Help reverse engineering Supa Puss mod - get better mix taper?

itsGiusto

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I like the sound of my Way Huge Supa Puss, but the mix knob is unusable. It goes from 0%-wet to way-too-loud-wet in the first 1/32 turn. I sent it to Analogman to mod it, though I stupidly did not take a picture of the board prior to his mod. I'm trying to figure out what he did, so I can change the taper again. Though his mod made the taper better, it's still pretty bad.

I know he used the logarithmic taper-trick from "The Secret Life of Pots". I can see that the taper-resistor is changed, since the solder color is different, and it looks "blobby". This resistor is now a 10k, though like I said before, I don't know what was it used to be. I also don't know if there are any other changes he made to the board - nothing stands out as such, but it's possible.

The mix-pot itself is an MN100k, a dual-100k pot. I'm unsure why a dual pot is needed, which is why I'm a little hesitant to go changing the taper-resistor again. Does there need to be some consistency between the two pots in the dual pot?

I've been staring at the board all day to try to figure out the circuit, but unfortunately, I'm not great with understanding SMT components, and dual-sided boards. The traces are hard to follow, and some of them go under the board-mounted pots.

So, in short, this is really all I know about the mix-circuit:

3nEIzHlCyaaZTr_uaTOK2ptJKign9Hkd0g9Sd-7JaRlc-JTSbwLjl7HiA6h68eeuNUAZ3Fc7LmTqfqc0ThF5uVy9yiK5VrQAK22-hre753ofqO1M1F-kIHDFjT-99wbDP__8XJ6cMlw8ztpVS028ugRB=s0-d-e1-ft



Does anyone have any insight into this? Is there anything I can reliably do to improve the taper? Why is there a dual-pot? Would changing that 10k resistor to a smaller one help?
 

itsGiusto

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I'd go larger ... 15 - 20K
Cool, I can potentially try that. It's just harder to test, cause I can't simply clip in another resistor to reduce the parallel resistance. I'll have to actually remove the resistor in question.
But what's your reasoning for making it larger?
 

ICTRock

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ideally, you're looking for a resistor 15% of the pot value, if 10K isn't quite right then you'll want to go higher not lower
 

itsGiusto

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Alright, I figured it out! The circuit ended up being very similar to this diagram, except Right is wet and Left is dry:

pots-f9.gif


I did some audio-probing of the circuit to confirm. I was able to get an actually usable taper by putting a 10k resistor between GND and Right-Output. Now it's actually usable, though I may wish to put in an even lower value resistor instead to make the wet-mix volume even more discrete.
 
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delay dude

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Alright, I figured it out! The circuit ended up being very similar to this diagram, except Right is wet and Left is dry:

pots-f9.gif


I did some audio-probing of the circuit to confirm. I was able to get an actually usable taper by putting a 10k resistor between GND and Right-Output. Now it's actually usable, though I may wish to put in an even lower value resistor instead to make the wet-mix volume even more discrete.

@itsGiusto: the picture of the diagram does not show up. Could you please tell me, where you have added the 10k resistor? If you would try a lower value resistor, would you choose a 6,8k or even lower? Thanks for your help!
 

itsGiusto

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I think this was something like the original image:
pots-f7.gif

If I remember correctly, I think you want to put the resistors between legs 1 and 2 of the two pots of VR2.
I have no clue what resistor value would be good, I've never experiment with resistor values and taper. I'm sure 6.8 would be fine.
 

delay dude

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Is it possible, that analogman removed r79? I´ve read some comments on two different forums, that removing r79 changes the mix taper?
 

Korken

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Is it possible, that analogman removed r79? I´ve read some comments on two different forums, that removing r79 changes the mix taper?

Hello,

I own a Supa Puss since a couple of months and noticed the mix knob has this supa short run and in almost all settings I get an annoying feedback. Last night I thought, maybe someone else has this issue too. I found the posts you mention, telling that the mod Dunlop makes is to remove R79 and issue is gone. Well, I know how to do that, so I did it. And right, issue is gone! Now I can use my mix knob in full run: full counterclockwise is only dry, full clockwise is only wet and at noon is 50/50.

I can just point out that the R79 is a supa supa tiny SMD (surface mount), so you must have the right tools or supa good eyes for the mod. The R79 is located under the mix knob, so you must remove not only the main screws on the bottom, but also the female in, out and exp pedal plugs, to be able to bend a little a pcb that is inside.

I hope that can help someone having issues with the rebel mix knob.

Supa Puss is now supa good.
 
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