Quote from: thunderaxe on March 19, 2025, 09:27:48 PMcan someone let me know what would be the factors in deciding the value of the volume pots, or of volume pots in general?

Not loading preceding stage while simultaneously not loaded by succeeding stage..
(all-time-classic 1/10 rule of thumb..)
e.g. A 100k pot lug 3 could be connected to an amp of 10k output impedance and wiper to next circuit of 1M input impedance..

Of course, this isn't always obtainable so you either have to use your friends(buffers) or take into account particular loading effects..
e.g a 10k pot connected to CE amp of 10k Collector resistor turns this resistor to 5k equivalent one..
(effectively halving particular stages' voltage gain..)

okay, here's the revised schematic with the following changes thanks to the very helpful advice i've gotten here:

-VREF filtering cap is increased to 100uF
-GAIN pot lug 3 is tied to the wiper
-VOLUME and CLEAN pots are increased to A100K
-summing amp input and feedback resistors are increased to 1M

sounds great on the breadboard, next step is to build the circuit additions on vero and try to fit it all in a box!

Quote from: antonis on March 20, 2025, 05:33:07 AM

Quote from: thunderaxe on March 19, 2025, 09:27:48 PMcan someone let me know what would be the factors in deciding the value of the volume pots, or of volume pots in general?

Not loading preceding stage while simultaneously not loaded by succeeding stage..
(all-time-classic 1/10 rule of thumb..)
e.g. A 100k pot lug 3 could be connected to an amp of 10k output impedance and wiper to next circuit of 1M input impedance..

Of course, this isn't always obtainable so you either have to use your friends(buffers) or take into account particular loading effects..
e.g a 10k pot connected to CE amp of 10k Collector resistor turns this resistor to 5k equivalent one..
(effectively halving particular stages' voltage gain..)

are there any resources where i can learn how to calculate the input and output impedances of a stage? it's easy when there's just a series resistor but for something like the output of the tone or clipping stages i'm just not sure how the resistors in their configurations would add up or be considered parallel, etc.

Quote from: thunderaxe on March 20, 2025, 02:41:11 PM-GAIN pot lug 3 is tied to the wiper

You mean Lug 1..
(but correctly wired on schematic..)

Quote from: thunderaxe on March 20, 2025, 02:41:11 PMnext step is to build the circuit additions on vero and try to fit it all in a box!

Quote from: thunderaxe on March 20, 2025, 02:50:20 PMare there any resources where i can learn how to calculate the input and output impedances of a stage?

Without messing with imaginary numbers and complex formulas:

Sit at the point of interest and look around..

Locate all the paths to AC or DC ground..

Adopt conventional  assumptions, like:
Infinite impedance of NFB wired op-amp's inputs, zero impedance outputs, JFETs/MOSFETs zero gate current (hence infinite impedance), Emitter resistor seen at Base multiplied by h_FE, Signal source impedance seen at Emitter divided by h_FE, Power supply  considered AC ground, Big caps considered shorts, etc..

Proceed to calculation of Thevenin equivalent impedances..

Place all those impedances in parallel..

okay so let's take the output of the clipping stage as an example. there is a 100K resistor traveling from there back to the "virtual ground" node, as well as antiparallel diodes and a capacitor...
should i calculate their impedances parallel to each other, or just consider them parallel to or part of the zero-impedance NFB op amp output, and thus zero impedance total?

The formula for Zout is: Z_out(CL) = Z_out(OL) / [1 + A_(OL)*B], where Z_out(OL) = open-loop(*) output impedance, A_(OL) = open-loop(*) gain and B = feedback fraction with (*) => no feedback configuration..

With Z_out(OL) = 500R (hypothecical), A_(OL) = 110dB and B = (GAIN pot + R22) / R10, I let you calculate output impedance..  

okay i don't need to do the math to know that that's a very small number divided by a very large number, making an extremely small number. so my tone stack calculations where i just defaulted to an input impedance of zero would be pretty accurate.

and for the output of the tone stack, maximum resistance would be with both treble and bass pots at max, so 12.4K on one side and 83K on the other for an approximate parallel resistance of 10.79K, so the 100K pot is the right call.

for the clean side, it's just straight off the op amp buffer so practically zero, so probably any value would work? but if one side of the mixer is going to be 100K then both sides may as well be.

right?

There is another consideration for sizing a pot in an audio circuit and that is, the capacitance to ground will affect the ability to pass high frequencies (treble) without noticeable loss.  You have the capacitance from the wiper and the carbon element to consider.

If you have a 1 megohm control with 10 pF across it, the circuit rolls off at 15923 Hz but if it is connected to an output cable and the total is 100 pF, then there is a rolloff at 1592 Hz, noticeable for a guitar.  This is why it is good to do what you have done here - put the level control before a buffer so the performance is consistent no matter what is connected.  This also applies to a level control at the input and an input cable.  An input buffer helps here, too.

From a noise point of view, smaller resistors than 1M would be better.

Swapping 100K pot/1M resistor for 10K pot/100K resistor is probably an improvement. Whether you'll notice is debatable. You would if that stage were first, with all that gain after it, but last in the chain, there'll be noise from the gain stages anyway and you might not care.
100K feedback resistor allows you to put the cap back in which is a bonus. We don't really want to be amplifying out-of-band signals if we don't have to.

Quote from: ElectricDruid on March 21, 2025, 07:08:34 AMFrom a noise point of view, smaller resistors than 1M would be better.

Swapping 100K pot/1M resistor for 10K pot/100K resistor is probably an improvement. Whether you'll notice is debatable. You would if that stage were first, with all that gain after it, but last in the chain, there'll be noise from the gain stages anyway and you might not care.
100K feedback resistor allows you to put the cap back in which is a bonus. We don't really want to be amplifying out-of-band signals if we don't have to.

that did occur to me, that using 1M resistors means i'm attenuating and then amplifying the signal, which is both wasteful and will introduce noise. i'll try it with 10K pots and 100K resistors and see if i notice a difference. having 1/10th the amount of introduced noise at that stage certainly seems good to me.

what would be the hypothetical downside to not following the 10:1 rule with the output of the tone section?

Quote from: thunderaxe on March 21, 2025, 07:50:59 AMhaving 1/10th the amount of introduced noise

Actually only 1/3.16th (square root of ten) if you're talking about Johnson noise..

what would be the hypothetical downside to not following the 10:1 rule between the output of the tone section and the volume pot?

Quote from: thunderaxe on March 21, 2025, 02:23:52 PMwhat would be the hypothetical downside to not following the 10:1 rule between the output of the tone section and the volume pot?

10:1 rule stands for acceptable voltage attenuation..
This presupposes that there is no current draw issue..

e.g.
Suppose an ordinary op-amp powered via +/- 15V of 1 Ohm output impedance and a load of 10 Ohm..
1:10 rule is fine but what about output current..??
or same power supply BJT TO-92 Emitter follower of 25mA working current, hence 1 Ohm output impedance driving a load of 10 Ohm..

P.S.
For active tone controls, like in your circuit, you have to take into account output total load (Volume pot in parallel with tone setting circuitry) for not driving output into saturation

i had another thought: in the original circuit C6 is 10uF, forming a ~3Hz HPF, essentially just blocking bias voltage from going to GND and not affecting the audio signal in any real way. i changed this to 1.5uF because i wanted a 20Hz HPF somewhere before the clipping stage given the big shelving bass boost that the tone stage is capable of producing. but now that i think about it, the curve of the low shelf has a resonant peak around 100Hz that drops off below that, so i don't think that 20Hz HPF is actually necessary.

in another thread someone suggested skipping the cap and just tying to VREF instead. any thoughts on that? pros: it'd save a part. any cons?

!!!..DON'T PUT GARBAGE TO Vref..!!!

Quote from: thunderaxe on March 17, 2025, 11:07:16 PMhi all,

i'm working on my first unique(ish) design -- it's an adaptation of the keeley rotten apple, which is itself an adaptation of the EHX op amp big muff. the input, sallen-key and clipping sections are largely the same, then i've added a tone section based on the neve 1073, a clean blend, and as a bonus i used the clean blend path as an optional buffered bypass. it works and sounds good on the breadboard but can you give it a once-over and let me know if i've made any mistakes or if you have any suggestions for how i could do this better or smarter?

just to preempt questions about this: having the clean level control after the distortion volume is deliberate. from a usage standpoint i would've preferred it to be before the volume but doing it this way prevents me from having to boost the clean signal ~22x and clipping the op amp to get it to a level where it can compete with the distorted signal.

any and all advice is appreciated.

thank you!

Hey, this design sounds awesome! The combination of Keeley Rotten Apple, EHX Big Muff, and Neve 1073 is a solid base. Your clean blend placement after distortion makes sense to avoid overloading the op-amp. Just a couple of things to check: ensure the tone stack doesn't interfere too much with the clipping section, and watch out for any signal integrity issues with the buffered bypass. Other than that, it sounds like you're doing great! Keep it up!

Quote from: antonis on March 22, 2025, 03:40:03 PM!!!..DON'T PUT GARBAGE TO Vref..!!!

what if i made a second VREF called "garbage dump" and used it for this, as well as lug 1 of the VOLUME and CLEAN pots? 

but really, if i'm already dumping VOLUME and CLEAN to VREF and it isn't causing any problems as long as i have a 100uF filter cap on there, would this really cause any problems?

Quote from: simka on March 22, 2025, 05:21:37 PMensure the tone stack doesn't interfere too much with the clipping section, and watch out for any signal integrity issues with the buffered bypass.

can you elaborate on these a little? what should i be looking out for?