TS-808 Pin 2 Connection Variation

[Phend]

Hello:
Building a TS-808 Tube Screamer I have noticed a different connection of Pin 2 on the IC.
One scheme goes thru the 0.047 cap and the 4K7 resistor to Vr. Other goes to ground.
Any comment on such a difference ? Thank you
See two references.

[anotherjim]

It makes no difference to this circuit operation. It matters if the cap is polarized (this won't be) then ground is the best connection for the -end of the cap. It can be that the PCB layout was easier for the RC to terminate to Vr or ground.
A small outside chance exists of the cap becoming leaky so it behaves like it has a resistor across its plates - then Vr connection will prevent a DC offset in the opamp output. If the cap returned to ground, the offset will push the output to the + supply.

[Phend]

Vr is of course +4.5v in lieu of 0.0v

[anotherjim]

Vr is of course +4.5v in lieu of 0.0v

And so? Either choice points that path to AC ground and a cap doesn't pass DC.

[antonis]

For AC purpose, just replace C7 with a short..

[Phend]

Thank you, I am learning and this is a good lesson !
AC DC in the same circuit at the same time is probably one of the most miss understood, hard to visualize (if you can visualize electricity) aspects of, in this case, effects.  I understand the capacitor basics, but didn't see that "short". 
Aron mentions in a GGG writeup, one modification to the 808, is to Jumper C5 which is a capacitor.  It would be a short to ground. Which means to me that that part of the circuit is shorted to ground. 

[antonis]

AC DC in the same circuit at the same time is probably one of the most miss understood, hard to visualize (if you can visualize electricity) aspects of, in this case, effects.

Not so hard if you understand some basic principles..
Search for : "Superposition theorem/principle"..

P.S.
You can see below a BJT CE amp broken down into its DC & AC constituent parts..

[anotherjim]

The reason there is a cap at all is for frequency dependant gain it's a 720Hz high pass. So the cap goes in whatever it "grounds" to.
It's probably counter-intuitive at first, but even the +9v rail could be used as AC ground provided it has a bypass cap to 0v. This bypass cap is often there also as a power supply filter. In practice, you want opamp Vref connections isolated with their own bypass cap to minimise power noise being reflected in the signal so Vref gets double filtered - first with the +supply cap and then at the voltage divider output cap. An opamp can self-cancel noise across its power pins very well, but any noise to the inputs will get operated on just as signal does.

[m4268588]

In case of a high-impedance signal source, Vref rather than GND can reduce noise from PSU. It's more noticeable if the C3 CAP is large.


This does not apply to TubeScreamer because emitter follower sig src.