Fuzz pedal bypass with no volume drop that almost works

[Ashurbanipal]

Built Anderton's EPFM bass fuzz. It's two inverting op-amp stages with LEDs in the second section. There's a lot of a gain, so instead of true bypass, I'm trying to switch to a third op-amp stage that duplicates the second but does not have the LEDs, so that the clean and dirty sounds will be the same volume. It works great for about 60 seconds, then both dirty and bypassed stages weaken in volume, hum, and get undesireably distorted. The problem corrects upon disconnecting power and reconnecting, but reappears in 60 seconds or so.

I would guess I need to change capacitor values somewhere, but I don't really know what to do other than just guess, which is taking a really long time. Does anyone know what's going on?

[bioroids]

Hi! How did you connect the switch?

Miguel

[Bernardduur]

Do you have a schematic? I never saw one of it.....

[Ashurbanipal]

Here's my schematic.

The switch disconnects after the first stage coupling cap and connects to my rigged 3rd stage. It also disconnects after the 2nd stage coupling cap to the ouput and connects the 3rd stage coupling cap to the output. So power is always connected to both 2nd and 3rd stages. I'm guessing some capacitor is discharging in or after the first 60 seconds of having power due to being the wrong value or ground problems in my rigged design. But that's beyond my current knowledge of how to solve.

If I remove the 3rd opamp stage entirely, it works great as expected, just that true bypass is so much lower in volume as to be unuseable. Thanks so much if you can solve this.

[bioroids]

Wouldn't be easier to put a volume pot after the 2nd stage so you can adjust the level to match the bypassed signal? According to your schem you're gonna have a huge boost on bypass mode (and it will be also much louder than the effect signal because you have no limiting leds).

Other than that, I cant spot any obvious mistakes on the schematic, maybe I'm missing something...

Luck

Miguel

[Ashurbanipal]

Actually I do have a 10k volume pot that I forgot to draw after the 2nd stage output coupling cap and before the switch. Turning it way down to match the weeny bypass mode mucks up the tone to my picky ears. My third stage has a 1M trimpot at the moment, which is turned way down probably to around 50k. If I or no one here can figure this out, I may just build a separate booster within the same box for the bypass. Thanks.

[bioroids]

Mmm maybe you should use a 100K volume pot, with a series resistor, like this:

2nd stage output ---> resistor  ---> volume pot

You choose the resistor value so that, with the volume pot at max, you get the appropiate max level. Maybe start with 100K.
This way can't suck any tone and you got a usable range from the volume pot.

If you use the "boost the clean signal" approach you have to keep in mind that you're not at standard guitar levels anymore. You are gonna be switching between boost and distortion. You can put just a switch for the leds if you want that, too.

How are you powering the opamps by the way? Are you using a bipolar supply (+9v and -9v) or the standard signle supply?  If its the last case you need to bias the +inputs at 4.5v, not ground. That would explain the intermitent behaviour.

Luck

Miguel

[Ashurbanipal]

Thanks. Will try the resistor/100k pot. Can you explain why this does not muck the tone?

Biploar power supply. I tried switching off the LEDs at one point, but the clean volume was too loud. I think I then tried switching the LEDs off with a lower resistor in the 1M feedback spot, but I don't really remember. So many options. Hopefully the 100k volume pot will work.

[bioroids]

Yes, you can switch the leds with its resistors and cap on one hand, and the apropiate resistor for equivalent clean gain on the other. That has to work and it only takes a SPDT.

I'm sure that the opamp can drive the 100K volume pot. Maybe the 10K was too low an impedance for that particular opamp and that makes it a less desirable tone (though the sound should be the same at any point of the pot rotation). I would certainly try that before adding additional stages of boost.

Luck!

Miguel