3 Band EQ w/ 386 Boost: Need Help Please.

[Faber]

Hello all.

I was looking through the EQ circuits page and found the 3 band EQ.  I downloaded the Tonestack, adjusted the values to my liking and came up with this circuit.  I don't know if the 386 is overkill and a transistor would work, but that's what I came up with.  Also, what should the pot value be between pins 1 and 8?

Sorry the image is so big...



Thanks!

[GibsonGM]

At first glance, this should work, at least to experiment.  The pot value seems unimportant...5k, 10k, whatever.  You can tweak it for your preference.  You might find that you'd like the sound better by using an input buffer followed by a gain recovery stage!  That tone stack (Marshall-type) will eat up some dB's, and you might have to drive the 386 a lot to get a good signal output.

Look at the schem for a Tube Screamer.  If you use the first transistor (buffer) stage, add  your tone control to its output (at the emitter), and follow the tone control with the last transistor of the TS, which is a boost/recovery stage, you should get pretty good results!    You could also use the 2 trannies, and run the output to a 386 and make a version of a Ruby amp, etc. It might overdrive pretty well.
Good experiment, though  )

[ambulancevoice]

i would say 1k for the gain pot
5k would work but may have to much range
i would use 1k, it will work fine

[trevize]

I wouldn't use a 386 in this context. Use a simple dual opamp. The first with gain setting and the second as a recovery stage after the tone controls.

[CGDARK]

I wouldn't use a 386 in this context. Use a simple dual opamp. The first with gain setting and the second as a recovery stage after the tone controls.

I agree with trevize, but make sure that the recovery stage has enough gain to compensate for the tonestack circuit.

CG

[trevize]

If you want to use the 386 you could make a fet buffer > tonestack > 386 as recovery stage with variable gain so it can "maybe" simulate power amp distorsion

[Faber]

Okay, based on what y'all are saying, the 386 is the wrong chip to use for this application.  I'm gonna have to figure out what these buffers are used for...  I'll look into a dual opamp.  Of course the 386 was originally intended to get the gain back to unity.  Should I use an opamp AFTER then?

[Ben N]

If you wat clean bost with tone controls, the suggestons above are corect; don't use a 386. If you want some tasty crunch with that, then a Smashdrive is very similar to what you came up with, and sounds great:

http://www.diystompboxes.com/pedals/schems/smashdrive.gif

[Faber]

Okay y'all.  Sorry I didn't clear this up earlier.

The only purpose of the 386/opamp/whatever is to gain back the power lost from the EQ circuit and get the gain back to unity.  It's going to go into an overly ambitious multi-effects circuit and I wanted an EQ for it.

Now that we've brought up buffers, I just realized I've always heard about them and never knew what they were.  Is there a good article that explains them... or just a really smart person willing to explain them?

[Ben N]

No need to reinvent the wheel: There is a lot of discussion here about buffers; just use the Search function. There are also articles at AMZ and, I believe, Geofex--check for Links above.

A 386 is wrong because it is not an opamp, it is an audio power amp, meaning that it delivers enough current to drive a speaker. But that is useless for what you want. It also has a low input impedance, which means it is likely to load down a relatively high output impedance, like a guitar, resulting in signal loss and tone sucking. It is also likely to distort when you feed it with an unbuffered guitar signal--great for a dirtbox, not so much for a clean boost-eq.

With a dual opamp, you can make your eq with high input impedance, low output impedance, plenty of voltage gain (the kind you want) to recover your post-eq signal, low noise and low distortion.

Have you looked at the Tonemender and Mr. EQ projects at runoffgroove.com?

[ambulancevoice]

dual opamp sounds good
use the first opamp at the input before the eq and and another after the output
just my idea...

[Faber]

Yeah, the Tonemender is basically what I'm going to do.  Thanks guys.  I'll have to look up those buffers.  I tried earlier, but didn't find much.  Of course, I didn't look very hard...