Blend control for fuzz pedal

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dspruill

Well-known member
Joined
Oct 11, 2006
Messages
161
Location
Jacksonville, Florida U.S.A
Hi everyone, I have been bread boarding a fuzz pedal, and I have the next to last transistor stage parallel feeding  2 different output transistors to see which one I like the best. So I like them both and thought it would be fun to blend them together. Any advice to a noob? I know some pedals just use a pot with the wiper as out, but I know that is not the best way, even though it sorta works!

Thanks,
 
> e next to last transistor stage parallel feeding  2 different output transistors to see which one I like the best.

The final transistors load the next-to-last stage. And maybe not the same way. Especially in fuzz-work.

You may find that you like the output of "A" best, but when you take "B" out, "A" does not sound the same.

I do agree with instant (and blind) A/B testing. I don't think what you have is a truly elegant experiment, many uncontrolled factors.

Being breadboard you can swap the parts in a few seconds. This is IMHO not as good as an A/B switch worked by a silent friend, you lose focus and memory in the swap-time, but is perhaps another thing you should try.

If you just one to choose one-or-the-other, I'd be thinking a switch not a pot. You are not going to have a blend pot in the final build.... or are you? I'm sure it is worth a try. I expect unless a transistor is "bad", the variations of blend will be very subtle, not heard by the audience after the 3rd beer. But maybe you'll hear something.
 
dspruill said:
Hi everyone, I have been bread boarding a fuzz pedal, and I have the next to last transistor stage parallel feeding  2 different output transistors to see which one I like the best. So I like them both and thought it would be fun to blend them together. Any advice to a noob? I know some pedals just use a pot with the wiper as out, but I know that is not the best way, even though it sorta works!

Thanks,
It would work as long as the pot resistance is higher than the output impedance of the transistors (mainly governed by the collector resistor, if the stage is common-emitter)., and as long as the resulting output impedance is not grossly high. Typically a 47k (or 50k) pot should obey both constraints. The resulting output impedance would be about 20kohm worst-case.
In more general terms, blending applies only to signals that are in-phase. That should be the case in your design.
Indeed, PRR's caveats must be taken into account.
 
If you want to be able to blend 100% "left" or "right" take a look at RG Keen's blender circuit:

http://www.geofex.com/Article_Folders/panner.pdf

There's some gain loss there unfortunately, though there may be ways to handle that. But if you don't need the attenuation to be completely uniform, you can get it down to only 1/2 gain instead of ~1/3 gain like he has. For that you can nab the output from the echoplex. It's a 500K pot and 100K resistors. You can shrink those if you need to.

A single pot panning the wiper (as output) will have dead spots in the middle and more bleed between the settings, and there will also be a weird jump when you pan totally to one side. This could make your output inconsistent and will mess with your gain (which PRR alluded to). Some blended settings will be lower gain. And a fuzz is all about FUZZ, right? I really dislike this kind of control; even if it works "okay" for most peoples' purposes, it's just lousy compared to other options.

Of course, all this is general -- you didn't post a schematic or even say what kind of fuzz you're modifying.

Something else you can do that I think would work better: split the signal to both transistors and then blend the outputs of the two transistors. You can use RG's setup even easier that way; a -6dB or even -12dB cut at the output of the effect is pretty likely to be peanuts to a fuzz, which is usually enough gain to spit out all 9V (and if you're making e.g. a fuzz face or tone bender, just modify the output to take the signal from the collector instead of splitting the output with a 470R/8k2). If you really need to get back up to maximum output, add another transistor with 2x gain. No gain loss so you've got all the fuzz.

You could also use a dual gang volume pot on the Fuzz outputs (wired counter to each other) with some resistance to separate them, but this might need an output buffer to work best and there's no real benefit in this case compared to the RG Keen circuit.
 
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