Adding active treble control to Expandora causes issues with tone knob

[nate433]

I'm trying to add an active treble control to an Expandora but when I do and turn the tone knob fully CCW it kills the sound and gets almost gated. There's no other modifications made to the pedal at this time. Position of the treble knob doesn't seem to change anything. I'm using the same active treble control seen in the DOD Punkifier. I confirmed it's the treble control doing it since when i bypass it the tone knob is fixed.

My goal is to add more tonal control to the expandora since i'm not in love with the stock filter. I've tried bandaxall and other 2 knob control schemes, so i thought being able to boost/cut the treble after the filter could give some interesting options. Red is the entire tone control block, blue is the added treble control, yellow is the original filter

 

[nate433]

I tried putting a 100nf cap in between the filter, before the 22k resistor and i fixed the dying sound. BUT it screwed with the overall tone quite a bit. It also introduced a weird fizz that pulses in an out.

 

[jesuscrisp]

Other than adding that coupling cap you also need a bias resistor to Vref for the op amp stage that is your tone control. Also coming off your tone control there is zero reason to have a 1M series resistor and 1M volume pot in the next stage, you reduce both to 100K or less.

 

[nate433]

Other than adding that coupling cap you also need a bias resistor to Vref for the op amp stage that is your tone control. Also coming off your tone control there is zero reason to have a 1M series resistor and 1M volume pot in the next stage, you reduce both to 100K or less.

Ok i'll try lowering the 1M, that's just how the stock Expandora is.

For my own understanding, what is the resistor to VREF doing? I know it's common to have to set up 4.5v for op-amps to work, but what is the extra resistor doing in this case? Along the same line, why are the bias resistors in the drive sections 470k but the one in the tone control is 1M? I looked back at the Punkifier schematic i noticed it doesn't have one on the tone control but there is a buffer right before it that has a bias resistor.


Would that all be considered part of the tone block and I shouldn't have left it out? Easy enough to add back in since right now there's an unused half of the 4558 sitting empty.

 

[jesuscrisp]

For my own understanding, what is the resistor to VREF doing? I know it's common to have to set up 4.5v for op-amps to work, but what is the extra resistor doing in this case?

Well how do you think is the op amp setting itself up to bias? Exactly, through that resistor...

Along the same line, why are the bias resistors in the drive sections 470k but the one in the tone control is 1M?

These resistors bias and also set impedance. Depending on what comes before it and what op amps you are using, you might want higher impedance, but also too high impedance might not sit well with certain op amps and you get slight misbiasing/loading.

I looked back at the Punkifier schematic i noticed it doesn't have one on the tone control but there is a buffer right before it that has a bias resistor.

In the Punkifier there is no coupling cap between the tone control and whatever comes before it, while in the Expandora there is:

The volume control after it is an inverting op amp that doesn't need a bias resistor, but they wanted to have similar impedance as the usual Rat JFET buffer is why they used 1M for R10. This is generally not great in terms of design, and neither are so many other impedance choices in the Expandora, but it's up to you if you want to "fix" them or not.

 

[nate433]

Well how do you think is the op amp setting itself up to bias? Exactly, through that resistor...

I guess there's my confusion. I thought biasing was setting the voltage to 4.5 (VREF) in the power section. Why does it need the additional biasing resistor (beyond setting impedance)?

I also just realized that is a different op-amp in the photo i linked. Since it's using the same pins and the tone. So i guess I'll try setting up 1.1 as 2.2 is in the origianl?

 

[Brett]

Why does it need the additional biasing resistor (beyond setting impedance)?

You may find the answers (and more) by taking a peek at @Chuck D. Bones's Opamps for Dummies posts:
Part 1: https://forum.pedalpcb.com/threads/opamps-for-dummies-part-1.18833/
Part 2 (this contains a primer on opamp biasing): https://forum.pedalpcb.com/threads/opamps-for-dummies-part-2.18899/
Part 3: https://forum.pedalpcb.com/threads/opamps-for-dummies-part-3.19162/
Part 4: https://forum.pedalpcb.com/threads/opamps-for-dummies-part-4.19470/