3 band eq to 2 band Eq?

[suryabeep]

Pardon me if this is a super-noob question:
Can I make this 3-band eq a 2-band eq by removing the input buffer, Mid pot, the two 3k6 resistors, the 22n and 5n capacitors, and connecting the 'south' end of the 11k resistor to the - input of the op amp?
I'm thinking of sticking it in place of the Tone stack of an OCD

[Rob Strand]

Do you like the sound of the treble and bass controls on this particular 3-band?
When you convert to 2 band to 3 band the controls no longer behave the same.
By changing the caps you can make it a bit closer.

[suryabeep]

By changing the caps you can make it a bit closer.

I'd breadboard it but I just spent four hours breadboarding a failure of a distortion and I'm too tired to go back to the garage now :|

I drew a mock-up, did you mean something like this?:

[Rob Strand]

Yes that's the idea.  You probably don't need the 150k on the opamp + input as it will just add noise.

To make the 2-band sound a bit closer to the 3-band:

Bass cap:  68n to 82n

Assuming 100k treble pot:
Treble Cap:  2n2 to 2n7
Treble resistors:   3.3k   (instead of 1.8k)

You should play with the caps to get the sound you like.
With the original cap values it might actually sound better even though it doesn't behave like the 3-band at all.

[suryabeep]

Thank you so much, Rob!
btw, I'm still fuzzy on some basics - why does the 150k add noise?

[Rob Strand]

Thank you so much, Rob!
btw, I'm still fuzzy on some basics - why does the 150k add noise?

You have to know a lot to understand why.

Here's main idea:

A resistor by itself:
- All resistors create noise (this is physics, called thermal noise or Johnson noise).
- Higher resistor values create noise more than smaller values.

In a circuit: How much noise a resistor in a circuit creates is tricky because it depends on where is appears in the circuit, how it is connected, and what it connects to.   The simplest cases are:
- For a resistor in series with a signal source the noise adds to the signal.
- For a resistor in parallel with a signal source it contributes no noise if the signal source is low impedance but can contribute some noise when the source is high impedance.

In your case the resistor is a high value and is in series with the opamp input.   The noise from the resistor goes straight into the opamp input.   The opamp will amplify it a bit then that noise gets added to your signal.  Without the resistor the circuit still works, so you are adding noise for no reason.

[suryabeep]

Ah, I see now. Thanks!

[PRR]

> did you mean something like this?:

Major missing wire!

[suryabeep]

oof thanks for pointing that out, Paul!