle as a sight,
and aim toward the middle of the sound source (see
illustration on subsequent page). The microphones
should be of the same make and model.
An advantage of this technique is better rejection of
back-side room noises that include the audience and
slap-back or echo. The X-Y technique is often used to
record noisy live events.
Another popular stereo micing technique is referred
to as the “Middle-Side” technique. This method combines two microphones of different response patterns
positioned along the same horizontal plane. One is
a cardioid mic which is aimed directly at the frontcenter of the orchestra. The second microphone has
a figure-eight pattern positioned so that the axis of
the lobes are lateral, facing left and right. An electronic matrix circuit processes both microphones and
provides Left and Right channel outputs. This method
vectorially extracts the left and right channels from
the sum and difference signals (the middle cardioid
and side figure 8 patterns).
Generally, M-S is practiced with specially built microphones that are sold with the M-S processor. The mic
preamps are normally integral to the processor. However, certain M-S processors allow you to use external mic preamplifiers so you could introduce a Model
207. When doing so, just be sure to set the gains of
the two channels reasonably close and do not change
them during the recording. With M-S recording, the
stereo separation will change if either the middle or
side gain should be changed.
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instruction Manual
Using the 207
This attempts to record sound like the ears hear it
using two closely spaced microphones either worn
on a person’s head near the ears, or located inside of
a dummy head with simulated ear canals. It attempts
to capture sound waves with the refractions that
occur naturally due to the shape of the head. These
days, binaural microphones are commercially available that come with a matched dummy head. However, many incredible recordings have been made with
two small identical omnidirectional mics mounted to
eyeglass frames.
Distance X
For those recordists that do much of their work in
binaural, the Model 207 is perfect. Its low noise
and clear open sonic quality generate recordings that
are incredibly realistic and beautiful, catching all the
nuances and details.
There are more microphone techniques than we can
cover in this manual. Many reference books exist that
go into great detail. Here are a few: “Modern Recording Techniques” by Huber and Runstein, “Sound
Reinforcement Handbook” by Gary Davis and Ralph
Jones, and “Introduction To Professional Recording
Techniques” by Bruce Bartlett. If you are one who
enjoys the challenge of recording, we believe you will
truly appreciate the outstanding performance benefits afforded by the Model 207.
Cardioid Mics
Figure 4-5 X-Y TECHNIQUE
(Example: Neumann KU-100)
Cardioid Mid
Bi-Directional Sides
Figure 4-6 M-S TECHNIQUE
Page 16
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
5.0 Warranty & Service
5.1 Limited Warranty
One year from date of purchase
All defects in workmanship and materials. The following are not covered:
a. Voltage conversions
b. Units on which the serial number has been defaced, modified, or removed
c. Damage or deterioration:
1. Resulting from installation and/or removal of the unit.
2. Resulting from accident, misuse, abuse, neglect, unauthorized product modification or failure to
follow instructions contained in the User’s Manual.
3. Resulting from repair or attempted repair by anyone not authorized by Aphex Systems.
4. Occurring from shipping (claims must be presented to shipper).
This warranty will be enforceable by the original purchaser and by any subsequent owner(s) during the warranty
period, so long as a copy of the original Bill of Sale is submitted whenever warranty service is required.
We will pay for all labor and material expenses for covered items. We will pay return shipping charges if the
repairs are covered by the warranty.
No warranty is made, either expressed or implied, as to the merchantability and fitness for any particular purpose. Any and all warranties are limited to the duration of the warranty stated above.
Aphex Systems’ liability for any defective unit is limited to the repair or replacement of said unit, at our option,
and shall not include damages of any other kind, whether incidental, consequential, or otherwise.
Some States do not allow limitations on how long an implied warranty lasts and/or do not allow the exclusion
or limitation of incidental or consequential damages, so the above limitations and exclusions may not apply to
This warranty gives you specific legal rights, and you may also have other rights which vary from State to State.
If it becomes necessary to return this unit for repair, you must first contact Aphex Systems, Ltd. for a Return
Authorization (RMA number), which will need to be included with your shipment for proper identification. If
available, repack this unit in its original carton and packing material. Otherwise, pack the equipment in a strong
carton containing at least 2 inches of padding on all sides. Be sure the unit cannot shift around inside the
carton. Include a letter explaining the symptoms and/or defect(s). Be sure to reference the RMA number in your
letter and mark the RMA number on the outside of the carton. If you believe the problem should be covered
under the terms of the warranty, you must also include proof of purchase. Insure your shipment and send it
Aphex Systems, Ltd.
11068 Randall Street
Sun Valley, CA. 91352
PH: (818) 767-2929 FAX: (818) 767 -2641
Aphex Systems Ltd. Model 207
Page 17
instruction Manual
6.0 Specifications
Switch Setting:
Maximum Level:
Nominal Preamp Gain:
Transformerless, NPN active balanced
2KΩ nominal
Greater than 70dB @ 60Hz
20 to 65dB minus OUTPUT TRIM loss
8 to 53dB minus OUTPUT TRIM loss
Input Z:
Maximum Level:
Nominal Preamp Gain:
1/4” Phone Jack
Unbalanced JFET Amplifier
-infinity to 65dB minus OUTPUT TRIM loss
-infinity to 38dB minus OUTPUT TRIM loss
Output Z Balanced:
Output Z Unbalanced:
Nominal Level
Maximum Level:
Output Trim:
XLR-3M and TRS 1/4” phone jack
Impedance Balanced (may be used unbalanced)
XLR:66Ω − TRS:1200Ω
XLR:33Ω − TRS:600Ω
+21dBu Unloaded
0 to -12dB
-10dBV (-7.8dBu) Unloaded
+6.75dBV Unloaded
0 to -12dB
+0,-0.5dB 30Hz-30KHz
-129dBu (Input Shorted)
10Hz - 22kHz, -79dB or better
10Hz - 22kHz @ -11dBfs , <0.18%
12AT7/ECC81 Dual Triode
1/4” TRS Phone Jack
-10dBV (316mVRMS)
Between front-end and Tube Output Stage
Frequency Response:
Tube Type:
Connector Type:
Nominal Operating Level:
Point of Insertion:
Refer To Sections 3 & 4
Power requirements: Unit is electrically supplied through an agency approved IEC-type datachable primary power cord. The voltage
rating and connectorization meet governing standards where units are sold.
Power Consumption (maximum): 12 watts
Dimensions: 19” W x 1.75” H x 8.25” overall depth (482.6mm W x 445mm H x 209.6mm overall depth); depth behind front
panel: 7.5” (190.5mm)
Net Weight: Rack-mounted: 6lbs. (2.73kg)
Shipping Weight: 9lbs. (4.1kg)
All specifications are subject to change without notice.
Page 18
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
Basic Description
A two-channel electronic preamplifier for microphones and instruments comprising a transformerless balanced
microphone input and a high impedance unbalanced Instrument input for each of the two channels. The preamplifier shall comprise a Reflected Plate Amplifier tube-type transformerless output stage. The preamplifier
shall further comprise an operating level switch to select between nominal output levels of +4dBu and -10dBV.
The two preamplifier channels shall be operable independently. There shall be two industry standard output
jacks per channel, an XLR 3-pin male, and a 1/4” TRS phone jack. Additional specifications are stated below.
Microphone Input
The input jack shall be a standard XLR-tupe 3-pin female. The input impedance shall be nominally 2000 ohms.
All normally operated controls for the mic input shall be placed on the front panel. The mic preamp shall comprise the following features: 1.) Selectable +48VDC Phantom Power; 2.) Selectable Polarity; 3.) Selectable 20dB
Pad; 4.) Selectable 70Hz 12dB/Octave Low Cut Filter; 5.) Selectable Peak Limiter; 6.) Continuous Gain Control.
The selectable peak limiter shall be a type that limits the mic signal before any amplification to prevent input
overload. The input stage shall be transformerless and actively balanced. The preamplified microphone signal
shall be sent through an INSERT jack to the output stage to facilitate the use of outboard signal processing.
Instrument Input
The input jack shall comprise a 1/4” phone jack. The input impedance shall be 1 megohms. The input stage
shall comprise a low noise JFET integrated circuit amplifier.
Insert Jack
A 1/4” TRS phone jack shall be provided at the rear panel for inserting outboard signal processing for the
microphone input only. The insert jack shall comprise an unbalanced send and an unbalanced return that operate at nominal -10dBV levels. The point of insertion shall be between the microphone preamplifier and the tube
output stage.
Physical Properties
The device shall be packaged in an all metal chassis measuring 19” (482.23mm) wide, 1.75” (44.42mm) high,
with an overall depth of 8.25” (210mm). Depth behind the front panel shall be approximately 8” (203mm).
The device shall have a net weight of approximately 6lbs. (2.73kg) and is capable of mounting in a standard
electronic equipment rack.
The unit shall have a self contained power supply operating from the ac line. Primary voltage, connectorization
and agency listings shall be appropriate to meet local requirements.
Aphex Systems Ltd. Model 207
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instruction Manual
7.0 Appendices
Appendix A: Balanced and Unbalanced
Lines and Operating Levels
Interfacing all types of equipment with balanced
and unbalanced lines and can sometimes be troublesome. First you have to somehow connect balanced
to unbalanced and then you have to deal with different levels. This tutorial will teach you about the
principles of balanced and unbalanced lines, wiring
standards, and how to effectively interface them.
Professional audio equipment usually comes equipped
with inputs and outputs that are balanced using
3-pin XLR connectors and sometimes 1/4 inch phone
jacks as well. This equipment most often is designed
to operate at +4dBu, a professional industry standard. That translates to a magnitude of 1.23 volts
RMS (Root-Mean-Squared).
wire is the source and which is the return alternates
accordingly. In this regard, balanced and unbalanced
lines are the same. They both need two conductors.
What makes a system unbalanced is when one of
the wires is formed into a tube that wraps around
the other conductor, without touching it, such that
the outer conductor can be said to “shield” the inner
conductor. This describes all of the coaxial cable used
for video, cable-TV and radio as well as most of the
high fidelity audio cables.
Figure 1 Balanced Line Model
Consumer gear has unbalanced I/O as standard, usually on RCA jacks. The normal operating signal level
follows the IHF (Institute of High Fidelity) standard of
-10dBV, or 0.316 volts (316mV) RMS. Converting to
dBu dimensions, this works out to be the same as
-7.79dBu. There is therefore a difference of 11.79dB
between pro and consumer operating levels.
There is the notion that some king of earthly “ground”
exists out there that sinks all the noise and acts as
some kind of a shield. You see wires connected to
ground rods and water pipes that are supposed to
get a good ground. This is not a correct interpretation of grounding from an audio standpoint. Proper
grounding of equipment and wiring is important and
you will gain a better understanding of that as you
read along.
Balanced -vs- Unbalanced
Every audio signal is connected through a circuit. The
circuit must contain two conductors to create a complete return path. In other words, a signal voltage is
conducted to a piece of equipment by injecting a current into a wire. That current has flow though to the
destination through the wire and return back to the
source through another wire. Since audio is an alternating voltage, swinging through negative and positive polarity, the current through the two conductors
changes direction each alternate half cycle. Which
Page 20
Figure 2 Unbalanced Line Model
If both conductors are identical insulated wires that
are twisted together, then they form a balanced line.
This describes telephone lines, microphone cables,
and most professional audio cables. Typical balanced
cables include an additional shield wrap around the
twisted pair, but this is not strictly required for balanced lines to work properly.
Many people, because they have more experience
with unbalanced wiring, think that balanced is confusing. Believe it or not, balanced lines are really
easier to understand than unbalanced. There is no
grounding issue with balanced, and the way it works
is perfectly natural and simple. Balancing naturally
rejects hum and noise and eliminates all sorts of complications in interfacing.
Balanced transmission works something like this.
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
Your balanced input stage looks at the two wires
and detects only the potential (voltage) difference
between them. Anything that is the same on the two
wires (for all practical purposes as seen measuring
from ground) is called a common mode signal and
is cancelled out by the differential amplifier. Figure 1
illustrates how the hum is induced into both wires
equally and therefore is cancelled out.
Since the balanced line has wires that are twisted
together, each wire tends to pick up the same amount
of induction from external sources. Induction will
create no significant voltage difference between the
wires, hence the noise (or hum) will not be picked up
by the differential input stage.
It can be seen that the signal generator driving the
twisted pair will cause a difference between the wires,
and that signal will be readily picked up by the differential input stage. One of the beauties of the balanced line is that it is completely independent from
ground. Nothing is connected to ground at all, nor
does it care about ground. Nevertheless, most professional cable has an overall shield wrap that is
intended to be connected somehow to ground. You
may well ask why, and the answer is less than glorious. Simply, nothing is perfect, not even balanced
cable. Under some circumstances the shield can overcome extreme interference problems that can’t be
adequately rejected by the twisted pair alone. Things
like 2-way radios, television transmitters, and light
dimmers can induce very heavy interference that may
be reduced by shielding. You are going to find virtually all balanced cables include a shield so you need
to deal with it, even if it is not actually needed. That
subject will be addressed a little later.
Unbalanced wiring works a little differently. Figure 2
shows the basic plan. In this case, the wires are not
twisted, they are coaxial. The unbalanced input stage
is somewhat like the balanced input stage because
amplifies a difference signal, but this time it is the difference between two non-symmetrical conductors.
To make things even less symmetrical, the outer conductor is connected to ground at both ends. The
principle is that the outer shield conductor shields
the inner conductor from induced noises. This can
only work well if the cable is relatively short and
the ground at each end of the cable is somewhat
Aphex Systems Ltd. Model 207
equal, i.e., there is no “grounding difference” that
can cause current to flow through the shield conductor. Grounding difference is a serious problem in
studios, because often the equipment grounds are
connected to power outlet grounds, and there can be
a significant difference of ac voltage between alternating wall outlet grounds. For this reason, unbalanced systems can sometimes never be made hum
free, and just changing one piece of equipment in
a studio can cause hum to appear somewhere else.
When you are using unbalanced gear, it is a very
good procedure to power all your equipment from
one large power isolation transformer. At the very
least, make sure all equipment is powered together
off the same distribution panel circuit (same circuit
Appendix B: Dealing With Grounds and
Ground Loops
Many people equate this term with hum, and that’s
just about the bottom line of it. If you have a ground
sensitive system, like unbalanced audio equipment
for example, then hum will result from ground currents that flow from the ac power system. It is sometimes very difficult to isolate and stop ground currents
between unbalanced equipment, but it is quite easy
to clean up balanced gear. That’s why pro gear is
always balanced! The cost of balancing is that of
more expensive connectors, cable, and electronics
but the cost is worth it when you depend on your
audio quality. That’s why the Model 207 is equipped
with a fully balanced I/O. Now that we’ve sold you
on only using really expensive pro gear, lets show you
how to get away with the really cheap stuff! At least
from the standpoint of killing ground hum.
A ground loop is an ac current that has become
routed through your audio ground system. The current comes mainly from ground potential differences
that exist between different wall outlets that return
to opposite phases at the power distribution panel.
Secondarily, however, many pieces of equipment contain line filters and transformers that leak a small
amount of ac power into the chassis and ground
You may once have had the experience of getting
zapped by touching two pieces of gear at the same
Page 21
instruction Manual
6.0 Appendices
time. That illustrated the ground loop effect - straight through you! No matter what you do, you
may not be able to prevent some of your equipment
from generating ground currents. The most likely culprits are digital products because they use switching
power supplies that require heavy line filters to prevent conducted EMI from going out of the box. Filters so employed very often take the ground leakage
current right up to the UL safety limits. Although it
won’t kill you, that is a lot of ground loop current for
audio cables to handle.
There are basically three ways to attack the problem
of a ground loop. First is to eliminate it from its
source, and the second is to re-route it through
another path. The third is to balance out your unbalanced audio interfaces.
Identify the Sources
A good way to identify grounding problems is to use
a multimeter to check the ac voltage between the
chassis of your various gear when no audio cables are
hooked up and all gear is plugged in and switched
on. Just start touching the two probes to the metal
chassis of different pieces of gear. Ideally, you should
always see zero volts. Warning! You may see as much
as the whole line voltage between two different chassis! It does happen. This voltage between chassis will
be responsible for your ground loop problems. If
you find there is more than about 1 volt between
equipment grounds, you should start looking for a
Commonize the Power
Try plugging all of your equipment into the same
outlet strip. Get one that has enough outlets in one
strip or string more than one together. Of course, you
need to make sure you don’t overload the one ac circuit your strip is plugged into. If the load is too great
for one circuit, use a second or third circuit that is
tapped off the same 120 volt phase in your distribution panel. That means all outlets should be on odd
or even numbered circuit breakers. That’s because,
as you go down the column, the circuit breakers
tap into alternating legs of your incoming electric
power. Be sure you’re always on the same leg. You
can tell you’re on the same leg by measuring the ac
voltage between the hot slots of the different outlets
you’ve chosen. It should be very low or zero. That will
remedy 50 percent of the cases.
Page 22
Check the Cord Polarity
For products that have 2-wire power cords, try reversing one of the power cords in the socket. That may
reduce the ground current generated by the internal
electronics of the offending gear.
Redirect Ground Loops
Sometimes it just comes down to brute force grounding. That means providing such heavy, low resistance,
ground current paths that little current is left to flow
through your audio grounds. You can try adding
heavy gauge, for example 12 gauge, copper wire
from chassis to chassis. You will need to locate a
metal screw that solidly binds to the metal chassis of
the gear. You may even need to drill a hole through
the chassis and install a screw yourself. Equipment in
rack shelves can have their chassis grounded to the
metal rack frame by a heavy wire and the frame itself
can act as a brute force ground. You just have to try
everything you can think of. Usually a combination of
all these methods will be needed to completely clean
up a badly humming audio system.
Balance Out the Audio
Remember, balanced lines are inherently hum free.
If you can balance out your unbalanced equipment,
you will be able to stop the hum.
Pseudo Balancing
You will find in Appendix D an interconnecting
method called Pseudo Balanced. This works when
connecting an unbalanced output to a balanced
input. This breaks up the ground loop by requiring
the shield to be grounded only at one end. For best
results always ground the shield only at the receiving
Level Interface Units
Aphex manufactures the Model 124 Level Interface
box which is designed to electronically convert two
unbalanced inputs and outputs into two balanced
inputs and outputs, and at the same time translate
the -10dBV IHF unbalanced levels to the pro +4dBu
balanced levels. This cost effectively gives your nonprofessional unbalanced equipment a fully professional I/O equal to the world’s best pro audio gear.
Seriously consider putting one of these on each
unbalanced piece of gear you use.
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
Avoid Transformers
The use of balancing transformers is an option, but
you will invariably lose audio quality due to transformer limitations. Try everything else first.
Appendix C: Proper Wiring Techniques
A true balanced line should be used wherever your
equipment allows. Use “twisted pair” shielded cable.
For unbalanced wiring you should use high grade,
low capacitance shielded wire for best results. If you
have an unbalanced output but have a balanced
input, the “pseudo-balanced” configuration may help
deal with ground loop hum. This method and others
are illustrated in Table 2.
The 3 pin XLR, 1/4” (63.5 mm) TS mono phone and
the 1/4” (63.5 mm) TRS stereo phone are the most
commonly used line level connectors in pro audio.
Less common is the use of the “RCA” phono jack,
which is essentially a consumer type connector. The
XLR and the TRS are three conductor and are used for
balanced connections. The TS and the RCA are two
conductor and are used for unbalanced connections.
In addition to the three main contacts on an XLR
there is also a grounding lug contact. This lug is connected to the connector’s case (shell). In all Aphex
products audio ground and chassis ground are one
and the same. Aphex products that use XLR connectors tie Pin I to the XLR case automatically. Therefore it is not necessary to use the XLR case-ground lug.
This also makes possible the use of XLR ground drop
adapters (see Note 3).
TABLE 1: The wiring convention shown is now standardized in 17 countries including the USA. Please
note that any equipment that still uses Pin 3 as positive on XLR connectors is not adhering to the standard.
The three main contacts on an XLR (or TRS) and
the accepted wiring assignments shown above are
Aphex Systems Ltd. Model 207
only part of the picture. The standard for terminating ground is Pin 1 (Sleeve). But which ground? It
could be connected to audio signal ground or chassis
ground depending on the method of grounding used
by the equipment manufacturer. In all Aphex products audio ground and chassis ground are one and
the same at all I/O jacks. This is just good, common
sense engineering practice (which is what you would
expect from us, course). Unfortunately, many products
are designed so that the noisy currents from the shield
drain into signal ground instead of chassis ground.
This practice creates a real hum and noise problem for
end-users. The appropriate overall grounding scheme
of an audio system would be a lot easier to predict
without this problem1.
The standard balanced line wiring recommendation
from Aphex Engineering is this: In the majority of
cases maximum noise rejection occurs when the
shield is connected to the input ground only (especially in locations with high levels of RFI). That means
the sending end shield should be left disconnected.
However, if you already have cables with the shield
connected at both ends, go ahead and try them
out. If you are connecting a fairly simple audio
system it may be fine as is.
A word on optional shield connections: Connecting the cable shield of a balanced line at both ends
creates unnecessary ground loops which may carry
noise and hum currents that can be amplified. Connecting the shield only at the sending end (instead
of the receiving end) may exaggerate common mode
noises at the receiving input stage. It can actually
increase RFI and noise more than having no shield
at all. Because of the “Pin I Dilemma” (mentioned
above) you may be forced, in some situations, to
experiment with how the cable shield is connected
to ground to eliminate a pesky hum or radio interference problem. It might be good to try XLR ground
drop adapters (see Note 3) as a method of trying
these conflicting methods out and being able to
change easily if necessary.
Regardless of inaccuracies, it has become more or less
standard over the years to refer to balanced lines as
Page 23
instruction Manual
3-Pin XLR
1/4” TRS Phone
Standard Wiring Convention (Balanced)
Ground/Shield (Earth, Screen)
Positive (Signal, High, Hot)
Negative (Signal Reference, Return, Low, Common)
1/4” TS Phone
Standard Wiring Convention (Unbalanced)
Center Pin
Positive (Signal)
Ground/Shield (Signal Reference/Return)
low impedance and unbalanced lines as high impedance. The fact is, however, that both balanced and
unbalanced lines are operated at low impedance in
modern practice owing to the fact that all output
stages have become low impedance. A few exceptions might be outputs from passive mixers, instrument pickups, electric guitars and some keyboard
synthesizers. It is generally ideal to drive any audio
line from a low impedance and receive into a high
impedance. Generally, a minimum 1: 10 ratio is
possible. This is called “bridging”. This has become
modern practice and all balanced inputs are normally
running 10K ohms or higher impedance. Because of
these developments, it is now no longer as critical
to consider impedance when dealing with interfacing pro line level equipment (impedance “matching”
is mostly a requirement of the past).
A word on impedance and interfacing adapters:
If you are connecting between two line level devices
and they have different connectors (example: 1/4”
phone to XLR or vice-versa), you do not need to use
an impedance matching transformer. With very few
exceptions you are strictly dealing with a difference
in connector types and should only use hard-wired
adapters (or cables) for this situation.
APPENDIX D: Standard Cable Wiring
In relation to 1/4” phone jacks, you may see the
terms “TS” and “TRS” as abbreviations. Here is a what
Page 24
that means: TS refers to the Tip-Sleeve or “mono”
2-conductor type and TRS refers to Tip-Ring-Sleeve
or “stereo” 3 conductor type 1/4” phone connectors.
This applies to jacks (female connectors) and plugs
(male connectors).
Note: We recommend using only conventional
1/4” phone plugs with the Model 207 and with
all other audio equipment. Professional patch bay
cords using brass PJ055 telephone type plugs are
designed only for patch bays and will not make
proper contact with standard 1/4” phone jacks.
The following instructions show all the different ways
you will probably ever need to hook up your 207 as
well as any other equipment you may own. You will
see that connecting balanced outputs to balanced
inputs is ultimately simple and the same cable will
work for all flavors of output stages.
Connecting a balanced output to an unbalanced
input requires a little more knowledge and care.
You should refer to your equipment manuals and
determine the type of balanced output stage that is
provided, then use the correct “transition cable” as
depicted in this section. Improper transition cables
can cause crosstalk, hum, and distortion problems
within your system.
Believe it or not, there are at least 5 types of balanced output stages in use today. They may be placed
into two main classes: transformer balanced, and
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
transformerless balanced, usually called “active balanced”. Transformer balanced outputs are becoming
outdated because of their high cost and their sonic
limitations. However, they can still be found on a lot
of older equipment.
Within the transformerless class, there are several
types of circuits that are used by different manufacturers. These different types of output circuits all look
just about alike to any balanced line, but they act differently when driving an unbalanced line. You need
to observe the proper cable wiring for each type of
output circuit. We strongly recommend that you refer
to your various equipment manuals to find out what
is used in each case before hooking up to unbalanced
When connecting a balanced output to a balanced
input, however, you don’t need to know what kind
of balanced output you are dealing with. Simply
treat it generically.
OK for Microphones
Standard store-bought cable. Shield grounded at both ends.
Positives: Good for microphones.
Negatives: May cause ground loops through the shield grounds if used
to connect equipment together.
Preferred for Line Levels
Shield grounded at receiving end only.
Positives: Stops ground loops and reduces noise.
Negatives: None
1/4” TRS Phone to 1/4” TRS Phone Balanced Cables
Standard store-bought cable. Shield is grounded at both ends.
Positives: Both ends are interchangeable.
Negatives: May cause ground loops through shield contacts.
No Connect
Custom cable. Shield is grounded at receiving end only.
Positives: Stops ground loops and reduces noise.
Negatives: Should be oriented so lifted shield is at sending end.
XLR to 1/4” TRS Phone Balanced Cables
From an Output
To an Input
Female XLR
Male XLR
To an Input
From an Output
Stereo Phone Plug
Stereo Phone Plug
No Connect
Stops Ground Loops
No Connect
Stops Ground Loops
Aphex Systems Ltd. Model 207
Page 25
instruction Manual
Voltage Balanced Outputs (Used on the 207)
It was mentioned that there are several types of balanced output stages in use today. The following diagrams show you how to properly unbalance each
type of output. If you follow these instructions, you
should have no problems.
Unbalancing loses half the output level.
You lose 6dB of gain.
Female XLR
Don’t Ground or Connect Pin 3
Transformer Balanced Outputs
Mono Phone Plug
Unbalancing loses no output level.
You retain full gain.
Female XLR
Ground Pin 3 Directly to Pin 1.
Alternatively, Carry Pin 3 Through Twisted Pair Cable
and Ground at Other End
Impedance Balanced Outputs
Mono Phone Plug
Unbalancing loses no output level.
You retain full gain.
Female XLR
Servo Balanced Outputs
Pin 3 Doesn’t Matter
OK Grounded or Not Grounded
Mono Phone Plug
Unbalancing loses no output level.
You retain full gain.
Female XLR
Ground Pin 3 Directly to Pin 1
Do Not Carry Pin 3 Through Cable and Ground
at Other End
Mono Phone Plug
Page 26
Aphex Systems Ltd. Model 207
two channel tube mic preamplifier
Standard Cable (Guitar Cord)
Mono (TS) Phone Plug
Mono (TS) Phone Plug
Standard Method
Enhanced Method (Pseudo Balanced)
Advantage: Reduced Hum and Noise Pickup
Mono (TS) Phone Plug
Stereo (TRS) Phone Plug
(Guitar cord of Part 3 above usualy works just as well)
Mono (TS) Phone Plug
Stereo (TRS) Phone Plug
Not Used
Male XLR
Male XLR
Mono (TS) Phone Plug
Mono (TS) Phone Plug
Not Used
Male XLR
To Equipment Input
Ground Shield This End Only
Female XLR
Stereo Plug
To 207
Insert Jack
Ground Shield This End Only
From Equipment Output
Don’t Connect Pin 3
Mono (TS) Phone Plug
To Equipment Input
To 207
Insert Jack
Ground Shield This End Only
From Equipment Output
Mono (TS) Phone Plug
Aphex Systems Ltd. Model 207
Ground Shield This End Only
Page 27
two channel tube mic preamplifier
Patent Notice
This product is protected under one or more of the following Aphex patents.
Aphex Systems Ltd. Model 207
Page 28
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