neg feedback clipper: low cut shunting to +9v instead of Vb?

[Pojo]

I'm building something with the typical tube screamer type negative feedback clipping and added a low cut pot (like the Timmy has) that controls how much low frequencies are overdriven. The way my vero layout is working out so far (I'm still extremely bad at them), it's much more convenient to shunt to the supply rail instead of to the bias voltage rail which is how the Timmy and similar circuits do it. I've tried it on breadboard and it seems to work fine.

My question:

Is there anything I need to worry about doing it this way? I don't really understand the reason why if a fixed capacitor value is used it's ok to shunt to ground, but if you want to control it using a pot to vary between 2 caps you must shunt to the bias supply. So I don't know if it just simply needs to go to a positive reference and the actual voltage doesn't matter so much. I really really hope this is the case!

[R O Tiree]

Got a schem, please?

[Mike Burgundy]

Schem would be handy indeed. What I'm thinking though is:
A) for AC (DC blocked),  ground, Vref and the positive rail are all theoretically ground. There is a catch though - going to "ground" through the positive rail might cause all kinds of weirdness due to real-world battery properties such as internal resistance.
B) is there is a DC path between your shunt and whatever your "grounding", they will need to be at the same DC points (hence the usual Vref) or the pots will get scratchy, DC bias will change and battery use might rocket.

[Pojo]

Schematic Link here: http://imageshack.us/photo/my-images/542/m7hp.jpg/

The red circled area is the section in question.

I'm thinking since the capacitors would block any dc from getting any further that it would be ok? Looks like this would be situation 'A' would apply here. Now here's another question, as I was finishing out the layout it became even more convenient to tie C7 to +9V and tie C8 to Vb. Which also means that the 2 caps aren't linked together like shown in the schem, but my logic tells me that it doesn't matter as long as each cap goes to somewhere that the signal can 'escape'. The unknown in my head is whether or not the different voltage levels matter. Essentially as the pot is turned, the signal will be passing in changing ratios through both of the 2 voltages.

[slacker]

It should work connecting one to Vs and one to Vb, for audio signals both of those points look like ground so the different DC levels shouldn't matter. Like Mike said using Vs as an AC ground might lead to problems that won't show up until you've build it, personally I'd avoid doing it.
I'd rework the layout so that they could both go to vb or ground.

[Pojo]

Thanks for the help guys.

Slacker, I think (although I don't know why) in this situation actually using ground doesn't work...hence Vb is chosen instead. What's the actual difference between using Vb and Vs? Note that I'm not using the actual rail straight from the supply but the filtered Vs supply.

[slacker]

I don't think I've ever tried that fat control, and I've seen people say it doesn't work connected to ground but I can see any reason why it shouldn't. If the circuit is still on the breadboard try it and see.
In theory using Vs or Vb on your schematic are the same for audio signals, the DC voltages are difference but C1 and C2 are big enough to give audio signals a path to ground, so those points appear to be ground for audio. A possible problem with using Vs is that you could end up with some of the audio signal on the Vs line, this can then get back into the audio path through R6 or the power pin of the opamp and cause noise or oscillation.

[Pojo]

Ok makes sense...so maybe with Vb because of the voltage divider it makes it less likely to do so? (signal leaking into other parts of circuit).

[Pojo]

Hmm...ground works fine on breadboard...no difference to my ears

[Pojo]

  Ok I found the link I saw where I read about using Vb instead of ground.

http://www.muzique.com/lab/fatt.htm <--- go to bottom of page

And the only reason is because the mod he suggests is a switch which completely bypasses the .047uf cap in a stock TS..so the voltage in the feedback loop would leak to ground when the switch is closed.

I managed to rework the layout to have both caps shunt to ground....me thinks this should be good!