Understanding the Tube/UBE Screamers

[YouAre]

On the ROG UBE Screamer, there is a mixing stage to blend in clean signal at the end. I've never seen any mixing stages on the tubescreamer, but I have heard guys complaining that the tubescreamer sounds like it has "overdrive sitting on top of a clean signal" as if they can still hear the cleans.

This got me wondering if the feedback arrangement of the TS's gain stage is mixing clean and clipped signal. Is this the case? If so, does that mean that an inverter with feedback doesn't have this mixing characteristic?

Also, can anyone delve further into differences between the feedback of opamps and of inverters?

Thanks,
Murad

[Bill Mountain]

The opamp will pass all frequencies but only clip the ones that are boosted. The inverter will only pass the boosted frequencies.  This is a basic and almost wrong explanation but its the simplest one I could type since I'm using my phone.

[YouAre]

The opamp will pass all frequencies but only clip the ones that are boosted. The inverter will only pass the boosted frequencies.  This is a basic and almost wrong explanation but its the simplest one I could type since I'm using my phone.

Thank you! This makes sense to me, I'm just waiting for the punchline. What's the flaw in this logic?

[Bill Mountain]

I was referring to my over simplification.

[CynicalMan]

The inverters basically act like poor op amps here. Think of them as op amps with the non-inverting input connected to ground. The output is equal to the input multiplied by a fairly high gain and inverted, with clipping if it goes near the power rails.

The difference between the UBE Screamer and a Tube Screamer topology is that in a Tube Screamer, the clipping stage is non-inverting. In an inverting op amp stage or an inverter stage, the gain is -Rf/Ri (feedback resistance divided by input resistance). When the diodes conduct, they short out the feedback resistor, and the gain approaches zero. In an non-inverting op amp stage, the gain is instead 1+Rf/Ri. So if the diodes conduct and short out the feedback resistor, the gain is still at least 1. This stage cannot attenuate the signal, there will always be a clean signal with a gain of 1 unless the op amp itself clips. If the gain pot is maxed out, the op amp gain is at 100 and the clean signal is relatively low, but at lower gains you can definitely hear a clean signal under the overdrive. So I bet that's the effect that the UBE Screamer is trying to emulate with the mixer.

[ORK]

With the slight difference that in the traditional creamer the mentioned clean signal feeds through the same tonecontrol as does the signal when clipped.