What this handy small box enables you:
Quickly and precisely adjust all microphone inputs to optimum low noise and recording gain settings.
Designed for microphones with sensitivities between -57dBv/Pa and -30dBv/Pa.
Easy adjustment for microphone sensitivity and accurate level indication by using the mixers overload-LED.
All microphone channels will have the same 120dBspl sound pressure clip level, and a low noise level.

Prototype with more adjustment steps (not needed).

Live multi channel recording problem :
"How to adjust microphone gain for enough headroom, and optimal low noise level at all microphone channels ?"

Due to the small knobs, accurate microphone gain adjustment at some multi channel recording interfaces is difficult.

You could feed a test signal to every microphone input with a level of fi. -50dBv. Knowing the sensitivity of each microphone, then calculate what the Vu meter indication on the recorder should be. Then adjust the microphone gain for that VUmeter reading.

But during a hectic setup for a HDD multi track live recording session, this easily leads to stress, uncertainty and later when balancing and mixing, to not recoverable overload or noise problems.

A better and easier way is to adjust all channel gains for the same clip level, indicated by the clip-level-indicator-LED. Keeping in mind a good low noise level.

If every microphone input is adjusted to a clip level equal to 120dBspl microphone pressure level, this automatically results in an optimal microphone noise floor about equal to the recorders noise floor.

120dBspl clip level is enough for most recording situations.
More channel gain is unwise, it only results in higher noise level and less headroom.
If more headroom is wanted, simply reduce channel gain for a lower VU reading.

This handy generator is quickly set for microphone sensitivity.
No calculations needed.

    Specifications:
- The scale on the utility is simply calibrated for MICROPHONE SENSITIVITY, not for voltage, avoiding miscalculations.
- The low-Z, balanced 1 kHz square wave tone output is (adjusted to microphone sensitivity) -4dBv to -31dBv :
        - By means of S1 in one 12dB step, and
        - +5 to -10dB variable by means of P1.
- The unit is self-powered by one 9V battery (current drawn 3.5 - 5.3 mA, with phantom power off or on).
    A LED indicates operation and battery condition (> 8.0 V is OK and lit).
- The PCB design can be used double or single sided.
- For reliability and easy soldering, no SMD components are used, and the PCB has 0.4mm isolation between tracks and mass.

This utility can handle phantom power, it is designed for it.
But some microphone input circuitries have to small sized SMD bias resistors. Connecting with phantom power ON can result in overheating them, unbalance (sensitive to hum) and noises (cracking).
IMPORTANT: To protect the microphone input bias resistors, this utility should be connected while the mixers phantom power is switched OFF. After connecting the microphone you can switch it back on.
NB: a good audio circuit design does NOT use SMD components in microphone input circuitry NOR in microphones.

>> P1 is preferably a log. type <<
On this schematic, and the actual (improved) PCB, S1 switches only two levels, not three as seen on the photo above.
PDF

The LM324 needs at least 7V power supply for ample driving currents into D1-4. Above 8.0V battery voltage LED1 lights, indicating ample battery voltage, and the utility is switched "ON".

The output of OP1 is a square wave signal (ca. 1.1 kHz). D1-4 limit the peak-to-peak value of that signal, thus defining (nearly) symmetrical an stable 0dBv (+ and - 1.4Vpp) input level for the attenuator R7 to P1.

OP2 (1/4 LM324) delivers a reference voltage, which biases output stages OP3,4 into class A.
Tests of the design showed, that the DC output of OP2 had to be be isolated from the rest of the circuitry by R23, otherwise instability occurred (generating strong needle pulses).

C6 forms a low-Z path to mass for the limiting currents in D2/3.
If the value of C6 is to small, ripple on Vref will distort the output wave form, especially at low output voltages.

OP3 and OP4 deliver DC coupled, low impedance and balanced output voltages to the XLR-3m receptacle. Output voltage of OP3 is equal to, but in opposite polarity to OP4, thus resulting in 6dB gain. The DC output voltage is 4.5V.
No output separation capacitors are used, as they should be polarised types with large value. Expensive and difficult to get.

D5,6,8,9 prevent damage to the opamps when connecting to or disconnecting from phantom power.

    How to use fast and simple :
1. First determine the "official" sensitivity of the microphone to be used ("dBv/Pa", see manufacturers specs).
2. Set S1 and P1 on the utility for that microphones sensitivity :
        for instance for -42dBv/Pa :
           S1       P1
        (-35dB - 7dB) is total -42dB
                        or
        (-47dB + 5 dB) is total -42dB.
3. Turn the microphone gain fully anti-clockwise.
4. Switch-OFF phantom power
5. Connect the utility to the microphone input and switch it on.
6. Adjust the recorders microphone gain so, that the clip LED of the microphone channel just lit up.

Do NOT change this recorders microphone gain anymore ! It is now calibrated to 120dBslp max. sound pressure.

7. Disconnect the utility.
8. Connect the microphone to the calibrated input. If needed, switch phantom power on.
9. Proceed with the next microphone / input.

REM: if you want more headroom (a higher clip level), say 10dB more, than adjust microphone gain for a VUmeter reading 10dB lower than clip level.

The output of only ONE signal pin is used for adjustment.
The first calibration should be done using a calibrated oscilloscope, NOT a VU meter.
Rem: The output between both signal pins XLR2 and 3, as "seen" by the microphone input circuitry, is double that value.
On a rms meter, the output indication for square wave signals will be higher than measuring a sine wave signal with the same peak-to-peak value.
On the actual (improved) PCB and schematic, S1 switches only two levels, not three as seen on the photo above.

1. Mark the positions of SW1 (see photo):
    -35dB    at the max. position.
    -47dB    at one lower position.
    "~"         at one lower position (lowZ output but no signal).
    "Off"      at the lowest position (hiZ output).

3. Connect a calibrated oscilloscope (range 500mV or 1000mV) to XLR pin 2 (or pin 3) and pin 1 (mass).
4. Set SW1 to the "-35dB" position.
5. Adjust P1 for 500mVpp indication on the oscilloscope.
6. Mark the position of P1 with "0". Do not change this position of P1 from now on.

7. Switch the phantom power on the recorder/mixer to "OFF". Turn microphone gain fully anti clockwise.
After that, connect the XLR output of the utility to a microphone input. The mixer/recorder should run under software having a Vu meter indication.

8. Adjust the recorders microphone channel gain, for -10dB Vu meter indication.
From now on do not change this microphone channel gain.

9. (Read the software's Vu meter indications).
Change the position of P1 on the utility for a mixers Vu indication in 2dB steps lower than -10dB, and draw marks on the scale of P1 between -2dBvu and -10dBvu indication (f.i. -2 dB, -4dB etc. down to -10dB).
Change the position of P1 on the utility for a mixers Vu indication in 1dB steps higher than -10dB, and draw corresponding marks between +1 and +5dB on the scale of P1.

I removed the knobs and PCB from the housing, and scanned the the temporary scale on the top of the box.
In a drawing program i used that scan as background under a transparent copy of the top-silk.
In a third transparent (top) layer i drawled the definitive scale.
This layer was printed onto transparent sticker foil.
After sticking the foil onto the box, it was protected with photo-color-protection spray.

The theory, the relation between a microphones sensitivity, and this utilities output voltage.

I am starting from the definition of a max. permissible sound level into each microphone of 120dBspl.

For adjustment this way, the level diagram (left) shows the relation between :
- a sound pressure level into a certain microphone (dBspl), and
- the recorders Vu meter reading (dBu).

The diagram shows that the recorders clip level is adjusted to 120dBspl sound level, and that this is 26dB higher than the microphones "sensitivity level" (dBv @ 94dBspl or dBv @ 1Pa, specified by the microphone manufacturer).

For each different microphone, the utilities output therefore should be able to be adjusted 26dB higher than the microphones output at 94dBspl, for instance. The table on the right side shows some examples.

PCBs

Xray view
(red=bottom tracks, blue=through-hole, yellow=top silk).
The connections between S1, the battery and the PCB are made with short, insulated wires.
=> For stability reasons, the power connections from "battery-" and "switched+9"
MUST be soldered directly to both connections near C7 <=
At the XLR connector, the metal housing must be interconnected to its pin XLR1.
The connectors pin XLR1 should connected to the mass plane near IC1-11.
PDF

Click "PDF" for easy printing with accurate dimensions (paper size A6).

Bottom copper mask (600DPI)
Borderline dimensions 78.74mm x 49.53mm.
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Top copper mask (600DPI)
Borderline dimensions 78.74mm x 49.53mm.
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Topsilk, also suitable as drill-template for the housing (600DPI).
Use the crosses for punching drill marks.
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