Question--Filter Reference Pt. in TS-style Clipping Circuit

[Tall Steve]

The 47n/4k7 HPF in the Tube Screamer feedback loop is referenced to ground, but I noticed that in some TS derivatives like the BB Preamp it's referenced to the voltage divider (+4.5V).  Assuming the same CR values, what effect does this have on the gain and frequency response?

[samhay]

None, unless your source of Vref is very poorly designed.

[Tall Steve]

Thanks!  Just for clarity, is referencing the filter to Vref the same as sending all other frequencies to unity gain?

[ashcat_lt]

Thanks!  Just for clarity, is referencing the filter to Vref the same as sending all other frequencies to unity gain?

Ummm...no?   (Not completely sure what you're asking there)

The whole thing works exactly the same either way.  Without the cap it would make a huge difference, but with the cap there's no difference period.

[Tall Steve]

I was trying to ask if there was a difference in function or process between the two even though they yield the same result, like taking the interstate vs the highway. Sorry. We just started on op amps in my EE class.

[Digital Larry]

The idea is "what is the AC impedance of the node at high frequency"?  If the power supply has a large cap across it, the +Vcc is nearly the same as ground.  As would be any well bypassed reference point based on resistor voltage dividers.  So the quality of your bypass caps is important.  Realistically there might be tens of ohms difference, but hopefully the other circuit is several kOhms and so it doesn't matter "much".

[ashcat_lt]

The idea is "what is the AC impedance of the node at high frequency"?  If the power supply has a large cap across it, the +Vcc is nearly the same as ground.  As would be any well bypassed reference point based on resistor voltage dividers.  So the quality of your bypass caps is important.  Realistically there might be tens of ohms difference, but hopefully the other circuit is several kOhms and so it doesn't matter "much".

The AC circuit completes itself through the bottom of the power supply one way or another else you get no gain - straight to ground, though a resistor to ground, or through the power supply to ground.  A capacitor bypassing the point you pick gives it an even easier path. 

The DC circuit doesn't complete itself else you get the output drifting toward (and eventually just sticking to) one of the power rails.  The cap prevents that no matter where it connects.

[Tall Steve]

That answers my question! Thank you guys so much!