Author Topic: How can I add a Feedback Looper knob to a circuit???  (Read 3662 times)

Jasonmatthew911

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How can I add a Feedback Looper knob to a circuit???
« on: December 29, 2011, 09:58:31 AM »
Hi, I was looking to make a mini 2 knob fuzz similar to the Tonebutcher Pocket Pus mini pedal, and I read that the 2nd knob is controlling the feedback loop...I know how to make a feedback looper box to add to existing pedals with send and returns, but I wanted to know what would be the proper way to add a feedback loop knob to my existing LM386 fuzz circuit?...I tried putting a 10K linear pot directly from my input to my output, but that doesn't work right...Can anyone help with this?....I'm basically trying to make something like this:

http://www.youtube.com/watch?v=zbp6-5yv89o

Jasonmatthew911

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Re: How can I add a Feedback Looper knob to a circuit???
« Reply #1 on: December 29, 2011, 11:59:11 AM »
I read something about the circuit having to be in phase in order for the feedback loop knob to work right, or else it just cancels the sound...I think this is my problem, instead of giving me noise and oscillation by feeding the output back into the input with a 10K linear pot, it's just cutting my signal...I'm using the - input (pin 2) of an LM386 for my fuzz circuit, could this be my problem?...Is there any way I can do this, or do I have to do something differently?

Seljer

Re: How can I add a Feedback Looper knob to a circuit???
« Reply #2 on: December 29, 2011, 12:06:50 PM »
I'm using the - input (pin 2) of an LM386 for my fuzz circuit, could this be my problem?...Is there any way I can do this, or do I have to do something differently?

Yes, that would most likely be your problem. The LM386 is being used as an inverting amplifier. The output is 180 out of phase from the input. When you add feedback you're adding negative feedback which is all the time and is great for when you want to lower and linearize the gain. But if you want oscillations and other crazy stuff you need positive feedback, either by changing the way your LM386 is connected or by adding another inverting amplifier in the feedback loop (its probably easier to just do the first).

wavley

Re: How can I add a Feedback Looper knob to a circuit???
« Reply #3 on: December 29, 2011, 02:49:14 PM »
I'm using the - input (pin 2) of an LM386 for my fuzz circuit, could this be my problem?...Is there any way I can do this, or do I have to do something differently?

Yes, that would most likely be your problem. The LM386 is being used as an inverting amplifier. The output is 180 out of phase from the input. When you add feedback you're adding negative feedback which is all the time and is great for when you want to lower and linearize the gain. But if you want oscillations and other crazy stuff you need positive feedback, either by changing the way your LM386 is connected or by adding another inverting amplifier in the feedback loop (its probably easier to just do the first).

+1 on that, feedback loops can be funny things they don't really work that well for everything because there are a lot of pedals that are 180 out of phase with the input, which really doesn't matter because the ear can't detect a difference between a single signal and it's phase reverse... until you try to use it going to one of two amps or in a feedback loop, then you're really in trouble.
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Jasonmatthew911

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Re: How can I add a Feedback Looper knob to a circuit???
« Reply #4 on: December 30, 2011, 01:13:18 AM »
I just rebuilt it using the + input and it works great now...Crazy sounds from such a simple circuit once you add a feedback looper...Thanks again for the help.

Gurner

Re: How can I add a Feedback Looper knob to a circuit???
« Reply #5 on: December 30, 2011, 07:37:20 AM »

...., feedback loops can be funny things they don't really work that well for everything because there are a lot of pedals that are 180 out of phase with the input,

that shouldn't matter in this instance becuase what causes the oscillation is the lm386 output phase in relation to the LM386 input phase and providing you put the input signal on the LM386 +ve pin, then the phase will always be 'in phase' with the input & therefore you'll get oscillation/instability when you feed it back to the input.

wavley

Re: How can I add a Feedback Looper knob to a circuit???
« Reply #6 on: December 30, 2011, 09:34:33 AM »

...., feedback loops can be funny things they don't really work that well for everything because there are a lot of pedals that are 180 out of phase with the input,

that shouldn't matter in this instance becuase what causes the oscillation is the lm386 output phase in relation to the LM386 input phase and providing you put the input signal on the LM386 +ve pin, then the phase will always be 'in phase' with the input & therefore you'll get oscillation/instability when you feed it back to the input.

Exactly, he built it using the - pin and it was 180 out thus acting as negative feedback adjusting gain instead of a positive feedback loop just like Seljer said and I +1'd.  I've tried a lot of pedals in loops and when one doesn't work right I hook it up to a scope and 99% of the time it's because it's out of phase with the input.  Which he proved by using the + input instead

I'm using the - input (pin 2) of an LM386 for my fuzz circuit, could this be my problem?...Is there any way I can do this, or do I have to do something differently?

Yes, that would most likely be your problem. The LM386 is being used as an inverting amplifier. The output is 180 out of phase from the input. When you add feedback you're adding negative feedback which is all the time and is great for when you want to lower and linearize the gain. But if you want oscillations and other crazy stuff you need positive feedback, either by changing the way your LM386 is connected or by adding another inverting amplifier in the feedback loop (its probably easier to just do the first).

I don't understand why you would call me out on something we were actually agreeing on.
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