CALIBRATION PROCEDURE
This procedure has been done by the factory and should not be re-adjusted unless absolutely needed
and should only be done by a qualied technician. There are potentially painful high voltages inside
the VOXBOX so care should be taken even by good technicians. They have been known to get a
shock but they also know to use one hand and insulated tools or gloves.
The function of each trim is labelled on the diagram on the previous page (page 18). You will
need an audio oscillator and a high impedance AC voltmeter plus a medium small tip at screwdriver
and a medium (#2) Phillips screwdriver. After removing the top let the unit warm up for 30 minutes so
that your calibration will be actually valid. You should check basic signal ow and familiarize yourself
with the unit during this time.
1) The rst step is to make the EQ / DE-ESSER gain block be “unity gain” with no EQ or Limiting. Put a 1 kHz
+4.0 dBu (1.228 Volts RMS) tone from an oscillator into the INSERT INPUT and select INSERT.
Make sure EQ and DE-ESS is BYPASSED. Measure the EQ OUTPUT PHONE JACK. It should read
the same level as the input signal. If not adjust “trimmer 1” (EQ=UNITY GAIN) for +4.0 dBu or 1.228
volts AC RMS.
2) Use the VU meter select switch to read “EQ OUT”. The VU meter should read “0”. If not, adjust
trimmer 2 (0 on VU METER = +4.0 dBu = 1.288 VAC RMS).
3) Use the VU meter select switch to read “GR”. The VU meter should read “0”. If not, adjust trimmer 3 (ME-
TER GR ZERO SET).
4) Now put the oscillator into the LINE INPUT and the AC voltmeter on the PREAMP OUT PHONE JACK.
Put the PHASE switch into the middle position (LINE) and the GAIN switch to “45”. The lter should
be set FLAT and COMPRESSOR in BYPASS and both ATTACK and RELEASE to MEDIUM. Verify
that unity gain for the line input is corresponds on the “INPUT” knob to approximately straight up or
11:00. Set the INPUT for +4.0 dBu (1.228 volts) measured at the PREAMP OUT. Now take the COM-
PRESSOR out of BYPASS and turn up the COMPRESSOR THRESHOLD to maximum. Select “PRE
OUT” on the VU meter switch. The VU should also read “-8”. If not adjust “trimmer 4” (COMP CAL)
so that it does.
5) Same as above except switch VU to GR mode and adjust COMP GR CAL (trimmer 5). Now turn the thresh-
old so that there is 2 dB of compression. Verify that GR and audio “track” within .5 dB. 6) Now put
the oscillator back into the INSERT INPUT and the AC voltmeter on the EQ OUT PHONE JACK.
SELECT LIMIT on the De-ess rotary switch. Select “EQ OUT” on the VU meter switch. Now take
the DE-ESSER out of BYPASS and turn up the DE-ESSER THRESHOLD to MAXIMUM so that the
output signal is reduced 3.5 to 4 dB (or that 40% modulation dot)(-2 dBu or .614 AC volts) . The VU
should also read approximately “-3.5”. If not adjust “trimmer 6” ( DS / LIMIT CAL) so that it does.
You can also check De-esser operation by switching the oscillator to 3 kHZ and selecting 3 kHz on the
de-esser select switch. Adjust the DE-ESSER THRESHOLD so that the output signal is reduced 6 dB
which will read -2 dBu or .614 AC volts. The VU meter should read “-6” +/- .5 dB.
7) Same as above but switch VU to D-S and adjust METER DS CAL(trimmer 7). Verify DS and audio track. In
other words watch that “real” de-essing and “indicated” de-essing agree.
Step 4 sets up the compressor ratio, Step 5 makes the G-R meter agree. Step 6 sets up the LIMIT Threshold and
step 7 makes the D-S meter agree. Two units should be calibrated together for the LINK to perform accurately.
At the factory we use a test jig that injects a signal into the LINK jacks to ensure unit to unit sameness needed
for LINK to be very predictable. The above procedure will be close.
The calibration procedure is completed unless you want to double check, which we always recommend. In
that case return to step 1.
Each of the power supplies (Not B+) is regulated and there are no trims involved. The voltages should be:
+12 VDC for laments, relays & lamps / leds.
+15 VDC for ICs used in sidechains and meter drivers
-15 VDC for ICs used in sidechains and meter drivers
+300 VDC for vacuum tube B+ supplies.
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