How to reduce treble in a Green Bean

[DuctTapeRiot]

Hello all,

I have been working up a Green Bean for a good friend of mine who plays rockabilly and surf music.  He plays with the Treble on his amp pretty much maxed out, and is finding that with the TS engaged he is getting too much high end (too shrill).  I have been trying to figure this out on my own, but am scratching my head a bit.  I have read over RGs article on the TS, and everything on Bevis about input and output caps, filter networks etc.

So far I have increased C1 to 1u, which I believe should let more bass through.

As far as next moves I think I have two options:
- Increase the value of C4 a bit, again to let more bass through, I know this is what the Fat switch does I just thought if I increase C4 a bit I will get even more bass coming though.

- Option # 2, now this is where i am confused.  I left the bright switch out, but i didnt jumper the pads because I just assumed that the bright switch was "additive" (more bright when on) as opposed to subtractive (more bright when off).  But when I am comparing the schematic in RGs article, it seems like I may have been wrong as he is talking about the low pass filter formed by the 1K resistor and the 220N cap (R8 and C6 in the MB schematic), but C6 will not be active in my build as I left out the bright switch.

Does any of that make sense?? If I jumper the bright switch pads will that activate that low pass filter and cut some of the treble??   Is there other areas of the circuit that I could adjust to roll off some of the treble?

Also assuming I would want to adjust that low pass filter to attenuate more treble, am i correct in thinking that I would want to increase the value of the 1K resistor (R8)?? Would I also need to adjust the cap value (C6).  Anything else I need to take into account here??  Also does anyone have any good links for high/low pass filters in pedals??

[oldhousescott]

Yes, you will want to jumper the bright switch pads. After doing so, you may want to play with C6. Increasing from 220n will progressively shave off more highs (and eventually upper mids).

[mgwhit]

I think you'd be better off increasing the value of C3.  This will let more highs through as negative feedback and should tame the treble.  oldhousescott is certainly right about C6, too.

[DuctTapeRiot]

Thanks guys! mg, can you explain a little more how that works, I read up on negative feedback in op amps (im still figuring out the theory parts of all this), and I think i am understanding how it is used to set gain, adjust impedance etc, but I am not seeing anything in the stuff i am looking at on how it would act to filter the treble???

[oldhousescott]

Capacitors and inductors (chokes) both act as frequency-variable resistors. Capacitors are high resistance at low frequencies, and low resistance at high frequencies. Inductors are the opposite of this.

Resistors at the input and in the feedback loop of opamps determine the gain of an opamp stage. In particular, for a given input resistor, a higher valued resistor in the feedback loop will cause a higher gain, and conversely, a lower valued resistor, a lower gain. So when you put a capacitor in the feedback loop, at low frequencies it looks like a very high resistance, and so the stage will have a high gain at low frequencies. (There's usually a resistor and/or pot in parallel with the cap which ultimately determines the low frequency gain of the stage). At high frequencies, the cap looks like a low resistance, and so there's less gain at high frequencies. (In this case, the low resistance of the cap at high frequencies dominates the paralleled resistor/pot).

So, all that to say, a cap in the feedback acts like a high frequency cut.

Sometimes the cap value is very small (10 - 50pF) in which case the high frequency cut may be beyond the upper range of hearing. In this case, the caps are being used to provide stability and prevent oscillation, but that's another story for another day.