Retro Instruments | 176 | User`s manual | Retro Instruments 176 User`s manual

Retro Instruments, Inc.
User’s Manual
Tube Limiting Amplifier
Retro Instruments, Inc.
Tube Limiting Amplifier
User’s Manual
2008 Retro Instruments, Inc.
P.O. Box 5066
Modesto, CA 95352-5066 USA
(209) 810-3344
Manual Revisions
Print Date
Initial Release
December 29, 2008
2008 Retro Instruments, Inc.
All rights reserved. No part of this document may be reproduced, transmitted,
transcribed, stored in a retrieval system, or translated into any language in any form by
any means without the written authorization of Retro Instruments, Inc.
Printed in U.S.A.
User Manual Part Number – 176001-01
XLR, RCA, Xcelite, UA are trademarks of their respective companies.
Retro attempts to provide information that is accurate, complete and useful. If you
require further information or find discrepancies in the text within, please contact Retro
Retro Instruments, Inc.
P.O. Box 5066
Modesto, CA 95352-5066 USA
(209) 810-3344
Section 1
1.1 Your Limiter
1.2 Applications
1.3 Specifications
1.4 Safety Considerations
Section 2
2.1 Operating Environment
2.2 Power Connections
2.2.1 AC Line Voltage Selection
2.2.2 Fuse
2.2.3 Line Cord
2.3 Input Connection
2.4 Output Connection
2.5 Stereo Link
2.6 Grounding
Section 3
3.1 Power Switch, Indicators and Interlock
3.2 Amplifier Active/Bypass Switch
3.3 Input Level Control
3.3 Output Level Control
3.4 Compression Ratio Control
3.5 Attack and Release Controls
3.6 Sidechain High Pass Filter
3.7 Asymmetry Switch
3.8 Meter Select Switch
3.9 Stereo Linked Operation
Section 4
Principles of Operation
4.1 Amplifier Circuit Board
4.1.1 Input Transformer Stage
4.1.2 6BC8 Variable Gain Stage
4.1.3 Interstage Transformer
4.1.4 12AX7 and 12BH7 Amplifier Stage
4.1.5 6AL5 Detector and Ratio Switch
4.1.6 Asymmetry Switch
4.1.7 Sidechain Highpass Filter
4.1.8 Output Attenuator
4.1.9 Test Facilities
Power Supply Circuit Board
4.2.1 Mains Input
4.2.2 Power Transformer
4.2.3 5Y3 High Voltage Rectifier
4.2.4 OB2 Gas Discharge Tube
4.2.5 Bleeder Resistor
Section 5
Gain Reduction Meter Zero
Internal Balance Test Switch
Balancing the 6BC8 Gain Reduction Stage
Balancing the Amplifier Stage
Noise and Distortion Tests
Frequency Response Tests
Hum and Microphonics
Section 6
Alignment and Maintenance
Schematics and Drawings
Board Layouts and Schematics
Recall Sheet
Section 7
Support and Service
7.1 Service
7.2 Support
7.3 Replacement Parts
7.4 Warranty
7.5 Factory Service Instructions
Section 1 Description
This section provides a general description of the 176 Tube Limiting Amplifier. Please
review this information before installing or operating the 176.
1.1 Your Limiter
The 176 Tube Limiting Amplifier is designed to provide the quintessential character of
vintage tube compression in the modern recording studio environment. It is modeled
after the superb UA 176 limiter and adds new features and conveniences. The Retro 176
uses high quality components that should provide many years of trouble-free service.
Your limiter has been through a full burn-in and testing procedure at the factory.
Ease of use is key with user-friendly front panel controls and easily recallable settings.
No matter what settings you use, the great sonic traits of the 176 will enhance your
1.2 Applications
The Retro 176 is designed for great versatility in many applications. Because of the way
real tube compression handles dynamics, the 176 enhances dimension, depth and detail.
Independent tracks such as vocal, bass, acoustic guitar, piano fill out nicely without harsh
restraint. The 176 is great for tracking, mixing and multibuss compression applications.
Possibly the best application for the 176 is use on the stereo buss, buffing it to a glossy
finish. The Retro 176 adjusts tone in a way that equalization cannot.
1.3 Specifications
Frequency Response
Harmonic Distortion
Noise Level
Gain Reduction
Maximum Gain
Input Impedance
Output Impedance
Minimum Input Level
Normal Operating Level
Maximum Operating Level
Maximum Operating Temp.
Physical Dimensions
Power Requirements
Flat within 0.5 dB from 20-20,000 Hz.
Below 1% from 20-20,000 Hz at 0-15 dB Gain Reduction
Greater than 76 dB below normal operating level
20 dB of available gain reduction
32 dB with input and output controls set maximum
600 Ohms - floating transformer balanced
600 Ohms - floating transformer balanced
–26 dBm@2:1 ratio –12 dBm@12:1 ratio
+4 dBm
+20 dBm
55° C
Standard 19” rack mounted. 2U, 3.5” high, 9.5” deep
40 Watts at 115/230 VAC, 50/60 Hz.
1.5 Safety Considerations
The Retro 176 is to be used in a metal equipment rack with adequate ventilation. The
vacuum tubes become hot. The AC mains power should be disconnected prior to
servicing. For safety, the front door of the 176 is equipped with an interlock switch that
interrupts the AC mains power when the door is opened. This switch should not be
defeated. Only trained technicians should open the front door to gain access to the
internal adjustments. Tubes should not be removed or inserted with the power on. An
accidental breakage of the glass envelope can expose high voltage on the plate of the
This product is grounded through the AC Power Cord. For safety, do not lift or remove
the ground from the AC Power Source.
Section 2 Installation
2.1 Operating Environment
Mount the 176 in a standard 19” equipment rack. Always install the bottom rack screws
first to support the unit. Please allow room above the top ventilation holes and above the
tubes. Additionally, there are ventilation holes on the sides to draw in cool air. Be aware
of the heat sources in your rack to allow proper cooling. Heat will shorten the life of
your equipment.
Audio equipment is sensitive to magnetic fields caused by nearby power supply
transformers. If you experience 60 Hz. hum in any of your gear, try unplugging adjacent
gear first.
If you purchased a matched pair with the link panel, mount the panel between the two
units and connect the RCA cables to the Couple jack on each 176.
2.2 Power Connections
2.21 AC Line Voltage Selection
AC Mains Voltage is applied to the IEC standard AC receptacle on the rear of the unit.
Set the Red AC Voltage Selector switch on the rear of the unit to 115 or 230 Volts for
your mains voltage. You can use a small flat screwdriver.
Power Fuse
Voltage Selector
2.2.2 Fuse
The fuse is a 1A fast blow 3AG type for 115 or 230 Volt Mains Voltage. Disconnect the
power cord before changing the fuse.
2.2.3 Line Cord
You can use a compatible IEC power cable for 100-120 Volts or 200-240 Volts per your
country’s specification.
2.3 Input Connection
Audio Input is a standard female XLR connector. It is transformer balanced and floating,
meaning there is no path to ground from pin 2 and 3. Unbalanced sources connect the
center conductor to pin 2 and the shield to pin 3.
2.4 Output Connection
Audio Output is a standard Male XLR connector. It is transformer balanced and floating,
meaning there is no path to ground from pin 2 and 3. Unbalanced loads connect the
center conductor to pin 2 and the shield to pin 3.
2.5 Stereo Link
An RCA jack on the rear panel allows strapping of multiple 176 limiters. An optional
stereo link panel is available to couple two units.
Coupling Jack
2.6 Grounding
The 176 is primarily grounded through the AC Mains. For safety, do not disable the AC
ground. The chassis is also grounded through the rack ears to the rack. The XLR
connectors are grounded on pin 1. Pin 1 is only used to drain the shield of the audio
cable on one end of the cable. The exception to the rule is a microphone cable, where pin
1 is used to carry ground to the microphone case.
Section 3 Operation
3.1 Power Switch, Indicators and Interlock
Upon initial installation, apply power. The red indicator and meter should light
immediately. If not, check power connections. Make sure the front door is securely
closed. Check that the voltage selector is set to the proper voltage. Finally, check the
fuse. If the fuse is blown, the likely cause is the fuse itself or a damaged 5Y3 rectifier
After a minute the meter should read near zero in the GR setting. The 176 is now ready
to use.
3.2 Amplifier Active/Bypass Switch
This switch lets you easily evaluate what the 176 is doing. It provides a hard-wired
bypass of the 176 for instant A/B comparisons.
3.3 Input Level Control
The Retro 176 has a fixed threshold. The gain reduction is adjusted by the Input Level
control. The input is optimized for standard +4 dBm line levels to provide 0-20 dB gain
3.3 Output Level Control
The Output Level control matches the Output Level of the 176 to the Input Level or
provides less or more amplifier gain as the user requires.
3.4 Compression Ratio Control
The ratio can be adjusted in four steps from 2:1 to 12:1. Lower ratios will put a
compression signature over a wider dynamic range. Higher compression ratios are best
to hold the signal level down. The ratio control also changes the threshold. The
threshold axis point is at 10 dB gain reduction. This results in similar gain reduction at
the various ratio settings when doing approximately 10 dB of gain reduction, however at
less amounts of gain reduction the lower ratio settings will start compression sooner than
higher ratios.
3.5 Attack and Release Controls
The Attack and Release controls determine how quickly the dynamic range is adjusted by
the compression. The effect of these controls can be heard as well as observed in the
meter action. The Retro 176 has extended adjustment of the attack and release
Slow Attack lets more impact through. Fast Attack holds down peaks and sounds more
The Release control set faster adds energy and density. Slower settings give gentler
control and retain more of the original source dynamics.
The Attack knob has a pull switch that disables the Retro 176 program-controlled time
constant. The original UA 175B and 176 have a single time constant. By pulling the
Attack knob OUT you can have that mode. The program-controlled time-constant (with
the knob IN) makes the Retro 176 gain reduction more natural, better controlling
dynamics with less distortion.
3.6 Sidechain High Pass Filter
The Sidechain HPF is a very effective tool to eliminate pumping on low frequency
material. It allows the compression to work more effectively controlling dynamics in the
midrange frequencies. Bass frequencies will pass through with little or no control, so it
really depends on your application. The Sidechain HPF is ideal for vocals. It is disabled
by a switch at the minimum setting.
3.7 Asymmetry Switch
Many voices and instruments are highly asymmetrical waveforms. The Asymmetry
Switch selects whether the detector is full-wave, detecting both positive and negative
sides of the waveform or half-wave, which detects only one side.
With the Asymmetry switch set to + or −, you generally hear the compression open up.
You will notice the most effect on signals that look spiky on your editor screen.
Generally the spikes are facing one direction, up or down. The other direction may be
smooth. The Asymmetry Switch determines which edge of the wave the compression
will ride on.
3.8 Meter Select Switch
The Meter Select switch provides metering of the Input and Output levels and the Gain
Reduction. The Input and Output meters are referenced to a standard operating level of
+4 dBm. It is simple to check your input and output levels for easy setup and A/B
comparisons with the Bypass switch. The Input Meter is fed directly from the Input
XLR. The Output Meter is fed directly from the 176 Output Control before the Amplifier
Bypass switch.
Normally the meter is kept in the GR position.
3.9 Stereo Linked Operation
Stereo linked operation is generally only desired when the stereo image must not wander.
For example, when dialogue is slightly panned and you are using heavy amounts of
compression. If the 176’s are unlinked, the panning would tend to move to the center.
For contemporary music recording, we prefer the 176’s unlinked. The unlinked stereo
pair actually enhances the stereo cues and create a wide soundstage. Instead of being
heavy-handed on the mono mix, they have better dexterity to reward the dynamics of the
independent left and right channels.
Section 4 Principles of Operation
4.1 Amplifier Circuit Board
4.1.1 Input Transformer Stage
The Input from the Female XLR Input connector feeds the bypass side of the Bypass
switch and the input pad and input transformer. The input pad provides 20 dB of loss and
transforms the 600 Ohm input to the proper 120 Ohm transformer impedance. The input
transformer feeds the grids of the 6BC8 variable-mu gain reduction stage through the
Input Level control in a balanced 200K Ohm arrangement.
4.1.2 6BC8 Variable Gain Stage
The 6BC8 gain is determined by a negative bias applied to the grids through the Input
Level control. This control voltage is derived from the detector and timing circuitry. The
6BC8 stage is powered by a regulated power supply using the OB2. This circuit is
balanced with VR2 across the cathodes and VR3 across the plates.
4.1.3 Interstage Transformer
The interstage transformer separates the gain reduction stage from the amplifier stage. It
provides a distinctive load for the variable gain stage to work into. Alternatively, the
interstage transformer can be bypassed which changes the tone and character of the
4.1.4 12AX7 and 12BH7 Amplifier Stage
The amplifier stage is a fully push-pull circuit. The first stage, a 12AX7 and the second
stage, a 12BH7 are closely tied through a dual balanced negative-feedback loop. This
provides gain and balance stability and provides low distortion and a low virtual driving
impedance for the output transformer. VR4 trims the balance of this amplifier and serves
as a simple way to verify correct operation.
4.1.5 6AL5 Detector and Ratio Switch
The output transformer has four sets of input taps to accommodate the four ratio settings.
Each set of taps is brought out to the ratio switch. The ratio switch then feeds the 6AL5
full wave rectifier through the Sidechain Highpass filter network and Asymmetry mode
switching. Bias for setting the compression threshold and ratio is derived from a resistive
ladder also on the ratio switch. The 6AL5 detector output feeds the attack/release
controls and timing capacitors before driving the gain reduction stage.
4.1.6 Asymmetry Switch
The asymmetry switch raises the bias and threshold of the negative or positive side of the
detector depending on the position of the switch.
4.1.7 Sidechain Highpass Filter
The Sidechain HPF applies additional resistance across the detector input coupling
capacitors effectively creating a first order highpass filter. The filter is disabled through a
switch when the filter control is set at minimum.
4.1.8 Output Attenuator
The Output Level is implemented immediately after the output transformer. The Output
control is integrated into a T-Pad that provides proper loading to the output transformer
and attenuates the output to the required user setting.
4.1.9 Test Facilities
The Retro 176 has an integral balance testing system. The static test injects a commonmode 60 Hz signal into the push-pull amplifier. The dynamic test injects a dc voltage
with 60 Hz component to observe balance during gain reduction. It allows the technician
a way to balance the amplifier without test equipment. See the Aignment and
Maintenance section for procedures.
Power Supply Circuit Board
4.3.1 Mains Input
The AC Power enters the 176 through the IEC power receptacle. It travels to the fuse,
the door interlock switch and the front panel User Power switch. Finally it reaches the
Voltage Selector switch and the Power Transformer.
4.3.2 Power Transformer
The Power Transformer provides several voltages that are used in the 176. It has
additional shielding and low losses to operate cool and efficiently. It accepts 120 or 240
Volts at 50/60 Hertz. It provides 5 Volts and 600 Volts to the high voltage rectifier. It
also has a center-tapped 6.3 Volt 3 Amp output for the amplifier tube filaments.
4.3.3 5Y3 High Voltage Rectifier
The 5Y3 is a commonly available full-wave power rectifier. The original design used a
5AR4 that can also be used and provides a slower turn-on to allow the filaments to warm
up. A common failure is arcing in the tube, which can blow the power fuse.
4.3.4 0B2 Gas Discharge Tube
The 0B2 is a cold-cathode, glow discharge tube. In combination with a 10K Ohm
resistor it provides a regulated 108 Volts to the 6BC8 gain reduction stage. It will glow
orange or purple depending on the variety. This tube can fail two ways; it stops glowing
and the gain reduction meter rises to +3, or the light dances in the tube causing a rumble
or squeal in the audio signal. With normal gain reduction this tube will dim slightly.
This is normal.
4.3.5 Bleeder Resistor
There is a 270K Ohm bleeder resistor across the first power supply capacitor. In the
event of a choke failure, the input filtering capacitor will discharge. Under normal
conditions the power supply will not carry residual voltage seconds after power is
interrupted. Always check the power supply capacitors for stored voltage when
performing repairs.
Section 5 Alignment and Maintenance
Balance Test Switch
Meter Zero Cathode Bal. Plate Bal.
Amp. Bal.
All adjustments should be performed by a qualified technician.
5.1 Gain Reduction Meter Zero
Use a small Xcelite Greenie or precision adjustment tool to make internal adjustments.
The Gain Reduction Meter can be adjusted to zero to compensate for normal aging of
components over time. With no input applied and the release control set to 50, adjust
VR1 so that the meter sits exactly at the 0 dB mark. The Release control can have a
slight effect on the zeroing. The asymmetry switch should have no effect on the meter
zero. If it does, the 6AL5 tube may be at fault.
5.2 Internal Balance Test Switch
The Internal Balance Test Switch has three settings. For normal operation, it must be in
the center position. The Top position (static test) is used to set VR2 and VR4. The
Bottom position (dynamic test) is used to set VR3. The Meter Zero VR1 may have to be
trimmed after balancing.
5.3 Balancing the 6BC8 Gain Reduction Stage
Poor balance will be apparent on compression attack, causing a ripple sound or a thump.
Set the Meter Selector to Output. Set Interstage Switch to IN. Set Release to 100 (fast).
Set Output Level to 100 (full). Set Balance Test Switch to Up (Static Test).
Adjust VR2 for minimum indication on the VU meter.
Set Balance Test Switch to the Bottom position.
Adjust VR3 for minimum indication on the VU meter. Sometimes this will not be a full
dip. Only an extremely matched 6BC8 will dip fully.
Set Balance Test Switch to Up (Static Test). Adjust VR2 for minimum indication on the
VU meter. The meter should dip to −20 dB at the null. Continue to the next step to
balance the amplifier stage.
5.4 Balancing the Amplifier Stage
Only perform this adjustment after balancing the 6BC8 stage. Continue by setting the
Interstage Switch to OUT. Adjust VR4 for minimum indication on the VU meter. The
meter should indicate −10 dB or less. If it does not, suspect the 12AX7 and 12BH7. An
unbalanced amplifier can cause thumping on attack in the Interstage OUT operating
Upon completion of balancing, BE CERTAIN TO PLACE THE TEST SWITCH BACK
IN THE CENTER POSITION or you will hear hum modulating the audio signal.
5.5 Noise and Distortion Tests
To check the noise of the 176, send a 1 KHz tone into the 176 and adjust for 3 dB of
compression. Compression Ratio should be set to 12:1. Adjust the Output Level for a 0
dB reference on your analyzer. Remove the input signal and decrease the Input Level to
0. Read the residual noise on the analyzer. It should be less than –76 dB. If it is not,
remove the 6BC8 and see if the noise remains. If the noise vanishes, suspect the 6BC8
tube or 0B2 tube. If the noise remains, suspect the 12AX7 or 12BH7.
Distortion can be measured by feeding +4 dBm signal into the 176. Adjust the Input
Level for the desired amount of gain reduction, then measure the distortion at the output.
Higher amounts of gain reduction increase the distortion slightly. Faster Attack and
Release times cause increased distortion at low frequencies.
5.6 Frequency Response Tests
Frequency response tests should be performed below the threshold of gain reduction to
keep the compression from adjusting the amplifier gain. Frequency response should be
flat within +/ − 0.5 dB from 20-20,000 Hz at all settings.
5.7 Hum and Microphonics
Hum in the 176 is generally due to tube degradation or failure. Many tubes don’t make it
out of the gate, so remember that when replacing them. Tubes used in the 176
amplification are dual-section. If they are not matched well or one side fails before the
other, the 60 Hertz AC filament voltage can induce hum into the amplifier.
Noise tests will show that the 176 has effects of microphonics like all tube equipment.
Microphonics are caused by vibration in the tubes. Sometimes tubes can whistle, ring,
squeal or make other space-like sounds. Mostly, they will make sound when you tap
them. A healthy tube may make a tink or bonk sound or nothing at all. An excessively
microphonic tube will make spastic sounds when tapped which can help isolate an
intermittently noisy tube. The 6BC8 will always exhibit some microphonics because it
is the first stage of amplification and has a relatively tiny signal passing through it.
Section 6 Schematics and Drawings
Board Layouts and Schematics
Section 7 Support and Service
Retro Instruments provides all levels of support and service for your product.
7.1 Service
The Product Warranty outlines our responsibilities for defective products. We also
provide parts and service beyond the warranty period.
Before returning a product for repair, contact our service department by telephone at 209810-3344 or contact us through our service page at
We will give you instructions how to return your product for service. Please fill out the
Factory Service Form and include it with the product return.
7.2 Support
If you have a question or concern about your product you can contact our technical
support at 209-810-3344 Monday through Friday between 8AM and 6PM Pacific Time.
7.3 Replacement Parts
We usually stock all parts for our products. Most parts ship within 24 hours.
7.4 Warranty
Retro Instruments products are warranted to be free of all defects in material and
workmanship for one year from the date of purchase.
Within the period of this warranty, Retro Instruments will replace or repair at no charge
at its service facility any part which is defective in material or workmanship. This
warranty does not apply to any defects caused by negligence, misuse, or accidents, and
shall not be effective if the repair is performed by persons other than technicians at Retro
This warranty is effective only if the enclosed Product Warranty Card is fully filled out
and returned to Retro Instruments within 45 days of the date of purchase. Retro
Instruments neither assumes nor authorizes any representatives or other persons to
assume for the company any other liability in connection with the sale or any shipment of
Retro Instruments products. There are no other warranties, expressed or implied, given
by Retro Instruments extending beyond or different from the express warranty stated
Retro Instruments reserves the right to make changes and improvements in its products
without incurring any obligation to similarly alter products previously purchased.
This warranty shall be void in the event this product is materially altered after shipment
of the product from Retro Instruments, except this warranty shall not be void if such
alteration is authorized in writing by Retro Instruments for the individual customer.
7.5 Factory Service Instructions
To obtain factory service, complete this page and include it with the unit.
Name ______________________________________
Company __________________________________
Shipping Address __________________________
City, State, Zip _____________________________
Phone Number ______________________ e-mail ________________________
Model _____________________ Serial Number _________________________
Describe Problem:
Special Instructions:
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