Changing Boost of Non-Inverting Amp

[jwblant]

Hey ya'll.  I've started a build that is a clone of another pedal but I've added some different things such as an active tone stack between the gain stage and the amplification stage.  Because of all the extra components inside in the circuit, the voltage drop is a lot greater than the original pedal.  I've found the pedal sounds fantastic but it has almost no volume to it.  Long story short, I want to increase the gain of the output amplifier.  It is a non-inverting amp, except with R1 and R2 having a capacitor in place with them.  Normally this wouldn't be a big problem but changing a resistor and leaving the capacitor is going to affect the frequencies.  So, if I change a resistor do I only need to change the capacitor that is in series with it or do I need to change both capacitors?

Thanks for all the help yall!!!

[ashcat_lt]

Changing either resistor will change the frequency response and you may need to change one or both caps to compensate.

Edit - Okay, maybe that wasn't all that helpful.  R11 has a much bigger impact on the cutoff frequencies than R10 does.  When changing R10, it's more like you're making the slope of the filter on either end steeper, whereas changing R11 moves the whole frequency plot around.  The filters where they are now are really just barely touching frequencies which a guitar can produce anyway.  I'd say make R10 bigger for more gain.

[jwblant]

Well I was thinking of changing R10 from 150k to 390k or 470k.  If I did this, would there be a need to change c8 as well? 

[ashcat_lt]

I don't think you'll notice a difference unless you're playing a piezo pickup directly into a full-range system like a PA or something.  The frequencies that cap rolls off are on the very edge of what brighter passive pickups can produce, and most guitar speakers start to fall off a bit below there.  It actually might help to "de-fizz" any distortion/overdrive you get along the way.  It's honestly probably quicker and easier to try it and see, but I think you'll be fine.  If you really want to be safe go ahead and make those caps a little smaller.

[amptramp]

Your -3db rolloff point is 48253 Hz.  This is set by the 150K and 220 pF feedback components.  The low end gain is unity for very low frequencies but the -3 db turnover is at 40.8 Hz as set by the 39K and 0.1 µF components.  Maintaining the same product of resistance and capacitance will keep the same rolloff and turnover points.  You can use the calculator here:

http://www.muzique.com/schem/filter.htm

to perform these calculations.  If you just change a resistor or a capacitor without adjusting the corresponding component, these frequencies will change.  Note that there will be anumber of rolloffs and turnovers in a typical pedal / amplifier chain, so do not reduce the rolloff or increase the turnover all that much.  If you had four items in the chain with htese frequency breakpoints, you would get -12 db at these frequencies and the frequencies you normally use would be affected.