Graphic EQ's

[MullisMan]

I'm building my first overdrive pedal and instead of going with a standard passive eq section, I would really like to go with an active EQ.  I'm basing my pedal off of the Wamp-o-drive in Brian Wampler's DIY book, and that uses the JRC4558.  In that same book when he goes over the graphic eqs, he has a JRC4558 at the end of the graphic eq circuit as well.  Does this mean I'll need two op amps in my circuit?  I didn't know what that would do to my space issues, since i'm also going to try and use JFETs on the input and output.  The Wamp-O-Drive uses both sections of the 4558 already so I don't see any other way around adding another IC in.  Also, does anybody know what BJT's are commonly used for active eq sections?  I only want a bass and treble control.

[MikeH]

Yeah, you'll need another opamp.  Really you can think of them as completely independent circuits.  Graphic EQs require lots of opamps and unless you want to make something really versatile like the EHX graphic eq, a 3 band active eq will probably suit you just fine.  There's a great one at runoffgroove called the "tonemender".  You could likely work it in in place of whatever passive eq you already have, or just tack it on the front or back of your existing circuit.  You'll get different results putting the eq in different places, but the end of your circuit will probably be the most useful

[MullisMan]

Awesome, thanks! Does it usually matter what BJT I go with? I planned on doing as you said, swapping the passive eq section out for the active at the end of the circuit, and probably not going with the JFET on the output.  Now i'll just have trouble deciding which caps to use in between sections of the circuit.  Thanks again.

[MikeH]

For coupling caps I usually start at 0.1uF and go down from there, although 0.1 usually works just fine.  You'll likely have to adjust some cap/resistor values in the active eq you go with to suit the distortion you're making, but it shouldn't be that hard.  The tonemender is super versatile and the article at runoffgroove explains what values do what.

edit: really you could build the distortion and the eq as separate circuits with their own switch, and use them together OR independently.  Afterall, we're looking for versatility here, right?

[Paul Marossy]

Does it usually matter what BJT I go with?

I think the only important consideration is that they be low noise types, like a 2N5089 for example.

[MullisMan]

I'll be sending Steve at Smallbear my order soon then.  Thanks again Paul.

[Paul Marossy]

No problemo.