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 Forum index » DIY Hardware and Software » Circuit Bending
getting my spring reverb to feedback, any tips?
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SineHacker



Joined: Mar 09, 2010
Posts: 99
Location: United Kingdom

PostPosted: Sun Jul 01, 2012 10:05 am    Post subject: getting my spring reverb to feedback, any tips? Reply with quote  Mark this post and the followings unread

I went through town today suffering abuse and confrontations from skinheads and drunks so I could buy a plastic tub and try this out:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

I am using an LM386 to drive the speaker and a cmos 4049 circuit from Nicolas Collins designs for the piezo contacts. I tried a LM386 for the contacts as well but the 4049 was much cleaner and I could use a cap to put a little top-end roll off on the "wet" sound this way as well.

It sounds nice, but the reverb effect only lasts about a second, which is ok - but it would be great if I could make it longer, I was wondering if there is a way i can bleed the wet signal back to the amp side for a feedback effect?

also, I had to separate the chips into two circuits, otherwise a lot of the dry signal seemed to spill through into the output, I guess this is more down to noisey ground from the LM386 - any tips on fixing this so I can run it off one battery?
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SineHacker



Joined: Mar 09, 2010
Posts: 99
Location: United Kingdom

PostPosted: Sun Jul 01, 2012 12:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

someone suggested I invert the current before feeding it back through the circuit, the 4049 I am using is an inverter but I don't know too much about them - I use them for preamps and distortion a lot - I have 5 empty gates on the chip, do you think I could use one of them to achieve this?

I don't really get the datasheet, but I will have another look through it in the mean time

edit- actually I read the datasheet again and it says "logic level conversion" so I guess an audio signal is too complex for this.

the lm386 has a + and - input so if I do use another lm386 and just swap the inputs I use on one... fuck it I should just give it a go!
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JingleJoe



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PostPosted: Sun Jul 01, 2012 10:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah! Don't let those vagabonds bring you down or stop you from doing SCIENCE! Force your way against the flow of the river of society to achieve your goals! Very Happy
I feel like I should mention that the lm386 is a specialized op-amp IC, these have two differential inputs which are added or subtracted from one another depending how you look at them, then multiplied by the gain. In the case of the lm386, gain is automatically set to 20 and one of the inputs can be tied to ground and the other used with no problem, because the output is biased to half Vs Smile which is a technique used in a lot of other op amp circuits to make them work from a single power supply, I'd strongly recommend a 100uF power supply filter cap, even with a battery, for the lm386 if you don't have one already.

Now as for feedback, that can be done very simply, I think a circuit diagram is in order (see attachments) that's what I would do, you will probably need extra amplification or attenuation at different points in the circuit but it will depend on your circuit.


reverb concept.png
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reverb concept.png



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bubzy



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PostPosted: Sun Jul 01, 2012 11:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

http://electro-music.com/forum/topic-53738.html Smile
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Psyingo



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PostPosted: Sun Jul 01, 2012 11:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

JingleJoe wrote:

I feel like I should mention that the lm386 is a specialized op-amp IC, these have two differential inputs which are added or subtracted from one another depending how you look at them, then multiplied by the gain. In the case of the lm386, gain is automatically set to 20 and one of the inputs can be tied to ground and the other used with no problem, because the output is biased to half Vs Smile which is a technique used in a lot of other op amp circuits to make them work from a single power supply, I'd strongly recommend a 100uF power supply filter cap, even with a battery, for the lm386 if you don't have one already.


Imma let you finish but the 386 isn't really an Opamp. It's made mostly just for amping audio to drive a speaker. Most of the things you could do with an Opamp simply don't apply to the lm386. At least not easily, and not without extra circuitry.

Calling a 386 an Opamp is like calling a bike a car, both have wheels, right?



feedback won't help with your decay time, like it does with echoes/delay, it would likely build up into typical feedback noise. You can chain reverb tanks to get longer reverb.

You might also want a buffer in the feedback loop as well.
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SineHacker



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PostPosted: Mon Jul 02, 2012 2:41 am    Post subject: Reply with quote  Mark this post and the followings unread

bubzy wrote:
http://electro-music.com/forum/topic-53738.html Smile


this is very helpful thanks, I'm going to move my conversation over to that thread
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JingleJoe



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PostPosted: Mon Jul 02, 2012 5:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Psyingo wrote:

Calling a 386 an Opamp is like calling a bike a car, both have wheels, right?

No.
I never said it was an op amp like any other op amp, you should note that I said it is a specialized op amp. You are right, it is specialized for high current output, it is also internally biased and has internal feedback resistors to set the gain to 20. I can't believe I have to repeat myself like this, I couldn't have made it any more clear that it was different from an op amp.
Psyingo wrote:

feedback won't help with your decay time, like it does with echoes/delay, it would likely build up into typical feedback noise.

Do you have an example of that? If your feedback is attenuated as it would be in the diagram I posted, then decay time or reverb time would be increased without the risk of self oscillation.

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Psyingo



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PostPosted: Mon Jul 02, 2012 7:31 am    Post subject: Reply with quote  Mark this post and the followings unread

JingleJoe wrote:
Psyingo wrote:

Calling a 386 an Opamp is like calling a bike a car, both have wheels, right?

No.
I never said it was an op amp like any other op amp, you should note that I said it is a specialized op amp. You are right, it is specialized for high current output, it is also internally biased and has internal feedback resistors to set the gain to 20. I can't believe I have to repeat myself like this, I couldn't have made it any more clear that it was different from an op amp.
Psyingo wrote:

feedback won't help with your decay time, like it does with echoes/delay, it would likely build up into typical feedback noise.

Do you have an example of that? If your feedback is attenuated as it would be in the diagram I posted, then decay time or reverb time would be increased without the risk of self oscillation.


even mentioning that the lm386 is a "specialized Opamp" may cause confusion and have people think that the 386 could simply be dropped into an Opamp circuit and have it work. It's best just to call it an audio amplifier to avoid confusion.

I have no hard facts regarding feedback extending decay time. I had fiddled with it years ago. I think if extending the decay time of spring reverb were this easy than it would be common practice and the need for longer springs wouldn't be so great. Of course no need to take my word for it, try it out. Feedback is a fun effect to play with in reverb tanks, anyway.
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JingleJoe



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PostPosted: Mon Jul 02, 2012 11:21 am    Post subject: Reply with quote  Mark this post and the followings unread

If people drop an lm386 in as a replacement op amp without looking at the datasheet it's thier own fault for not researching thier parts properly before using them. Also I don't know why anyone would do that after I clearly state that it does different things than an op amp. Don't you think people have the attention span to read a post in full? Rolling Eyes

Longer springs can help in extending delay but so can a good dose of attenuated feedback.

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