What does this resistor do? Plus a couple other parts

[WGTP]

http://aronnelson.com/gallery/WGTP/SuperSat?full=1 

Short story.  I had my first PV Bandit stolen and went to buy a new one.  I got the newest Special because it was louder but found out when I got home that the tone controls only worked on the clean channel and the "thick" setting on the distortion channel was fixed.  Now after all the years I'm going to UNFIX the thick control, especially since "thick" is no longer cool.

The .22uf/1.5K RC network is rolling off some highs and boosting bass.  I think C21 - .015uf is probably knocking off some more highs  I'm not sure if C30 - .001uf is large enought to roll off some highs I can hear or not.

R37 - 22K to ground is it reducing or increasing bass, or just setting the impedance of the circuit at this point.  Is it softening the clipping?  Thanks for your help.   

[bioroids]

For the little I know, and with the partial schem shown, I don't think R37 does anything. The low frequency cutoff with it and the 2.2uf cap is near 4hz... it also sets the DC point at ground, together with the gain pot. I assume the line at the bottom is also gnd, where the clipping diodes go.

It looks very similar to that R48 resistor... how's the rest of the schem?

Now is when someone who actally knows chimes in with a better answer 

Luck!

Miguel

[Connoisseur of Distortion]

well, it also forms a voltage divider... but i can't tell you if that's relevant or not. could be, considering that there are diodes immediately afterwards.

[WGTP]

I can't find the rest of the schematic now    I'm thinking there is some reason PV put that there, but not sure why.  I guess I'll disconnect it and see if the earth stops rotating.  The diodes etc. are connected to V1/2.  If it is part of a voltage devider, it would seem to incourage more distortion from the op amp, and less from the diodes???

I will probably by-pass one diode for assymetrical/lower level clipping and increase the 1.5K resistor for more highs and mids.  Thanks.   

[Sir H C]

The .22uF and 1.5k are a pole-zero, so you lose some gain to the point where the 1.5k resistance dominates the circuit and then it levels out.  So yes, this is a rolloff, and it boosts below about 500 Hz if my math is right.  Given the ratio of 1.5k to 10k impedence, that gives you an idea of how big the boost is.

The .001uF should do nothing you can hear.  It is in a 1.5k system impedance, so you have a rolloff around 100kHz if my quick calculation is right.  This is more for stability and to keep it all from ringing.

22k is setting a DC reference I would guess.  Not much else for it to be there.

C21 is probably sending out the signal somewhere else (such as a reverb tank) or rolling something off, though I doubt that because why wouldn't they just draw it to ground right there?

[bioroids]

The diodes etc. are connected to V1/2.

That's strange, one side of the diodes to v1/2 and the other to ground? It seems the opamps are ground referenced, running by a bipolar supply

Miguel

[Sir H C]

Those diodes are going to a ground node.  1/2 rail in this case is the local ground for them.  Look at R40/C27.  That is 1/2 the gain setting for the op-amp and it goes to the same reference point as the diodes.  Since the 4558 is a dual op-amp I guess the other 1/2 shows the voltages it is running off of.

Oh, the technique with the cap in series with the resistor is cool for high gain applications as the DC offset of the op-amp is not gained up by the gain stage, in this case up to 3000 or so.  So 1 mV offset would be huge at the output, but with that cap, the DC gain is unitiy so the gain is much lower.

[WGTP]

OK, I'm found the rest of the schematic and made a couple of changes.  You all are correct about the diodes.  A suprise is that C21-.015uf is in a feedback loop back to the other feedback loop (negative I presume) that I'm thinking rolls off some more highs.  The next stage is a TS type tone control using the second part of the op amp.  Whew Hew, they are socketed.  I tried bypassing 2 of the diodes with a single GE, but can barley hear any difference with all those highs rolled off.  I think the op amp is doing most of the distorting.

I'm going to try raising R95/1.5k to increase the frequencies above 500Hz.  Also have to try it without the C21 feedback or cut it in half, or try a cap to ground in it to reverse the effect.

Sorry for the confusion and thanks for the comments.   

[Sir H C]

If you just pull the resistor, you kill the bass boost and kick up the overall level.  That would be the easy way to first not kill so much highs.

[WGTP]

If I was ambitious, I might drill a hole in the back of the amp and put in a 50K pot in series with the 1.5K resistor.  That is an interesting way to do bass boost, it might be useful for other things like Distortion+, Rat, DS-1, etc.  Also the feedback loop deal deserves futher evaluation.  It is after the Post Gain, so it increases as the Volume of the amp is turned up.