Aion Electronics L5 Preamp Manual - Page 7

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Adjustments & Calibration

The L5 Preamp has two different calibration trimmers: one that sets the range for the compressor circuit

threshold knob, and one that sets the level at which the overdrive kicks in. Fortunately, both of these factory

adjustment procedures are well-documented and they do not require any special equipment.

For these, all you will need is a multimeter and a signal generator, and there are a few free smartphone apps

that will work for a signal generator. Turn both trimmers all the way down to start with. Note that for both of

these trimmers, we are calibrating the audio signal level, so you will use the AC setting on your multimeter.

Distortion trimmer

The distortion trimmer should be set first. The procedure calls for a 1kHz 30mV sine wave to be inserted

onto pin 2 of our IC10, and for the trimmer to be adjusted until you measure 4.4V AC on pin 6 of IC10. For

convenience, there is a pair of pads marked "TEST" in the top right corner of the board where you can insert

your signal. The "+" pad connects to pin 2 of IC10 and the other is connected to ground. I would solder short

pins to these pads (about 1/2" in length) to act as 'posts' for alligator clips to attach to. (The clipped leads from

a 1N4004 diode would be perfect for this.)

For the signal generator, I used a free iPhone app called "Signal Gen". The advantage to this one was that it

lets you specify a gain in decibels. Even though the dB level is nowhere near accurate (it doesn't even take

into effect the headphone/speaker volume!), it's at least useful for making fine adjustments to the level and for

giving you a number to remember.

I hooked up a 3.5mm male-to-male headphone cable and turned up the phone volume to maximum, then

set the frequency to 1kHz and the gain to -29.6dB, which is the dB level specified in the Lab Series service

manual. I then measured the voltage of the output with a multimeter (black lead to the sleeve, red lead to the

tip). The measurement was around 20mV AC. I ended up at the -23.4dB setting in the app to get 30mV. This

probably varies widely depending on the phone, so measure yours to make sure you are getting the right

signal level before using it to calibrate the distortion trimmer. (Also make sure the headphone volume on your

phone is all the way up!)

Compressor/limiter trimmer

For the compressor/limiter trimmer, with the 30mV sine wave signal still inserted into pin 2 of IC10, turn the

master volume up all the way and turn the compressor on. Then turn the compressor knob up to about the

2:00 position (⅔ of the way up) and touch your probe to the "PCB OUT" pad, the output of the circuit. Turn the

trimmer until you measure 1.17V AC.

A note about the compressor trimmer: I found that when using the L5 preamp as a pedal, the minimum signal

level to trigger the compressor was too high, even on the compressor's lowest setting. I ended up dropping the

compressor trimmer to zero to make the minimum setting as low as possible, and this felt much more usable to

me. You'd never turn this knob anywhere near all the way up anyway.

CA3080

The original factory schematic for some of the Lab Series amps specify a "custom 3080" for the compressor

circuit. While there is no way of knowing what this "custom" designation referred to, it's likely that they were

simply selected by the manufacturer to ensure they met a minimum performance requirement in certain areas.

It's definitely not referring to the actual silicon being customized by the manufacturer to Moog's request.

Silicon manufacturing processes have come a long way since 1977, and newer devices are much more

consistent than they were when they were first developed. It is most likely the case that the later improvements

in the 3080 manufacturing process would have made the "custom" selection unnecessary. So unless you are

using a chip that's actually from the 1970s, it's probably okay to just use what you can find without any regard

for its specifications.

L5 PREAMP