So this is where I am with my Pandora...

[thunderaxe]

Here's what I did with mine...
I connected a C5K CONTOUR pot + 47uF cap + 22R resistor in series and then put them in parallel with C9. With the pot fully CCW, it's the normal Expandora bass rolloff. As the pot is rotated clockwise, more and more bass comes in until at full CW the response is basically flat from 50Hz on up.

View attachment 2114

I also added a CLIP switch, similar to what reubenreub did here.

BTW, all of those cap changes you made affected the frequency response below 4Hz. C14 might help a little if you're driving a real low impedance pedal. The bass response is really all about C8, C9, R6 & R7.

hi @Chuck D. Bones, old thread i know but i had a quick question about this mod -- i get the basic idea of how it works, but i'm not clear on one thing -- does turning the C5K pot work by changing the series resistance to ground, which in conjunction with the 47uF cap changes the corner frequency of the high pass? or, does it work more like you're blending in a signal path that has a lower fixed corner frequency, and thus more like a low shelf?

 

[Chuck D. Bones]

It's more like a shelving filter. Here's the freq response. This version uses C1K for the BASS knob. C1K has a smoother sweep than C5K.

 

[thunderaxe]

It's more like a shelving filter. Here's the freq response. This version uses C1K for the BASS knob. C1K has a smoother sweep than C5K.

View attachment 84281

would C1K still be the right pot if i go with the 100uF cap? i'd like to be able to dial in green russian big muff levels of low end at the extremes

 

[Chuck D. Bones]

Yes. For extreme bass, you might even want a bigger cap, like 220uF if you have one. Check the DC voltage at IC2 pin 6. Should be very close to 1/2 the voltage on pin 7. Large electrolytic caps can be leaky and will upset the bias which will affect the tone. I recommend tantalum for C8, C9 & the Bass cap.

 

[thunderaxe]

i have a more general question about this kind of negative feedback loop high pass if you don't mind:

i understand that in the negative feedback loop, everything is kind of topsy-turvy backwards world because the signal that passes through it gets cancelled out, whereas the signal that gets blocked from passing instead gets passed forward to the output. the higher the resistance in the feedback loop, the more signal gets passed to the output; a high pass filter in the feedback loop functions as a low pass filter for the output, etc.

but a high pass filter that functions -- as the ones in the feedback loop here do -- by shunting treble to ground -- isn't that signal lost either way? it isn't getting cancelled out by going into the inverting input, but it isn't getting passed to the output either, it's getting shunted to ground.

 

[Feral Feline]

^^ “Extreme bass” … I like that.

 

[Chuck D. Bones]

I really need to write the next installment of Opamps for Dummies...

You're right about the feedback loop working the opposite from how a circuit that's not in a negative feedback works.