L5 PREAMP DRIVE CHANNEL
9
BUILD NOTES, CONT.
Calibration
The L5 Preamp has two different calibration trimmers. To set these, all you will need is a multimeter and
a signal generator (which can be a computer or smartphone).
Note that all voltages are taken in AC, not DC, since we are dealing with audio signal levels. The voltages
are also all RMS rather than p-p, so confirm that your multimeter measures RMS if you’re not sure.
Before you start, turn both trimmers all the way down.
Distortion trimmer
The distortion trimmer should be set first. This sets the level at which the overdrive kicks in. The
procedure calls for a 1kHz 30mV sine wave to be inserted onto pin 4 of IC5, and for the trimmer to be
adjusted until you measure
4.4V
on pin 5 of IC5.
For convenience, there is a pair of pads marked “TEST” right underneath the “Frequency”
potentiometer’s pads where you can insert your signal. The “+” pad connects to pin 4 of IC5 and the
other is connected to ground. It’s recommended to solder short pins to these pads (about 3/4” in length)
to act as ‘posts’ for alligator clips to attach to. (The clipped leads from a 1N4004 diode work well for this
since they are more rigid than normal component leads.)
If you don’t have a signal generator, look for a smartphone or Mac/PC app that allows selection of wave
type, frequency and gain level. These come and go, so we can’t recommend anything specific, but there
are several free ones available at any given time and they all do the same basic thing.
From here, hook up a 3.5mm male-to-male headphone cable and turned up the phone or computer
volume to maximum, then set the frequency to 1kHz and the wave type to sine. Using a multimeter set
to AC millivolt mode, adjust the volume in the app until you read 30mV. (Don’t rely on the app to tell you
the output signal level; they have no way of knowing the actual real-world level.)
Now, use alligator clips to connect the sleeve and the tip of the headphone cable to the two wires
coming from the test pad. Since it’s AC, the polarity does not matter. This will insert the signal to pin 4 of
IC5 so you can adjust the trimmer as specified earlier, targeting 4.4V on pin 5.
Once you know the correct factory setting, feel free to adjust the trimmer up or down and see if you
prefer it in any other position (but consider first marking the trimmer with a Sharpie so you can get
back to the calibrated setting). Since this is a pedal adaptation, you may find it worthwhile to adjust the
distortion so it comes on earlier than it did in the original amps.
Compressor/limiter trimmer
With the 30mV sine wave signal still inserted into pin 4 of IC5, turn the master volume up all the way
and turn the compressor on (switch in the “up” position). Then turn the compressor knob up to about
2:00 (2/3 of the way up) and touch your probe to the “PCB OUT” pad on the right side of the footswitch
board. Turn the trimmer until you measure
1.17VAC
.
This will get you in the range of the original amps. If you’re using the L5 primarily as a pedal, you may
want to set this lower so it’s more sensitive to lower-level signals.
