Transistors in opamp overdrives/distortions

[ItsGiusto]

Hi everyone. Just trying to understand how all these circuits function, and I have a theoretical question.

In just about every overdrive or distortion pedal schematic I see, there's a transistor at the front of the circuit before the main clipping stage, and another at the end shortly before the output. I was wondering what purpose these serve. They must do something useful if they're in every single one, but if opamps have such a ridiculous amount of gain on tap, are these really necessary? Or maybe these are doing something else that I don't understand, like acting as a buffer for the signal. I'm curious what would happen if you were to remove these from the circuit.

[bluebunny]

In just about every overdrive or distortion pedal schematic I see . . .

You mean, "just about every Tube Screamer variant"?      Yep, they're buffers.  You'll find many of these variants simply omit them, for example the "Boutique Tube Screamer" at Beavis (there are many others out there).

[ItsGiusto]

Oh ok, cool. What exactly are they buffering, or maybe what I mean to ask is why are they buffering? Is it for any reason in particular? The only context in which I'm familiar with buffers in an analog sense is that audio can sometimes lose fidelity especially in high frequencies when there are impedance mismatches or long runs of wire, and buffers can prevent this. Is that what they're attempting to do here?

[Johan]

They are there to change the impedance so that the signal doesn't degrade when hitting the rest of the circuitry or driving the added cable after your fx . You Can Think of them as current amplifiers. Adding strength without changing voltage amplitude. .. obviously not 100% true in every situation (someone will slap my fingers if i don't point that out) by close enough. .
J

[antonis]

Despite of the fact that all the above answers are true and correct..!!

there's a transistor at the front of the circuit before the main clipping stage, and another at the end shortly before the output.

It depends on the circuirtry of the individual transistor..

Without dealing with emitter followers and other "nasty" terms, you can discriminate them as amplifiers (if the signal comes out from the collector) or buffers (if the signal comes out from the emitter) - just a ROUGH optical indication on the schematic...

@Johan: You may slap my fingers too, if you like..

[GibsonGM]

Jack Orman did a TS with no buffers, called "Son of Screamer" (bottom of R.G.'s page).  Works great.   Sometimes you will find the buffers there almost like a "precaution" in case of long cable runs, etc.  Good practice, but not entirely necessary or "required" for good operation.

http://www.geofex.com/article_folders/tstech/tsxtech.htm

It's not always about gain (you're right, opamps can deliver TONS of gain)....often people are considering noise floor and signal degradation.  Buffers, as impedance matchers so to speak, also 'bootstrap' via feedback and <for lack of a better term>  "build up" your signal.  They're very useful, tho not always needed. 

[duck_arse]

if a pedal has soft bypass like boss, it will have fet switches, which like to be driven by low impedance buffers (and drive high impedance buffers, so my fingers don't get slapped). r.g.'s page ^ has it all there.