Understanding a Trim Pot

[awgearhart]

I just ordered two PCB's that incorporate a trim pot (which I have never dealt with before). One being the Small Clone from TonePad, and the other being the Super White Auto Wah from GuitarPCB. The Small Clone uses a 100k trim pot, and the SWAW uses a 50k trim pot.

Before I get the boards and start wiring, I wanted to understand the functions of each trim pot, and if it varies based on the pedal, what is the function of the trim pots in either of these pedals? I understand it's supposed to set the "bias..." but what does that necessarily mean? What effect does it have on the sound? Is there a particular sound you're supposed to go for or is it based on preference?

And if people have specific resistance values pulled from their trim pots in either of these pedals, please provide them here!

Any input would be great... thanks!

[Mark Hammer]

1) If there were "specific resistance values" that could be recommended/employed, then manufacturers would have used them in the original design.  Trimmers are used to adjust/compensate for "the unknown" and we'll-see-when-we-get-there.

2) All bucket-brigade delay chips, whether used in a flanger, a chorus, or a delay, require that the audio signal be sitting on top of a steady DC voltage.  So, the actual audio signal might only "swing" as much as +/-500mv, but the chip needs it to be 500mv above and/or below 14V, or 4V, or whatever that specific circuit is looking for, given the power supply.  The trimmer is used to adjust that basic DC voltage fed to the input of the delay chip, and the audio signal is added/subtracted from that DC "bias" voltage.
When setting the finished pedal up for use, the "ideal" DC bias voltage will be one that results in a clean, clear, delay signal.  Adjust the trimmer a little higher or lower, and the delay sound will lose some volume and become distorted.  Adjust even higher or lower than that, and the delay signal will disappear completely.  I will recommend lifting one end of the 22k resistor that mixes in dry/clean signal at the output stage.  That way, you will only hear wet signal, making adjustment by ear much easier.  Once you have the trimmer adjusted to obtain a decent wet signal, then you can solder in the lifted end of that resistor.
3) In the case of the Snow White, the Bias trimmer is adding a DC voltage on top of the envelope.  The range in the spectrum where the filter sweeps will depend on the combination of the envelope+bias voltage.  Where bias trimmer settings for the chorus will either be correct or incorrect, bias settings for the Snow White will have a range that allows for "season to taste", although above and below that range, the pedal may not sweep. in any audible way

[11-90-an]

For your biasing question... https://www.diystompboxes.com/smfforum/index.php?topic=124504.20

The Small clone's trimpot use is found here... https://www.diystompboxes.com/smfforum/index.php?topic=91024.0 (and if you want a little extra read the circuit analysis of a similar chorus  : https://www.electrosmash.com/boss-ce-2-analysis)

This is for the SWAW... here is a similar circuit (seems to have the same use of trimpot): http://revolutiondeux.blogspot.com/2012/07/mad-professor-snow-white-auto-wah.html

Hope this is helpful...

[Marcos - Munky]

Also, since you didn't used a trimpot before, they're basically a small pot. But while pots are used for controls you change the settings based on the sound you want (like the speed of a tremolo, or the eq of a overdrive), trimpots are "set and forget about it" pots. That means you can use them instead of a pot for anything you set and forget about it.

[awgearhart]

Thank you all for the responses!

My interpretation of your feedback leads me to believe the trim pot should be adjusted to where the signal is mainly CLEAN and CLEAR with a little room for taste.

[Marcos - Munky]

To make it simple, adjust it to the best sound in your opinion. Yeah, electronics have rules and laws. But for guitar electronics, often the "most correct" isn't the "best sounding". On low voltage circuits, just adjust the trimpots to taste. The worse it can happen is to burn a led or a ic, and this is not too common to happen.

Of course, this "rule" doesn't apply to tube bias setting. You don't want to kill a brand new quartet of 6L6.