Transcendence Boost input section

[lowpitch]

What is the purpose of R2, R3 and C2 in the input section?

 

[JTEX]

An attempt at an RF filter. You can probably omit all 3 (jumper the resistors) and nobody will know. C2 value is too low to have much of an effect anyway.

 

[JTEX]

Now that I took a better look, I wonder what C4 is doing there. A 100nF cap right across the output? That's odd. Can the op-amp drive that without oscillating? (Not that I haven't done much worse myself )

 

[lowpitch]

Now that I took a better look, I wonder what C4 is doing there. A 100nF cap right across the output? That's odd. Can the op-amp drive that without oscillating? (Not that I haven't done much worse myself )

I wouldn't know, you're the pro People keep bringing this one up as a great transparent volume boost so I guess it works. I do have the board and parts here, just haven't gotten around to building it.
There's more stuff I don't really understand the purpose of. The second op amp seems pointless. I also don't understand R6 and R7. Are they for stability? Couldn't they have just designed it around a single op amp, deleted IC1.2 and R7 and be done with it?

 

[JTEX]

Valid questions. R7 is often used after an op amp to insulate its output from the effect of a capacitive load. Otherwise it might oscillate. R6 is just weird, just another path around the op amp. Second op amp not really needed, but sometimes they use a dual part when a single is enough just because of better volume pricing when stocking just one part instead of two separate ones. Who knows?

 

[Alan W]

I wouldn't know, you're the pro People keep bringing this one up as a great transparent volume boost so I guess it works. I do have the board and parts here, just haven't gotten around to building it.
There's more stuff I don't really understand the purpose of. The second op amp seems pointless. I also don't understand R6 and R7. Are they for stability? Couldn't they have just designed it around a single op amp, deleted IC1.2 and R7 and be done with it?

The second "op amp" is part of the power supply, not in the audio stream. It's a "voltage doubler"—I think in this case it's used so that there is a 27V differential between rails—far more swing than you'd get with 9V. And the op amp that is in the circuit is one that really appreciates more voltage. Besides having built this, I have the Holy Fire boost that this is an offshoot of—it originally came with a 48V wall wart to power it.

 

[lowpitch]

The second "op amp" is part of the power supply, not in the audio stream. It's a "voltage doubler"—I think in this case it's used so that there is a 27V differential between rails—far more swing than you'd get with 9V. And the op amp that is in the circuit is one that really appreciates more voltage. Besides having built this, I have the Holy Fire boost that this is an offshoot of—it originally came with a 48V wall wart to power it.

The power supply I'm pretty clear on. It looks to be taken pretty much straight from the datasheet of the TI LT1054 (Figure 21. 100-mA Regulating Negative Doubler) with different diodes/capacitor values. It puts out +/- 18V minus some diode drops as far as I know. Pretty damn good and very useful for a clean boost!
Anyway, by second op amp I meant the second half of the OPA2134. You got IC1.1 – a non-inverting op amp with variable gain –, the output of which then goes to IC1.2 – a unity gain non-inverting op amp buffer. The output of both op amp circuits also weirdly gets mixed together via R6 and R7 which I just don't really understand why. But I guess it is what it is and if it sounds good, that's all that matters.

 

[mckillacuddy]

The second "op amp" is part of the power supply, not in the audio stream. It's a "voltage doubler"—I think in this case it's used so that there is a 27V differential between rails—far more swing than you'd get with 9V. And the op amp that is in the circuit is one that really appreciates more voltage. Besides having built this, I have the Holy Fire boost that this is an offshoot of—it originally came with a 48V wall wart to power it.

That’s the one I want to see. Know if there’s a schematic out there?

 

[JTEX]

R6 is the "clean blend" for more mojo

 

[lowpitch]

R6 is the "clean blend" for more mojo

Hah. I get that reference