Mix Pot Value..

[antonis]

Hi guys...  

I'm building a distortion circuit with 2 separate overdriving and clipping signal paths wich are mixed before tone control..

I wonder if the value of Mixing Pot (100K) is high so I shoud have significal signal "loss" and - if its true - should I reduce the pot value or keep it (100K) and reduce the value of the Volume Pot..

[bool]

47k would be good, 25k would be better, but 10k would be excellent.

... or strap 2 pieces of 10k resistors each across the existing pots' lugs ...

That should give a "much enhanced and subtler control" over the center mix values.

[antonis]

47k would be good, 25k would be better, but 10k would be excellent.
... or strap 2 pieces of 10k resistors each across the existing pots' lugs ...
That should give a "much enhanced and subtler control" over the center mix values.

10k was my original thought but had doubts about the top or bottom edge mixing points..
(should 10k be capable to "low down" the signal to allmost zero comparing to the other one..??)

P.S.
I like your idea of getting a dual log-antilog (or something like this... ) pot...

[samhay]

With a 10k pot, you will get roughly 10% (actually 1/11th) of your unwanted signal at each extreme of your mix pot. This is reduced by using a bigger mix pot, but then your tone stack gets unhappy as it is not driven by a low-impedance source...

[antonis]

So for getting a "clean" signal (and keep my tone stack happy..) I have to wire a 3 position switch...

(I guess the 9th knob will not be the cause for my son's unhappines.. )

[samhay]

Or put a buffer between the (100k) mix pot and the tone stack (which might actually work just fine as drawn - let your ears decide).

[bool]

Well ... the most important question is, do you want/need the 0%/100% - 100%/0% mix range or is the less precise mixing range acceptable.

If it isn't, you'll have to go with buffers (2 opamps) ... or with a virtual ground mixer (1 opamp) and a "balance pot" (a dual pot).

Or just buffer (1 opamp) the existing 100k pot and drive the tonestack from that.

[antonis]

Maybe I will use a dual op-amp (for buffering the signal before the tone control AND replacing the output stage transistor) or a quad op-amp for all stages...

[PRR]

I like the "Tremble" knob.

Though I think these are usually called, top to bottom, Treb Bass MID.

Have you BUILT it? Is it a real problem, or are we just bench-racing it?

> keep it (100K) and reduce the value of the Volume Pot..

I do not understand the logic here. Between MIX and VOL is that tone-stack, so MIX and VOL don't "see each other" much.

We have five cascaded impedances and want to scale them to play well together.

The opamp needs at least about 1K output; else the distortion is about the opamp not just the diodes.

The final buffer is about 250K input. Really R15 could be much larger like 100K, and R12 could be 1Meg, to give ~~500K input, but that's a small change.

With 500K load the Volume pot could reasonably be 500K.

The tone and volume is standard Fender but someone has scaled-down all tone impedances (and not all to the same scale). From experience, the volume pot may be equal to the Bass pot; conversely we may now raise the Bass pot to 250K or 500K. All else should scale in proportion. These pot values are now Stock Fender, however the drop-resistor R11 and the caps are scaled different from Fender. I assume because Fender typically did not use this behind a distorty stage, and this sure is after a distorty stage, so a different tonal balance is desired.

If R11 is 100K (Fender) or 220K (mod scale), then the tone stack input impedance is *roughly* 100K-220K (a little lower in treble, much higher for bass).

We want mix-pot wiper impedance.... wait. In Fender we know this tone-stack works fine when driven with 39K (12AX7 plate node impedance). Wiper impedance is 1/4 of pot resistance, so we can use like a 154K pot. Round down to 100K. There will be some dip at mid-turn, maybe not a lot, maybe enuff to notice.

As Samhay says, the leak-through at extreme rotation is like 1K against mix pot. For 1K against 100K the leak will be super small. (Not good enough to mix two very different programs, but plenty to mix one fuzz against another fuzz off the same axe.)

Offhand my sense of proportion says to raise tonestack impedance, maybe buffer impedance also, then buy both 100K 50K and 22K for mix-pot, and try it 3 ways.

However I am not convinced the original values are "wrong". And thorough re-design may need a LOT of try-and-see.

[antonis]

I like the "Tremble" knob.

Though I think these are usually called, top to bottom, Treb Bass MID.

The tone stack is copied from Fender Pro Roc amp but during testing I didn't like very much the cut off frequensies for individual tone area so I tried some compinations..
(I'm not using any simulation program so I just make all caclulations with pencil and paper - propably the values of filters need tweaking..)

As for reverse of Tremble and Mid pots, I've noticed (by ear) that they actualy must be switched...
(maybe my ears need cleanning or I've got a "peculiar" crossover in my brain.. )

P.S.
I completely agree about re-design and try...
(but I'll do it on custom made PCB so I want to first finish up the complete schematic and proceed with Eagle designed board..)