how do you use a trim pot for bias control on a pedal?

[TimWaldvogel]

i have no idea how to adjsut a bias on my pedals, and i have no idea how it affects sound

could someone maybe give me an example of using a trimpot bias on a mxr distortion plus schematic

here is the schematic?
http://www.kilback.net/homebrewtweaks/pedals/mxrdistp.gif

[aron]

Well, you could adjust the bias on the op amp. It's via the two 1M resistors that "feed" the 1M resistor going to the + of the op amp. Adjusting either of the 1M would adjust the bias to the op amp. I think I did this years ago and concluded it didn't add much to the circuit.

[TimWaldvogel]

thank you so much, i am still learning and i appreciate all help from people in this forum. its helpful to ask questions and get answers 

[aron]

No problem. I think what I did in my case was to put a pot and put 9V at one outer lug, then take the wiper and go to the op amp. The other lug went to ground. I put a cap to ground from the wiper - so it's just like the schematic except you can adjust it. Then I started turning the pot. I wish I had kept notes about doing all of this a long time ago.

[JKowalski]

- Biasing is necessary on transistors because they can't really do much without it. It is part of how they work.

- Op amps should always be biased at 1/2 of the supply voltage.  If it is not in the middle, then one side of the bias will have more headroom (avilable voltage) then the other side. There isn't much reason to do this. Adjusting the bias on an op amp wouldn't do anything, until you adjust it too far then you'll get hard clipping.  With split supplies, (+/-), your bias is just ground, since ground would be 1/2 of the range between the + supply and the - supply. The op amp just needs to be able to reach voltages above and below the bias of the signal, so that it can amplify without going outside of it's supply rail (it can't create voltages above what is fed into it, of course).

So you can see that with split supplies and a bias at ground, you can go negative and positive no problem. With a signle supply, for example +9V and 0, your best bias is 4.5V because it can go above and below 4.5v by 4.5 volts. (well technically less because most op amps can't reach all the way to the rails, usually like 0.8v away from it)

[TimWaldvogel]

so cutting it in half is done by the two equal resistors  from 9v, to input, to ground correct?

[JKowalski]

Yes, two equal resistors in series will result in 1/2 the voltage across them in the middle.