SignVideo ENG-44 Field Mixer Calibration

by Eddy Grabczewski

The aim of recalibration is to set the ENG-44 mixer output voltages to a standard value and to check the output limiting. Study our chart of Line Levels for the ENG-44 and keep this as a reference.

WARRANTY

You may feel nervous recalibrating your brand new ENG-44 and my advice is not to do so if you mixer is still under warranty. Return it back to the SignVideo if you have any problems during this period.

After warranty then you have little to loose in recalibrating the mixer yourself. Over time the output voltages may drift and you should know when this happens. Learning how to do this will give you more control and confidence over your recordings.

Since I have no inside knowledge of how SignVideo calibrates their mixer then I must own up to the fact and warn you that following these instructions is at your own risk.

EQUIPMENT REQUIRED

To perform a recalibration, you'll need the following equipment:

1. a digital voltmeter with an AC voltage scale from 0.0 Vrms to 10.0 Vrms. The leads should terminate in crocodile clips.

2. three short XLR (male) to XLR (female) cables. You'll need to connect the voltmeter to one of these cables, between pins 2(hot) and 3 (cold).

3. an audio signal-generator is useful for the second part of the recalibration procedure. If you don't have a dedicated generator then you try using a laptop with suitable software (e.g. TrueRTA) or a CD player with a specially prepared disc. Setup the signal so that it outputs a 1 kHz sine wave at 0.775 Vrms (0 dBu).

4. A Philips head screwdriver.

5. A small screwdriver with a non-conducting handle. This is used to adjust variable resistors.

6. A pair of headphones, to hear the line-up tone.

FAMILIARIZATION

1.Place the mixer on a table top, with the front panel knobs facing you. The top black aluminium panel is the one you'll need to remove. The screws are a bit stiff, but if you hold the Philips head screwdriver firmly and push hard and twist anticlockwise then they will loosen. There are many screws and you need to remove them all. Don't bother undoing the bottom panel. There is nothing of any interest under this panel.

2. Underneath the lid you'll find a circuit board of course. Study this and you'll see five variable resistors in blue boxes with the following adjacent labels printed on the green circuit board:

  • TONE LEVEL - adjusts the signal level of the line-up tone
  • LIMITER - adjusts the cut-in point of the limiter
  • LEFT LEVEL - adjusts the signal level on the left output channel
  • RIGHT LEVEL - adjusts the signal level on the right channel.
  • SLATE LEVEL - adjusts the signal level of the slate microphone.

3. The next thing to note is the VU meter's ten LEDs. Each one fades-in to a maximum brightness and then fades out gradually. By applying a 1 kHz sine wave at 0.775 Vrms between pins 2 and 3 of the MIC/LINE inputs (set to LINE LEVEL) it's possible to tabulate when each LED reaches maximum brighness. Setting the XLR Outputs to LINE level and adjusting the MASTER gain whilst measuring the LEFT OUT and RIGHT OUT voltages across pins 2 and 3. Here are my results; yours may differ slightly:

  • -20 LED = 0.250 Vrms (-10 dBu)
  • -10 LED = 0.400 Vrms (-5 dBu)
  • -7 LED = 0.500 Vrms ( -4 dBu)
  • -5 LED = 0.600 Vrms ( -2 dBu)
  • -3 LED = 0.800 Vrms ( +0 dBu)
  • -1 LED = 0.880 Vrms (+1 dBu)
  • 0 LED = 0.970 Vrms (+2 dBu)
  • 1 LED = 1.070 Vrms ( +3 dBu)
  • 2 LED = 1.200 Vrms ( +4 dBu)
  • 3 LED = 1.320 Vrms ( +5 dBu)

RECALIBRATION PROCEDURE

1. Calibrate the Outputs - in this procedure we'll calibrate the mixer so that when the MIC/LINE INPUTS (set to LINE LEVEL) receive a 1 kHz sine wave at 0 dBu, the mixer outputs a signal of 0.775 Vrms (0 dBu):

  • Set all mixer inputs and output to LINE level and each PAN switch to 'C'.
  • Rotate all the channel faders fully anticlockwise.
  • Insert the 1 kHz sine across pins 2 and 3 of MIC/LINE INPUT of CH4.
  • Connect the headphones. You should hear the tone.
  • Rotate the TONE LEVEL variable resistor almost fully anticlockwise.
  • Rotate the LEFT LEVEL and RIGHT LEVEL variable resistors fully anticlockwise. Then rotate them clockwise a quarter-turn.
  • Attach the XLR cable to LEFT OUT and tap pins 2 and 3 into the digital voltmeter.
  • Turn the CH4 fader halfway between mark 6 (approx 1o'clock).
  • Switch on the mixer.
  • Rotate the MASTER control clockwise until the digital voltmeter reads 0.775 Vrms.
  • If the voltmeter displays a different voltage then adjust the LEFT LEVEL variable resistor to give a reading that is as close as possible to 0.489 Vrms. This can be a tedious process and you may have to settle on anything between 0.481 and 0.497 Vrms, depending on how obstinate the variable resistor is being.
  • Attach the XLR cable to the RIGHT OUT and note the output voltage. If the voltmeter doesn't display the same voltage as for LEFT OUT (0.775 Vrms) then adjust the RIGHT LEVEL variable resistor to read as close as possible to 0.775 Vrms. This can be a tedious process and you may have to settle on anything between 0.771 and 0.779 Vrms, depending on how obstinate the variable resistor becomes.
  • You are now ready to calibrate the tone generator in the following step.

2. Calibrate the Tone Generator - in this procedure we'll calibrate the tone generator so that when the TONE ON switch is selected, a 1 kHz sine wave at precisely 0 dBu (0.775 Vrms) is sent to the mixer outputs LEFT OUT and RIGHT OUT:

  • On the front panel, switch on the reference tone using the TONE ON switch but be very careful not to knock the MASTER gain control.
  • Now measure output voltage on LEFT OUT. It should be close to 0.775 Vrms. If not then adjust the TONE LEVEL variable resistor until it is.
  • Now check the output voltage on RIGHT OUT. If this is not close to 0.775 Vrms then I suggest you calibrate the outputs (step 1) and calibrate the tone generator (step 2) once again.
  • You are now ready to calibrate the limiter in the following step.

3. Calibrate the Limiter - in this procedure we'll calibrate the mixer to limit the output signal to 1.946 Vrms (+8 dBu) - which is the Nominal Peak Level for an SQN mixer. The mixer should start limiting at 1.734 Vrms (+7 dBu). The LIM light on the front panel glows RED in proportion to the amount of output limiting:

  • Set all mixer inputs and outputs to LINE level and select each PAN switch to 'C'
  • Input a 1 kHz sine wave at 0 dBu into the MIC/LINE 4 input.
  • Connect the headphones. You should hear the tone.
  • Attach the XLR cable to LEFT OUT.
  • Tap pins 2 and 3 of the XLR cable into the digital voltmeter.
  • Switch on the mixer.
  • Rotate the CH 4 gain control clockwise to its maximum position (full gain).
  • Observe the reading on the voltmeter.
  • Adjust the LIMITER variable resistor until the voltmeter reads just below 1.946 Vrms (+8 dBu). The LIM light should now be glowing at full strength.
  • Rotate the CH 4 gain fully anticlockwise (no gain).
  • Attach the XLR cable to the RIGHT OUT and Rotate the CH 4 gain control clockwise to its maximum position (full gain).
  • Ensure that the output voltage of RIGHT OUT does not exceed 1.946 Vrms (+8 dBu). In my own tests, the voltage read 1.486 Vrms (+6 dBu), which is not symmetrical with LEFT OUT and slightly below the 3 VU LED level.
  • This completes the limiter calibration procedure.

FINAL REMARKS

The ENG-44 has an output limiter that works simultaneously on both LEFT OUT and RIGHT OUT channels, however our mixer outputs significantly different voltages on the LEFT OUT and RIGHT OUT channels during limiting - RIGHT OUT was limiting at 2 VU whilst LEFT OUT was limiting at 4 VU - a hardly satisfactory yet uncorrectable discrepancy.