QES Labs VARIABLE GM User manual

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VARIABLE GM
Stereo Vacuum Tube Compressor
Rev. 4 –Aug. 9, 2017.
OWNER’S MANUAL

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Thank you for purchasing our product.
We strive to provide you with a professional product, a toolbox you will use for
several years with satisfaction.
This unit has been carefully handcrafted by Val in Italy, choosing the finest
components available, aiming for your complete satisfaction.
It is of extreme importance that the unit be earthed !
You should use it with a FULLY functional earth well.
NEVER disconnect the earth from the receptacle.
Please make sure that your line mains voltage matches with the VariGm AC Inlet
indication.
FUSE:
You should replace defective fuse with 2AT (slow blow) for 230Vac line, or 4AT for
115Vac line.
Please carefully read this manual throughout before operating the unit.
ANY real tube equipment is sensitive to a high SPL environment. This may cause
microphonics in a recording situation. Make sure you are able to shock mount and or
place the unit in isolation if need be.
Be sure to have at least one rack space between any tube products from above and
below the unit. This will ensure your tube unit will not over heat. Overheating will
cause damage to the tubes and shorten their life span.
QES Labs reserves the right to make improvements or changes to it’s products at any
time, without notice.
Then have fun with your job.

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LOCATION AND VENTILATION
The Variable Gm Compressor must be installed in a stable location with ample
ventilation. It is strongly recommended, when rackmounting this unit, that you allow
enough clearance on the top and bottom of the unit such that a constant air flow is
allowed. The unit is cooled by the air motion (thermal conduction), heat
spontaneously flows from a hotter to a colder body.
Good ventilation is of the utmost importance.
UNPACKING
Unpack the unit carefully by removing all the custom foam packing material, then
make sure that all supplied accessories are present.
Carefully examine all items for any possibility of shipping damage.
Variable Gm 3U rackmount compressor
External P.S.U.
PSU umbilical cable
2-pc. IEC mains power cords
If the unit is damaged or fails to operate, notify your shipper and us or your local
authorities immediately.
It is prudent to retain the shipping materials for future use.
INSTALLATION
Make sure your mains voltage supply agrees with the voltage this unit is set for.
Make sure that the unit has the proper fuse installed and that the mains switch is OFF.
Connect the input cables to the 3-pin XLR female connectors on the rear panel.
The Input XLRs are transformer coupled balanced wired as follows:
PIN 1 : Ground,
PIN 2 : Hot (positive going phase),
PIN 3 : Cold (negative going phase).

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Connect BOTH the IEC mains power cords to PSU and main unit, then the umbilical
cable from the PSU, which should be installed on the floor, as far wawy as possible
from the rack mounted main unit. The umbilical cable sockets are 4-prong for the
heater supply, and 5-prong for the anode supply, and 12V supply.
1. OPERATION
Connect the unit before switching it on.
Make sure that all of the external connectors are plugged into the Variable Gm
sockets, then switch the unit on.
Power up the unit and allow it enough time to stabilize before using. It usually takes
30 minutes to 1 hour before everything has stabilized and for the unit to warm up and
sound at its best. And of course remember to leave 1-2 inches on top so that air can
flow up and away from it. Definitely avoid that the heat gets trapped with no place to
flow, this way you’ll literally bake up the internals, shortening lifespan of the caps
and tubes!

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2. FUNCTIONS
From left to right, the first stage of the Variable Gm is the Remote cut-off preamp.
The available controls are:
a) INPUT
It is a continuously variable pad which levels the incoming audio signal from the
Input XLR connector located at the back panel. In conjunction with the OUTPUT
control, it imparts the colour of the compression. The more signal you feed to the
remote cut-off stage (input stage), the more coloured and ‘saturated’ the
processed audio will be. It’s an attenuator from –inf. To 0 dB, and is active only in
LIMIT mode.
OUTPUT
It is 9 dB variable attenuator (pad), it will serve you to trim the output signal if you
need gain on the INPUT selector. It works in conjunction with it to determine the
compression action. 0dB point: input at -18, output at -9 (full CCW).
METER FUNCTION SWITCH
Toggles between IN, GR, OUT. In the first position (In), the VU meter indicates the
average (very similar to RMS) level of the signal present at the Input XLR connector
for that channel. A 0VU indication lets you know that a +8 dBu (1.947Vrms) signal
is present. Consequently, the +4dBu indication is found at the 50% on the percentage
scale.
GR: Indicates the amount of gain reduction. The movement deals with the current
variation of the remote cut-off tubes during the peak reduction action. The needle
stands at the 0 with no compression. Even if the current variation has a relationship
with the dB’s of compression, the scale is not read in dB directly. You will have to
learn the correct dB indication by ear. Usually when the needle is at -1, you’re having
about -2.5/-3dB of Peak Reduction.
OUT : Applies the same spec’s as in the IN position, but the meter is actually
monitoring the signal present at the Output XLR connector.
ATTACK
This controls the Attack time, the time when the compression action starts. The span
is from 25 ms, fastest, in full cw position, and 100 ms, slowest, full ccw position. The
time indication refers to the first time constant, i.e. when the reduction reaches 63%
of it’s final value.

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-4 –
RELEASE
It’s an 11 position selector. Timing indications indicate the first time constant (63%
of final value) of the time the compression action ceases. It is expressed in seconds.
THRESHOLD
This potentiometer (or 24-step rotary switch for the mastering version) indicates at
which level the peak reduction action starts. In full ccw position the compression is
out. At cw position the compression is the max. available.
It depends on the INPUT selector and the level of the incoming audio signal.
SIDE CHAIN HIGH PASS FILTER
It lets you cut the bass frequencies enter the side chain rectifier, so that the bass notes
are not governing the compression action. It does not affect the actual audio signal
path, just the Peak Reduction action. This switch is effective for bass-heavy music,
where you don’t want the bass driving the whole action of compression.
BYPASS
Two independently operated switches for each channel, controlling an audio relay
action. In bypass position the output connector is short circuited through the input
connector (Hard-Wired Bypass).
STEREO LINK
It toggles between:
DUAL MONO: the unit acts as two independent channels.
STEREO: the side chains are tied together, this way the stereo image is preserved.
When the switch is on STEREO position, you still have to manually balance, and set
equal, all of the selectors.
You can match them precisely by feeding the unit with a sine wave tone, and set the
levels exactly equal through the meters.
-5 -

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3. MEASUREMENTS
Noise floor

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TECHNICAL SPECIFICATIONS:
Frequency response (+0/-0.5dB):
Channel Out (14 dB gain): 10 Hz –20 KHz
5 Hz –40 kHz (-3 dB)
Signal to noise ratio (unweighted,50Ωsource Z, 10Kohm load, ref. +24dBu):
Greater or equal to 98dB, 20Hz-20KHz
Common Mode Rejection Ratio (C.M.R.R.) ref. 1KHz, 14dB gain:
70dB.
Total Harmonic Distortion + Noise (THD+N), 1KHz, 5 harmonics count:
Gain 14dB, GR out, Output Level +18dBu = 1.06%
Intermodulation Distortion IMD (SMPTE), 60Hz/7kHz, 4:1, ref. +4dBu:
Line amp: less than 0.1%
Input and Output Impedance:
Input: 20 Kohm
Output: 200 ohms
Maximum Gain, source 50Ω, load 10kΩ :
29 dB (±2dB)
Maximum Output Level, balanced, 20Hz-20kHz, load 10kΩ, THD+N less than or
equal to 1%:
+18dBu (±2dB)

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DECLARATION OF CONFORMITY
I declare that the model “Variable Gm” conforms to the following
standards adopted by the European Community:
EMC (electro magnetic compatibility) 89/336/CEE directive
LV (low voltage equipment) 73/23/CEE directive
on electrical safety (EN61010) and electromagnetic compatibility (EN61000).
RoHS (Restriction of Hazardous Substances) 2002/95/CEE directive.
QES Labs
di Valerio Riccelli
4 Via Ugo Niutta
Napoli, NA 80128
Italy.
Data / Date / Datum: 11/09/2007.
Firma / Signature / Signatur / Unterschrift
Direttore tecnico / Technical manager / Directeur technique.

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WARRANTY
QES Labs warrants this product to be free of defective parts and
workmanship for a period of 1 (one) year from the date of original
purchase, except for vacuum tubes (60 days).
This warranty excludes the following conditions: normal wear and tear,
misuse, customer negligence, accidental damage, adverse weather and/or
natural events, unauthorized repair or modification, cosmetic damage and
damage incurred during shipment.
This warranty is provided by the dealer where the unit was purchased, and
by QES Labs. In case of a valid warranty claim, buyer’s sole and exclusive
remedy and QES entire liability under any theory of liability will be, at its
option, to repair or replace the product without charge, or, if not possible,
to refund the purchase price.
The warranty is not transferable, and applies only to the original purchaser
of the product.
This warranty is in lieu of all warranties whether oral or written, expressed
or implied, including, but not limited to, any implied warranty of
merchantability or fitness for a particular purpose.
The warrantor assumes no liability for lost profits, property damage or any
other direct, indirect, special, incremental, incidental or consequential,
exemplary damages whatsoever which may result from failure of this
product, even if QES Labs is aware of the possibility of such damages.
In no event will QES Labs’ liability exceed the purchase price of the
product.
Any and all warrantees of merchantability and fitness implied by law are
limited to the duration of the expressed warranty.

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RECALL SHEET
SESSION:___________________________________________________
SONG: ___________________________________________________
PRODUCTION:________________________________________________
ARTIST: ____________________________________________________
ALBUM: ____________________________________________________
SOUND ENG.:_________________________________________________
SETTINGS FOR:_______________________________________________