Output buffers and volume control

[Lubdar]

Does any one have any good explanations or references to explanations about the placement of output buffers (or the lack thereof) and volume controls.
Some examples
1.) on the SLOW LORIS, the volume POT is after an output buffer
2.) on the  STANK, the volume pot is before the output buffer
3.) on the GREENBEAN, the volume pot is before the output buffer
4.) MUDBUNNY doesn't have an output buffer and there is only the volume control
5.) PEACEKEEPER doesn't have an output buffer and there is just the volume control


Any help would be greatly appreciated

[midwayfair]

(a) Does the designer care about the series impedance of the volume control and its interaction with the grid capacitance of the first tube?
(b) Is the pot value high enough to affect audio frequencies with that capacitance in the first place?
(c) Did the designer even take that into consideration at all?
(d) Did the designer feel it justified the cost and time of the extra parts to buffer the volume pedal?
(e) Is there something else in the design that requires the buffer?
(g) Was there space on the PCB for the buffer?

1. Placing the volume pot after the diodes may have altered the diode's behavior. I'm not totally 100% sure it would have been audible.
2. 500K volume pot is really high. The tone stack has a lot more resistance. Putting the buffer there prevents the tone stack and the volume pot when on a low setting from losing volume going into anything lower than a 1M of a guitar amp.
... I would have left off R4, though. The volume pot is already doing the same job and all that does is lower the volume pot's resistance to 450K; I'm not sure that's an audible change.
3. The output buffers were in the original Tube Screamer as part of the bypass. The pedal sounds radically different with the input buffer removed. It also sounds a little different with the output buffer removed ... but oddly enough it adds a bunch of impedance AFTER the buffer. I never completely understood the reasoning behind that except that they wanted a predictable series impedance specifically to cut some high treble going straight into a tube (due to the tube's capacitance forming a LPF with the 100K in the TS). That resistor is different in other versions.
4. The BMP probably doesn't care about impedance differences; it's mucking up the signal as much as humanly possible already.
5. I'm guessing the difference in sound wasn't enough to warrant the extra parts and PCB space.

[Lubdar]

Thanks for the insight!!!
From my perspective, I hadn't considered factors A and C, I've only ever played on solid state amps so I have yet to learn anything about how tubes work. For my own personal interest, I don't mind spending the time, cost, or space to test out extra parts. I'm trying to increase my understanding of the use of buffers in order to hopefully figure out ways to improve upon a very questionable design of a series of cascading reverse beta boost stages....

[midwayfair]

Thanks for the insight!!!
From my perspective, I hadn't considered factors A and C, I've only ever played on solid state amps so I have yet to learn anything about how tubes work.

Transistors also have internal capacitance. It's higher in germanium transistors, for instance. It's usually single digits for a lot of silicon transistors, higher for some FETs, etc.

[Lubdar]

Ahh gotcha, much appreciated    I've got the circuit breadboarded and I think I'll go back and switch buffers in and out of the circuit to see if I can hear any difference when considering the the placement of the output buffer.  I missed the opportunity to hop on your RUST BUNNY(and I have yet to read any posts about it), but I noticed you had both an input and output buffer, with the volume pot after the output buffer.

[midwayfair]

Ahh gotcha, much appreciated    I've got the circuit breadboarded and I think I'll go back and switch buffers in and out of the circuit to see if I can hear any difference when considering the the placement of the output buffer.  I missed the opportunity to hop on your RUST BUNNY(and I have yet to read any posts about it), but I noticed you had both an input and output buffer, with the volume pot after the output buffer.

The output buffer in that was optional to allow for some very extreme tone control values and a higher volume pot value if the alternate tone controls were used. I still have some Rust Bunny boards. (The input buffer was to allow for a buffered fuzz face build and some other things.)

[Lubdar]

Ahh definitely interested in your circuit...

[George]

i found it helpfull as well thanks