proco rat op amp biasing question

[Boner]

https://www.electrosmash.com/proco-rat#link3

Why did they connect C4 and C5 to ground and not 4.5 volts since the op amp is powered with a single power supply?

C2 I don't get either. Why is that connected to ground and not 4.5?


I thought the practice was that all "ground" connections for single supply op amp stages be the halfway point (reference) and not actual ground.

[antonis]

You thought wrongly..

+4.5V is for bias..!!
None of the above mentioned caps (or any other cap) has anything to do with bias..
+4.5 Volts are DC and caps block DC..
BUT
there is (usually) a cap (like C3) for +4.5V ripple rejection, which is also used as AC Ground..
(so caps to GND are connected there for layout convenience..)

P.S.
Caps connection to +4.5V (or Vref or Vcc/2) is terminologically wrong..
(they are connected to AC ground and NOT to 4.5VDC..)

[Boner]

So with a single power supply powering an op amp like this, you bias the entire non inverting amp with a single resistance (here its R2) to Vref and everything else associated with the amp has nothing to do with biassing and so these connects to DC ground?

and youre saying it doesnt really matter if you connect them to Vref anyways since they block DC? But they should be connected to 0Vdc to make it correct.

[antonis]

So with a single power supply powering an op amp like this, you bias the entire non inverting amp with a single resistance (here its R2) to Vref and everything else associated with the amp has nothing to do with biassing and so these connects to DC ground?

More or less, yes..
You can also bias non-inverting amp by direct coupling preceding stage op-amp output but let it be for the time being..

and youre saying it doesnt really matter if you connect them to Vref anyways since they block DC? But they should be connected to 0Vdc to make it correct.

More or less, yes.. 
"Correct" is a word with a trillion implementations..

[ElectricDruid]

https://www.electrosmash.com/proco-rat#link3

Why did they connect C4 and C5 to ground and not 4.5 volts since the op amp is powered with a single power supply?

C2 I don't get either. Why is that connected to ground and not 4.5?

I thought the practice was that all "ground" connections for single supply op amp stages be the halfway point (reference) and not actual ground.

Since caps block DC, what difference would it make if they did go to 4.5V instead? So it doesn't matter and you can connect them to ground just the same. This is one of those situations where the "normal practice" for converting a circuit to single supply can be ignored.

[ashcat_lt]

You can (and some circuits do) connect those caps to the 4.5V bias point, but for AC signals, that's literally the same as 0V "ground" because of that bias bypass cap that antonis mentioned, so...  One thing I can think of, though, is that the first two of the caps you're talking about (the ones in the feedback loop) are polarized electrolytics which may not be super happy about having both ends at the same DC voltage, and they're pretty big values, so might be a little tough to find a non-polarized alternative part.

[antonis]

so might be a little tough to find a non-polarized alternative part.

For a DIY project, it indeed should be a good idea (despite their over 3 times price..)

An alternative could be lowering their value about 5 times when raising gain resistors values by 5 times and also feedback pot value by the same amount..