No filtering in Quasar clipping?

[MullisMan]

Just wondering, why is the typical TS mid hump filter connected to Vr left out of the Xotic pedals clipping section?  Is it because of the active tone controls after the clipping section of the circuit? And if you were to put this back into the clipping section, would it make any difference?

[madbean]

The Quasar uses the inverted input of the op-amp rather than non-inverted like the TS. In this configuration, the clipping section does not utilize a cap to select the clipping frequency. If you want to emulate the mid range emphasis of the TS in the Quasar, make R8 1k and C9 220n.

[MullisMan]

Thanks for the reply Brian.  I guess my next question would be, why does using the inverting input make a difference?  I was under the impression that this just changed the phase of the signal? I just wondered if I could switch around the inputs, add the filter back in, and maybe make the cap switchable to change the cutoff frequency from the normal 720 hz to something that let a little more bass frequency pass though.


And one last question, what's with the weird biasing voltage used on the non inverting inputs of the op amp?  It's not the normal half of the supply voltage level i'm used to seeing.

[madbean]

There are a number of reasons to choose inverted vs. non-inverted inputs. In this case, most likely the inverted input was chosen because there is ample tone shaping available in the active Baxandall tone stack and setting a frequency range for clipping would be redundant. Additionally, the tone stack requires inverted input, so making the clipping stage inverted means the output from the op-amp is in phase with the input (phase shifting between stages don't matter).

There's a more subtle reason to choose one or the other, as well. Non-inverted inputs layer the clipped frequencies from the ensuing feedback loop over the input signal. This means there will usually be an element of clean, non-clipped signal in the overdrive. In the inverted set-up, this is not the case. The clipping in the feedback loop is applied to the entire (inverted) input signal.

[MullisMan]

Cool. Very good info.  Any idea why they don't use the standard voltage level for the Vbias on the non inverting inputs of the opamp?

[madbean]

Vb is actually pretty close to 4.5v if you think of R18 and R19 as a single resistor whose value is approximately 22k. Coupled with the preceding 22k resistor it creates a standard voltage divider.

The difference here is actually that they use a separate bias voltage to supply the base of Q1. The only reason I can think that this was done is to cut down on a potential noise factor. Since Q1 is a simple buffer, there is no special biasing requirement. You see something very similar in the "Darkside" which has more than one bias network to supply the different gain/buffer stages.