Issues with original design distortion

[antonis]

With a very rough estimation, you'll need about 12-13dB gain recovery after tonestack so you might have to by-pass R11 with a cap..
(I'm not familiar with J201 transconductance habits so you might need to partialy by-pass it for a Q2 voltage gain of about 4)

Holy crap another 13dB would make this pedal loud, it is already very loud as it is. I will give this a go and aim for 4V of gain. When you say partially bypass, can I add a cap+resistor in parallel to R11 and adjust the new resistor value accordingly?

If it's loud enough as it is, let it be..
(my guess was just a "paper circuit" guess..)

If you want to raise final stage gain, there are many Source resistor partially by-pass methods..
(below are drawn for BJTs but (3), (4) & (5) will get you into the point..)



P.S.

aim for 4V of gain

Gain is unit-less number..!! 
(ratio of same dimension elements)

[Voltzy]

If it's loud enough as it is, let it be..
(my guess was just a "paper circuit" guess..)

If you want to raise final stage gain, there are many Source resistor partially by-pass methods..
(below are drawn for BJTs but (3), (4) & (5) will get you into the point..)

Alright thanks for the diagrams I will give them a go.

I am planning on experimenting with my input/output DC blocking caps and the caps between stages. Is there anything I should look for in a value other than it creating a high-pass filter? For example I was thinking of changing my input cap to a 10nF as that will create a high-pass filter with R3 and filter any sub 15hz frequencies, will there be any unintended side effects? Also am I correct in assuming that C4 and C11 are creating a high-pass filter with (respective JFET source resistors + internal resistance of JFET)?

[antonis]

I was thinking of changing my input cap to a 10nF as that will create a high-pass filter with R3 and filter any sub 15hz frequencies, will there be any unintended side effects?

As long as there isn't any bootstrap capacitor (or other reactive element creating circuit prone to possible high resonant peak at some audible frequency), size of input capacitor should be set according to HPF corner frequency taste..
(no elementary analysis side effects..)

am I correct in assuming that C4 and C11 are creating a high-pass filter with (respective JFET source resistors + internal resistance of JFET)?

C4 with LM386 input impedance at pin3 and C11 with Volume pot (according to wiper setting)..
(HPF is created with a series cap and a shunt resistance comming AFTER the cap..)

C11 usually has relative high value due to unknown input impedance of succeeding effect..
R12 could be considered in parallel with C11 + R13 for output impedance calculation but, in practice, R12's low value dominates Volume pot value..

[Voltzy]

As long as there isn't any bootstrap capacitor (or other reactive element creating circuit prone to possible high resonant peak at some audible frequency), size of input capacitor should be set according to HPF corner frequency taste..
(no elementary analysis side effects..)

Excellent thank you! for some reason I had in my head that larger capacitors enable more bass frequencies to pass but it had just occurred to me that this is because they create a HPF and not an effect of the caoacitor itself.

(HPF is created with a series cap and a shunt resistance comming AFTER the cap..)

C11 usually has relative high value due to unknown input impedance of succeeding effect..
R12 could be considered in parallel with C11 + R13 for output impedance calculation but, in practice, R12's low value dominates Volume pot value..

It all makes so much sense now haha thank you.

So it is a good idea to have a large output capacitor compared to a small one to keep the output impedance on the lower side.

Thanks for the help!

[antonis]

for some reason I had in my head that larger capacitors enable more bass frequencies to pass but it had just occurred to me that this is because they create a HPF and not an effect of the caoacitor itself..

True & Correct..!!

A capacitor, by its own, doesn't affect anything related to AC ..
It's considered reactive element which means (in a brute aproximation) that it consumes no power..!!
(what is taken during one half cycle is given back during the other half cycle - contrary to a resistor which consumes power P=V*I at any instant, despite current polarity..)

So it is a good idea to have a large output capacitor compared to a small one to keep the output impedance on the lower side.

Not quite right..!!

Ouput impedance consists of what signal "sees" leaving output..
(just if it was turning its head there looking back..)
So we have a frequency depended impedance which consists of cap value, Volume pot value AND setting and impedance of whatever comes next..
If we were sure about next effect input impedance, we could precisely set output cap value according to our taste of HPF cut-off frequency..

[Voltzy]

Not quite right..!!

Ouput impedance consists of what signal "sees" leaving output..
(just if it was turning its head there looking back..)
So we have a frequency depended impedance which consists of cap value, Volume pot value AND setting and impedance of whatever comes next..
If we were sure about next effect input impedance, we could precisely set output cap value according to our taste of HPF cut-off frequency..

OK thank you, what I meant is that a larger capacitor has a lower impedance across the frequency range. It would be more preferable to use a larger output capacitor compared to smaller one to keep the output impedance low.

Will changing the value of the output cap actually have any noticeable effect on output impedance? It seems like changing R12 would give the greatest change in output impedance and changing C11 won't do much at all. I am still trying to wrap my head around impedance.

[antonis]

Will changing the value of the output cap actually have any noticeable effect on output impedance? It seems like changing R12 would give the greatest change in output impedance and changing C11 won't do much at all.

Quite right..
(but it depends on C11/R13 impedance ratio, 'cause they are set in series so their sum impedance is set in parallel with R12..)
Impedance on Q2 Drain is the parallel equivalent of R12 and C11+R13 BUT final output impedance is taken from R13 middle lug..
(OUT jack tip and next effect IN jack tip form a volage divider - OUT tip carries total impedance before it and IN tip carries total impedance after it..)

I am still trying to wrap my head around impedance.

Don't push it to hard..

Just sit on the point you want to estimate impedance and look back and forth..
Replace any reactive item with its equivalent resistance (at the frequency of interest), use series/parallel formulas to calculate equivalent resistance and use Thevenin's formula for resistive divider [ R_TH = R2/(R1+R2) ]..
Absolute (numerical) value of R_TH is your signal percentage at that specific point..!!
(or 1 - R_TH is your signal loss due to impedances voltage dividing effect..)

[Voltzy]

OK excellent thank you! you have been nothing but helpful.