Pro VLA II™
PROFESSIONAL TWO CHANNEL VACTROL®/TUBE LEVELING AMPLIFIER
USER’S GUIDE
IMPORTANT SAFETY INSTRUCTIONS – READ FIRST
This symbol, wherever it appears,
alerts you to the presence of uninsulated
dangerous voltage inside the enclosure. Voltage
that may be sufficient to constitute a risk of shock.
This symbol, wherever it appears, alerts
you to important operating and maintenance
instructions in the accompanying literature.
Please read manual.
Read instructions:
Retain these safety and operating instructions for future reference. Heed all warnings printed here and on the equipment.
Follow the operating instructions printed in this user guide.
Do not open:
Aside from two vacuum tubes, there are no user serviceable parts inside. Refer any service work to qualified technical
personnel only.
Power sources:
Only connect the unit to mains power of the type marked on the rear panel. The power source must provide a good
ground connection.
Power cord:
Use the power cord with sealed mains plug appropriate for your local mains supply as provided with the equipment. If the
provided plug does not fit into your outlet consult your service agent. Route the power cord so that it is not likely to be
walked on, stretched or pinched by items placed upon or against.
Grounding:
Do not defeat the grounding and polarization means of the power cord plug. Do not remove or tamper with the ground
connection on the power cord.
Ventilation:
Do not obstruct the ventilation slots or position the unit where the air required for ventilation is impeded. If the unit is to be
operated in a rack, case or other furniture, ensure that it is constructed to allow adequate ventilation.
Moisture:
To reduce the risk of fire or electrical shock do not expose the unit to rain, moisture or use in damp or wet conditions. Do
not place a container of liquid on it, which may spill into any openings.
Heat:
Do not locate the unit in a place close to excessive heat or direct sunlight, as this could be a fire hazard. Locate the unit
away from any equipment, which produces heat such as: power supplies, power amplifiers and heaters.
Environment:
Protect from excessive dirt, dust, heat, and vibration when operating and storing. Avoid tobacco ash, drink spillage and
smoke, especially that associated with smoke machines.
Handling:
To prevent damage to the controls and cosmetics avoid rough handling and excessive vibration. Protect the controls from
damage during transit. Use adequate padding if you need to ship the unit. To avoid injury to yourself or damage to the
equipment take care when lifting, moving or carrying the unit.
Servicing:
Switch off the equipment and unplug the power cord immediately if it is exposed to moisture, spilled liquid, objects fallen
into opening, or the power cord or plug becomes damaged during a lightning storm or if smoke odor or noise is noted.
Refer servicing to qualified technical personnel only.
Installation:
Install the unit in accordance with the instructions printed in the user guide.
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Pro VLA II
PROFESSIONAL TWO CHANNEL VACTROL®/TUBE LEVELING AMPLIFIER
IMPORTANT SAFETY INSTRUCTIONS – READ FIRST .......................................................................... 1
INTRODUCTION .................................................................................................................................... 3
INSTALLATION ..................................................................................................................................... 3
Unpacking .............................................................................................................................................................................................. 4
AC Power Hookup.................................................................................................................................................................................. 4
Analog Audio Connections ..................................................................................................................................................................... 4
FRONT PANEL CONTROLS and INDICATORS ...................................................................................... 5
Power ..................................................................................................................................................................................................... 5
Input Threshold Control .......................................................................................................................................................................... 5
Ratio Control .......................................................................................................................................................................................... 5
Output Level Control .............................................................................................................................................................................. 6
Attack Control......................................................................................................................................................................................... 6
Release Control...................................................................................................................................................................................... 7
Gain Reduction LED Meter..................................................................................................................................................................... 7
Bypass Switch ........................................................................................................................................................................................ 7
VU Meter ................................................................................................................................................................................................ 7
VU Meter Source Switch ........................................................................................................................................................................ 7
LED Output Meter .................................................................................................................................................................................. 8
The Link Switch ...................................................................................................................................................................................... 8
REAR PANEL I/O ................................................................................................................................... 9
Balanced Inputs...................................................................................................................................................................................... 9
Balanced Outputs................................................................................................................................................................................. 10
I/O Level Switch ................................................................................................................................................................................... 10
APPLICATIONS ................................................................................................................................... 11
Compressor/ Limiter ............................................................................................................................................................................. 11
Stereo Mix and Mastering..................................................................................................................................................................... 11
Vocal and Instrument Leveling ............................................................................................................................................................. 11
WARRANTY INFORMATION................................................................................................................ 12
SERVICE ............................................................................................................................................. 13
SPECIFICATIONS................................................................................................................................ 14
LIST OF FIGURES
FIGURE 1 – Channel 1 Controls ............................................................................................................ 6
FIGURE 2 – LED Output Meter and Link Switch................................................................................... 8
FIGURE 3 – Rear Panel ....................................................................................................................... 10
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INTRODUCTION
Thank you for purchasing Applied Research and Technology’s Pro VLA II. Offering a superb level of sound
quality, the Pro VLA II has a unique design that will enhance the sonic textures of your audio system for
years to come. The Pro VLA II is one of the finest audio compressors available. Developed in partnership
with studio and live-sound engineers, the Pro VLA II possesses a “sound” that is not available from any other
compressor on the market - at any price! Unlike typical compressors which use solid state feed-forward
circuitry to control level detection, the Pro VLA II is very musical. The nature of its operation is much like the
way your eye adjusts to light. Just as your eye transparently adjusts to changes in light, the Pro VLA II
adjusts to changes in signal level.
The Pro VLA II was designed and constructed with the absolute best components, assuring a lifetime of
quiet, reliable performance.
The Pro VLA II offers:
- Two channel opto-isolator (Vactrol®)/tube-based leveling amplifier
- Stereo linking of channels
- Master Volume and Balance control in Link Mode
- Hand selected 12AT7 Tubes
- Vactrol® - Analog Optical Isolators (AOI)
- Non-VCA design = transparent, classic sound
- VU metering of input and output levels
- LED metering of output level with peak-hold function
- XLR balanced inputs and outputs
- 1/4” TRS active balanced inputs and outputs
- Variable Threshold & Ratio controls
- Variable Attack and Release controls
- Variable Output Level rotary controls
- 10 segment Gain Reduction LED array
- 10Hz to 100KHz Frequency Response (+/-0.5dB)
- >100dB Dynamic Range
- Low noise (-99dBm ‘A’ weighted EIN)
- Internal power supply
- 3 year parts and labor warranty
INSTALLATION
The ART Pro VLA II may be used in a wide variety of applications and environments. In a rack-mountable,
all-steel enclosure, the unit is designed for continuous professional use. Mounting location is not critical,
however for greater performance reliability we recommend that you not place the unit on top of power amps, or
other sources of heat and/or strong magnetic fields. The tube circuitry needs about a minute to “warm up” and
stabilize from a cold power up.
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Unpacking
Your Pro VLA II was packed with care at the factory. The shipping carton was designed to protect it during
initial shipment. Please retain this carton for use in transporting the Pro VLA II when it is not installed in a
rack, or in the unlikely event that you need to return your Pro VLA II for servicing. The shipping carton should
contain:
- The Pro VLA II
- The owner’s manual
- User Registration Card
AC Power Hookup
The Pro VLA II has an internal power supply designed to operate at 100 to 125VAC, 50 to 60Hz. Units
manufactured for use outside of the United States of America have been modified to comply with the required
electrical specifications. Under no circumstances should the power cable be altered. If the cable becomes cut
or damaged, discontinue its use and have it replaced before operating the Pro VLA II.
The power source must provide a good ground connection, and the ground pin on the mains plug should
never be defeated.
Analog Audio Connections
Audio connections to and from the Pro VLA II are:
Balanced input:
[XLR] Pin 2 = Hot (+), Pin 3 = Cold (-), Pin 1 = Ground
[1/4”] Tip = Hot (+), Ring = Cold (-), Sleeve = Ground
Balanced output:
[XLR] Pin 2 = Hot (+), Pin 3 = Cold (-), Pin 1 = Ground
[1/4”] Tip = Hot (+), Ring = Cold (-), Sleeve = Ground
Unbalanced 1/4” cables may be used with no damage to the Pro VLA II. We recommend using only highquality cables equipped with the appropriate connectors.
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FRONT PANEL CONTROLS and INDICATORS
Power
The POWER switch turns the power on and off to the unit. The Pro VLA II should be turned “on” with all
monitor levels turned down to protect against any “thumping” caused by power up. Likewise, the Pro VLA II
should be turned “off” after turning all monitor levels down.
When the POWER switch is turned on, the VU meters will light. LEDs will light if its associated switch is in the
“in” position. Power on the Pro VLA II before any monitoring outputs or power amps are turned on.
Like all tube-based equipment, the Pro VLA II needs to warm up to operating temperature. The 12AT7
tubes will reach operating temperature within one to three minutes. During this warm-up period you may
experience sound variations due to the bias of the tubes stabilizing. This is normal and the Pro VLA II will
provide consistent results once the warm-up period is over.
If the Pro VLA II fails to power up when the POWER switch is turned on, check to see that its power cord is
plugged into an active outlet. If the unit still fails to operate properly, turn it off and unplug it. Then consult your
dealer or the ART Customer Service Department.
Input Threshold Control
The INPUT THRESHOLD control sets the point at which the Pro VLA II will act on a signal. Turning this
control counter-clockwise lowers the threshold (adding more compression to a signal). Turning this control
clockwise raises the threshold.
Proper setting of the INPUT THRESHOLD control is dependent on the input signal. The output of a guitar can
be –20dB to –10dB, whereas the level from an insert on a mixing console can be –10dB to +15dB. The easiest
way to set the INPUT THRESHOLD control is to start with the control fully clockwise. Slowly turn the control
counter-clockwise (lowering the threshold) until the yellow (0) LED light on the Gain Reduction meter begins to
light. Next adjust the control (either lower or higher) for the desired amount of compression. Use the Gain
Reduction meter as a visual guide to the amount of compression applied.
Ratio Control
The RATIO Control selects the “amount” of compression applied to the input signal once that signal reaches
or exceeds the threshold. This compression amount is expressed as a ratio of input to output. For example if a
4:1 compression ratio is chosen, for every 4dB over the threshold the input signal rises, the output level only
rises by 1dB. In this case if the input signal increased 12dB over the threshold, the output level would only rise
by 3dB.
In general, compression ratios of 10:1 and greater are considered “limiting”. The range of the RATIO control
is (2:1) to (20:1). The Pro VLA II may be used as either a compressor or a limiter with all of the following:
- Multitrack recorder, DAT machine, hard disk recorder, or analog recorder.
- In a mixer’s channel insert points.
- Between a microphone preamp and signal processors.
- Between preamplified electronic musical instruments (synthesizers, guitars, bass, samplers, acoustic
instruments with pickups) and other line-level equipment.
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FIGURE 1 – Channel 1 Controls
Output Level Control
The OUTPUT LEVEL control serves two functions.
The CH1 knob always controls the output level of CH1, but can also serve as a “MASTER” output level when
the Link Mode is active. The fully counter-clockwise position of the knob mutes the output.
The CH2 OUTPUT LEVEL control operates as a standard CH2 level control or as a “BALANCE” control when
the Link Mode is active. In Link Mode when the BALANCE control is turned fully counter-clockwise the CH1
output is fully on and CH2 output is fully muted. When centered, both channels have equal gain. When turned
fully clockwise the CH2 output is fully on and CH1 output is fully muted.
Attack Control
The ATTACK control sets the time it takes the Compressor/Limiter to respond to increases in signal level (by
reducing gain). You can use this control to shape the “front end” of the dynamics envelope.
One example is to listen to a snare hit and adjust the ATTACK control. A short attack makes the snare sound
“thin”. As the attack time goes longer (the knob is turned clockwise) you should hear more of the thump in the
compressed snare. The downside is that this creates an overshoot, (or “transient”), the length of which is the
time set by the ATTACK control.
Overshoots less than 1 msec. are very hard to hear even when they are clipped. If the ATTACK is set too
fast, the gain may be reduced too much and thereby create a “pumping” sound1.
1
“Pumping” in a Compressor/Limiter sounds like the signal is muted when it shouldn’t be.
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Release Control
The RELEASE control sets the time the Compressor/Limiter takes to increase the gain after the input level
drops.
Longer settings maintain the dynamics of the input signal, while shorter settings reduce the dynamics.
Shorter settings will also increase the apparent reverberation, and at extreme gain reduction settings, lead to
“breathing” artifacts2
Gain Reduction LED Meter
The GAIN REDUCTION meter displays the Pro VLA II attenuation action. The meter covers a very large
range while offering high resolution. The Yellow (0) LED illuminates when the input signal reaches or exceeds
the compressor detector threshold setting (INPUT THRESHOLD control).
Bypass Switch
The Pro VLA II BYPASS switch physically connects the input jack to the output jack on each respective
channel (also known as a hardwire bypass). The switch is lit when the unit is bypassed (unless power is off, yet
the analog signal still passes from input to output).
VU Meter
The input or output levels of the Pro VLA II can be monitored by the analog VU meter. The VU METER
switch selects the source of the audio for this meter. The ballistics characteristic of the meter provides an
accurate indication of the average signal level. If you need peak level info (critical in digital systems) use the
central stereo LED bargraph OUTPUT LEVEL meter.
VU Meter Source Switch
This switch selects either the input or output signal as a source for the analog VU meter, and lights when the
input signal is selected. This function allows the user to compare the input vs. output level.
This switch does not affect the LED bargraph meter’s audio source.
2
“Breathing” is the sound of the Compressor/Limiter turning up the gain so quickly you can hear breathing noises
between words during vocal processing.
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FIGURE 2 – LED Output Meter and Link Switch
LED Output Meter
This meter displays both average and peak levels of the output signal. The LED labels indicate how far the
output is away from full maximum output in dB.
In each bargraph, a single LED is held on for a little over one second, displaying the peak level. Some peaks
may not be audible, but can cause clipping in equipment following the Pro VLA II.
Average levels are indicated by a histogram style display. The average levels displayed are similar to the VU
meter when it is set to monitor the output level.
“0” on the meter corresponds to an output level of +16dBu when the I/O level switch is in the +4 mode. When
in the “-10” mode, this level becomes +6dBu.
The Link Switch
The two channels of the Pro VLA II can be configured for stereo operation by depressing the LINK switch.
Link Mode ties the output attenuation of both channels together onto the CH1 OUTPUT LEVEL knob, and
moves all control over “Attack” and “Release” times to the CH1 set of controls. The CH2 OUTPUT LEVEL
control becomes an output balance control. Link Mode ensures that each channel of the stereo input signal is
processed identically to prevent any shifting or distortion of the stereo image.
Bypass and meter-source select switch functions remain unchanged in “link” mode.
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REAR PANEL I/O
The Pro VLA II™ XLR connectors follow the AES standard of:
Pin 1 = Ground, Pin 2 = Hot (+), Pin 3 = Cold (-).
The balanced 1/4” phone jacks are typical TRS connections:
Tip - Hot (+), Ring - Cold (-), Sleeve - Ground.
Balanced Inputs
The Pro VLA II™ provides both an XLR and 1/4” input jack. Both jacks are balanced. The 1/4” jack can be
used with unbalanced signals by simply plugging in an unbalanced cable.
The inputs are designed for use with line level signals ranging from –30dBm to +20dBm. While it is possible
to plug an instrument directly into the Pro VLA II™, it is desirable to run the instrument into a preamp ahead of
the Pro VLA II™. This will provide a stronger signal and will keep noise to a minimum. Microphones must
connect through a microphone preamplifier (like the ART MPA GOLD) before connecting into the Pro VLA II™.
The input impedance of the input 10K Ohms.
NOTE: The Pro VLA II™ has a passive (hardwire) bypass. This means that the bypass works even if the
power is OFF.
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FIGURE 3 – Rear Panel
Balanced Outputs
The analog output of the Pro VLA II™ is available on both a 1/4” TRS balanced jack and an XLR jack. This
output is active pseudo balanced. The output has a 100 Ohm impedance into a balanced or unbalanced load.
The LED and analog VU meter monitor the level present at this output. “0” VU on the analog VU meter
corresponds to +4dBu (about 1.23 Volts RMS). “0” on the LED bargraph OUTPUT LEVEL meter corresponds
to an output level of +16dBu into a balanced load.
I/O Level Switch
The Pro VLA II™ can be optimized for either “professional” (+4dBu) or “consumer” (-10dBV) I/O levels.
When the switch is depressed the unit is in “professional” levels mode. This shifts the levels such that “0” VU
on the meter corresponds with +4dBu on the input or output.
When set to the “-10” mode, “0” VU corresponds to about –10dBV.
NOTE: +4dBu is equivalent to +4dBm into a 600 Ohm balanced load.
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APPLICATIONS
Compressor/ Limiter
The main application of the Pro VLA II™ is to control the dynamic range of an audio signal. Plug a line level
(post preamplifier or other gain stage) source into either input, and set the threshold and output controls to
provide the desired amount of compression to the input signal.
Stereo Mix and Mastering
Because of its low noise and excellent tonal qualities, the Pro VLA II™ is ideal for processing mixes when
recording to DAT, hard disk or analog recording devices. Used as a mastering device, the Pro VLA II™ is
capable of adding warmth and impact to the overall signal level. Pro VLA II™ is ideal for live use as well.
Vocal and Instrument Leveling
The musical nature of the Pro VLA II™ makes it ideal for use on a wide range of instruments and vocals.
Place the Pro VLA II™ into a channel insert, after a preamplifier, or in line with the direct output of a mixer
channel. Adjust the controls of the Pro VLA II™ to achieve the desired amount of compression.
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WARRANTY INFORMATION
Limited Warranty:
Applied Research and Technology will provide warranty and service for this unit in accordance with
the following warrants:
Applied Research and Technology, (ART) warrants to the original purchaser that this product and the
components thereof will be free from defects in workmanship and materials for a period of three years
from the date of purchase. Applied Research and Technology will, without charge, repair or replace,
at its option, defective product or component parts upon prepaid delivery to the factory service
department or authorized service center, accompanied by proof of purchase date in the form of a
valid sales receipt.
Exclusions:
This warranty does not apply in the event of misuse or abuse of the product or as a result of
unauthorized alterations or repairs. This warranty is void if the serial number is altered, defaced, or
removed.
ART reserves the right to make changes in design or make additions to or improvements upon this
product without any obligation to install the same on products previously manufactured.
ART shall not be liable for any consequential damages, including without limitation damages resulting
from loss of use. Some states do not allow limitations of incidental or consequential damages, so the
above limitation or exclusion may not apply to you. This warranty gives you specific rights and you
may have other rights, which vary from state to state.
For units purchased outside the United States, an authorized distributor of Applied Research and
Technology will provide service.
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SERVICE
The following information is provided in the unlikely event that your unit requires service.
1) Be sure that the unit is the cause of the problem. Check to make sure the unit has power, all
cables are connected correctly, and the cables themselves are in working condition. You may want to
consult with your dealer for assistance in troubleshooting or testing your particular configuration.
2) If you believe the ART unit is at fault, go to www.artproaudio.com. You may contact Customer
Service for more assistance, or directly request a Return Authorization for service in the “resources”
area of the website.
3) If you are returning the unit for service, pack the unit in its original carton or a reasonable
substitute. The original packaging may not be suitable as a shipping carton, so consider putting the
packaged unit in another box for shipping. Print the RA number clearly on the outside of the shipping
box.
4) Include, with your unit, a note with the RA number and your contact information including a daytime
phone number, preferably attached to the top of the unit.
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SPECIFICATIONS
Input Impedance
Balanced/Unbalanced ................................................................10K Ohms
Maximum Levels
Range Switch .............................................................................+4dBu/-10dBV selectable
Inputs .........................................................................................+20dBu (in +4dBu mode) / +10dBV (in –10dBV mode)
Output.........................................................................................+20dBu (R load > 10K, 0.1%THD)
Output Gain .................................................................................... +20dB max
Frequency Response ......................................................10Hz to 100KHz (+/- 0.5dB)
Dynamic Range .................................................................>100dB (20-20KHz)
THD @ 0dBm Out: .............................................................<0.1% (typical)
Equivalent Input Noise (EIN)
+4dBu mode ..............................................................................-99dBu ('A' weighted)
-10 dBV mode ...........................................................................-107dBu ('A' weighted)
Attack Time ......................................................................... 0.25msec. to 50msec. variable
Release Time ......................................................................150msec. to 3 sec, variable
Slope ......................................................................................Variable: 2:1 to 20:1
Max. Attenuation ...............................................................30dB
Dimensions .................................................................................... 3.50” H x 19.0” W x 9.17” D
Weight .....................................................................................10.5 lbs.
Power Requirements ............................................................... USA – 105 to 125 VAC / 60 Hz Export units configured for country of
destination.
Note: 0 dBu = 0.775 VRMS, 0 dBV = 1 VRMS
ART maintains a policy of constant product improvement. ART reserves the right to make changes in design, or make additions to, or
improvements upon, this product without any obligation to install same on products previously manufactured. Therefore, specifications
are subject to change without notice.
Vactrol® is a registered trademark of PerkinElmer Optoelectronics
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www.artproaudio.com
E-mail: support@artproaudio.com
© 2007 Applied Research & Technology
213-5004-100 e
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