Tube Screamer Schematic Question

[falco_femoralis]

Hi all, I'm tracing a few schematics and noticed something about the TS schem that I can't figure out. In the schematics of the TS online, the RC filter (4k7 res & 47nf cap) in the first op amp is connected to ground (R4 and C3 here)

But on the PPCB schem, they are connected to VREF. Same with the vol pot.



Other TS derived circuits, like the Moonshine, Il Diavolo, and Eternity are like this as well. Is there an advantage to referencing these to VREF as opposed to ground?

 

[psb962]

As no-one else has leapt in, all I can suggest is that it makes no difference as the capacitor C3 blocks DC, so it really doesn't matter what voltage level the 4k7 resistor is connected to - all that flows through it is AC (filtered off from the feedback loop).

 

[YourGuitarist]

There is no difference in circuit performance however from a layout perspective, ground will be tapped via a plane whereas vref would need a dedicated trace. Sometimes on 4 layer, one lay will be tied to vref but as far as I know, not common.

 

[Aentons]

I'm not sure but I think it depends on if the circuit is using a charge pump to achieve 18 volts. If so, there would be +9v and -9v and therefore 0v ground would be the middle of the range, like 4.5v vref in a regual 9v circuit.

 

[giovanni]

From the perspective of the signal (not DC), ground, Vref, Vcc and -Vcc are all equivalent because they are all ground for AC.

 

[YourGuitarist]

From the perspective of the signal (not DC), ground, Vref, Vcc and -Vcc are all equivalent because they are all ground for AC.

To add a caveat though, the size of the bypass caps dictates how "ideal" of an AC GND the rails and midpoints are. I've seen some filters be off spec because the midpoint was not low enough of an AC resistance to actual GND.