Question on RG's Tubescreamer 808 schematics

[Pyr0]

Looking at RG Keens "technology of the Tube Screamer" in the schematic of the clipping section, it shows the 47nF cap and 4k7 res going to ground, but in nearly all other circuits I search for they have it going to Vr (+4.5v)



further down the article, he shows Jack Orman's Son of Screamer, this one has it going to Vr



Which one is correct, or does it matter ?

-Alan

[R.G.]

It does not matter to the sound, given that the reference voltage is well designed. I guess, put another way, it could be considered that if it matters, the reference voltage isn't well designed.

The reference voltage is intended to be and acts as an "AC-only" ground with a DC offset. It's intended to be a source of DC with all AC on it "shorted" to ground.

[Pyr0]

Thanks RG, I thought it was something like that.

[karis12]

Thread bump for clarification's sake, hopefully this is fine.

I have just finished building a TS pedal using the schematic in ElectroSmash.com's article.

Just like RG's schem, the 47n + 4.7k goes to ground instead of Vr. Not only that, but the 100K level potentiometer's 3rd lug goes to ground as well, instead of Vr.

So far, the sound seems to be "correct" - the clipping doesn't completely go away with the gain pot at minimum, and the volume boost seems to be just about right for the average TS pedal (I own one, and have demo'ed a couple).

Are there any potential issues that might arise from this? Thanks in advance.

[slacker]

If you're using the electrosmash true bypass schematic it doesn't make any difference if the volume pot goes to ground or Vr, like R.G said above to the audio signal they're the same. In the original it goes to Vr because the FET switching needs to be referenced to that voltage.

http://www.electrosmash.com/tube-screamer-analysis

[nonoxxx]

For what I know in the high pass filter of the opamp loop , the cap block the dc , so it don't matter if you put it on ground, 4.5 V...
On a protoboard , I made the mistake to put it on the 9V rail , and it worked also

[antonis]

For what I know in the high pass filter of the opamp loop , the cap block the dc , so it don't matter if you put it on ground, 4.5 V...

To be more specific, that cap "rolls-off" DC gain to unity.. 
(no path for DC through gain resistor..)

On a protoboard , I made the mistake to put it on the 9V rail , and it worked also

Any DC source may be considered as GND to any AC signal..
(signal "sees" ground throught source's negligible internal resistance - in case of a series DC blocking cap, signal doesn't carry any DC offset voltage..)

More practically, GND is "seen" either via Vref regulating cap (in case of resistive voltage divider) or via output transistor (in case of Op-Amp stabilized Vref).