1 Meg Trimpot isp decimator purpose?

[njkmonty]

Is this for getting unity gain , out of the pedal  to adjust ?

or biasing to a particular voltage?  on the That 2181c chip?

https://guitar-electronics.eu/en_US/p/DECIMATOR-G-STRING-PCB/230

[diffeq]

That trimpot is connected to the "sym" pin. From THAT2181 datasheet (page 5):

Trimming

The 2181-Series VCAs are intended to be
adjusted for minimum distortion by applying a small
variable offset voltage to pin 4, the SYM pin. Note
that there is a 25 Ω resistor internal to the 2181
between pin 4 and pin 2. As shown in Figure 2,
Page 3, the usual method of applying this offset is to
use the internal 25 Ω resistor along with a larger
value resistor to form a voltage divider connected to
the wiper of a trim pot across the supply rails.
This trim should be adjusted for minimum
harmonic distortion. This is usually done by applying
a middle-level, middle-frequency signal (e.g.
1 kHz at 1 V) to the audio input, setting the VCA to
0 dB gain, and adjusting the SYM trim while observing
THD at the output. In the 2181, this adjustment
coincides closely with the setting which produces
minimum control-voltage feedthrough, though the
two settings are not always identical.

So it's there for both nulling the DC feedthrough (thumping) and adjust THD level. Not sure how it applies to this particular application. Decimator build document does not mention any trimming procedure. Are we supposed to do it by ear?

[DIY Bass]

I have seen a procedure for adjusting this on a different THAT chip and for a different application (compressor).  Basically the idea was to use a computer sound card plugged to the input and output, and adjust to give as much volume as you can without distortion.  Use a freeware audio analysis package to generate a signal through the audio card output and then measure THD at the sound card input.

[temol]

The build document says it's for setting unity volume between bypassed and engaged pedal.

T.

[PRR]

> That trimpot is connected to the "sym" pin.

It also connects to EC, a gain-change pin.

My suspicion is that it mostly changes gain. That agrees with the build doc.

[diffeq]

> That trimpot is connected to the "sym" pin.

It also connects to EC, a gain-change pin.

Oops. Missed that one.