Another Tube Screamer question

[kapsel]

Hi all,

I searched but couldn't find something relevant and also tried "The Technology of the Tube Screamer" from geofex, so apologies if I haven't noticed it.

I want to build a Tube Screamer (TS808 or TS9) and I noticed that in many schematics I find online, at the clipping stage, the 47nF capacitor and 4.7K resistor from pin 2 of the op-amp don't go to Ground but to 4.5V (Vb, from the voltage divider).

I imagine this is not a mistake, so is it something that is different between the many TS versions?

Thanks in advance.

G.

[anotherjim]

I don't believe it makes any sonic difference, but..
If going to Vref, then Vref MUST have sufficient AC bypass capacitance to take any signal current to supply 0V without that current affecting the Vref voltage.
If the pin2 capacitor if so large it is a polarized type, then it MUST have low leakage and return to supply 0V so the capacitor has DC polarizing voltage across it (Pin2 will have Vref via the feedback resistor).

[GibsonGM]

Hi Greg,

What Jim said, but also -

Do you know that 4.5V in this case is "virtual ground" to AC?  And that AC doesn't care which side of the power supply it returns on?    Just seeing if that's still a mystery to you.   AC just don't care...power supply caps are a virtual short to it. 

Because we elevate the wave with a bias voltage, it is 'riding around' 4.5V (not always the case, but often, that it will be 1/2 power supply)...it can rise above and below this level, which 'looks like' ground to it.

Same rules apply with using power supply ground, as long as we're consistent.   Like Jim pointed out, Vref is still going to be required on the input.