high end goes away when the gain knob goes down

[curran]

hey all!!!  I have a question about this build. I have built one of these nice little pedals and modified it to taste(hoo ray for DIY) BUT the gain control seems to roll off a bunch of highs when it is turned down below 3pm on the pedal. I usually have it set up at 3:30 anyway BUT I would like to know if I can put a cap/resistor across the gain 1+2 lug like a guitar volume to retain the high end in the tone of the circuit. the circuit is 150k/.001 in parallel on the guitar vol. would this work or it there something different I could to to the circuit itself?

[thelonious]

It depends on what the gain control is doing in the circuit. If you post a schematic, you'll get some answers.
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Edit: Oops, just noticed that this is in a dedicated of the forum, so it can only be about one thing. Here is the schem for reference for others:

[thelonious]

It looks like this gain control functions by dumping negative feedback (NFB) to ground. When you turn the gain up, there is less NFB because there is less resistance between it and ground. This is the opposite of a normal volume control, so I don't think a bright cap would work in this circuit.

When the gain control is at 0%, there is more NFB getting past the gain control, and that NFB is killing proportionally more lower frequencies (below 720Hz).

When it's at 100%, there is less NFB getting past the gain control, and the NFB killing proportionally less lower frequencies (below 62Hz), so it *ought* to sound "fatter" when it's turned up.

I don't understand why it would sound like it were rolling off highs. Maybe because less gain = less distortion = fewer harmonics/overtones that would fill out the high end?

Looks like you are going to need someone more experienced than me to answer this.  I say to try two things: 1) try the bright cap like you suggested and just see what happens. It's worth experiementing. And 2) try removing that gain control (disconnecting the .047 cap from the op amp) and replacing the 1M resistor with a pot. That is a more standard gain control. You might or might not like it better in this circuit.

[thelonious]

One more idea, which is really more like a workaround: replace R6 and C5 with a Stupidly Wonderful Tone Control (http://www.muzique.com/lab/swtc.htm) so that you can control how much high frequency content gets dumped at the end. That way you can add back in highs when you want to.

[curran]

I thought this was the discussion thread for the vooduu lab OD clone. not the 250. the 250 I built is great! the Vooduu is the one with the darkening up, as you turn town the gain

[thelonious]

Ah... ok. I found the 250 schematic in the introduction post, so who knows. The VLOD schematic is here:



It's a similar gain control, but the corner frequencies are different. You could try reducing C2 to 100p and/or replacing C4 and R6 with a passive tone stack and lowering the value of R9 to make up the gain like gaussmarkov suggests in the schematic.

But this circuit has been built a million times, so I'm sure there are other people on here who can help you more.