2 Simple questions about filters for Harmonic Tremolo

[LaloFP]

Hi guys!

I was looking at the Quaverato Harmonic Tremolo (https://zeppelindesignlabs.com/product/quaverato-harmonic-tremolo-pedal/) and I think I understand everything,
BUT there is two details that I would love to get clear about.

Question N° 1:
I get why HPF is going to V/2, because it's the virtual ground, the offset for the audio signal to be centered on 4.5v.
But why is the C3 cap going to GND? I think it would work going to V/2 too (and it seems more conceptually "coherent/symmetrical/simpler" to me), but I don't know if I'm missing something.



Question N° 2:
Is R15 there for a special reason? If I'm correct, it will influence the filters attenuating a little more of HIGHS.
Couldn't just be removed and get a more flat response? Or is something going on with the previous Opamp?



That's it! 

[ElectricDruid]

Question N° 1:
I get why HPF is going to V/2, because it's the virtual ground, the offset for the audio signal to be centered on 4.5v.
But why is the C3 cap going to GND? I think it would work going to V/2 too (and it seems more conceptually "coherent/symmetrical/simpler" to me), but I don't know if I'm missing something.

Since the capacitor blocks DC, it doesn't actually matter what the voltage at the lower end of the cap is. I agree that 4.5V is more conceptually coherent, but in general the actual ground has a lower impedance than the 4.5V midpoint, so it oftens works marginally better in practice.

Question N° 2:
Is R15 there for a special reason? If I'm correct, it will influence the filters attenuating a little more of HIGHS.
Couldn't just be removed and get a more flat response? Or is something going on with the previous Opamp?

I don't know. I can't see any good reason for it to be there either.

[antonis]

IMHO, R15 is just "forgotten" there..
(it serves as U2B output current limiting resistor in the good design practice guidelines..)

[ElectricDruid]

IMHO, R15 is just "forgotten" there..
(it serves as U2B output current limiting resistor in the good design practice guidelines..)

Yeah, that was my thought too. It looks like an op-amp that's feeding an output jack, and someone has just copied the whole circuit in from somewhere else and then tacked the new bits on the end where the output used to be.

[LaloFP]

Awesome guys, super clear! Thanks