Master volume topology

[catalin gramada]

Happy Hollydays,
I want to discuss here what is the best option for MV pot placement in the attached schematic and why, please. Can You help me? Thanks

[J M Fahey]

A is floating many Volts DC above ground, that alone is a problem.

B is more conventional and shows no problems.

[Chuck H]

Agree, and add that the term "master volume" as it is used for guitar amplifiers implies that it is as late in the circuit as practical. So B would allow clipping of the upstream stage shown in the schem where A would not. But if there were a more complete schematic and a description of the goals of the master volume (Is it just a master volume for a clean amp or is it there to allow preamp clipping at lower volumes?) maybe there is an even better option than what you have considered.

[catalin gramada]

Thank you, agree with both opinion and have to add, think...if the signal in front is strong enough it modulate the stream in CF cathode resistor even with potentiometer turned down...I suppose so it sucks So volume will never get close for certain signal amount in front in version A.

[J M Fahey]

You have not stated your goals yet

Master Volume is normally used in the Guitar World, to *allow* a certain stage to clip and overdrive and *after* that control output volume, even down to bedroom levels, even if MV is set to 0.

So in that context it does not "suck" , quite the contrary.

In any case, both versions will have basically same output signal level when set to 10, since the stage involved is unity gain.

In the conventional Tweed/Marshall cathode follower-driving-tonestack topology, CF contributes to tone signature, since when driving a complex impedance tone stack it adds unsymmetrical distortion: negative going signal is driven by (typically) 56k or 100 k resistor which limits output current big time, to, say, 1mA or less, while positive signal comes straight from cathode which can supply "anything you want" ... typically 4 or 5 mA.

But driving a 100k potentiometer is *easy* so even if heavily overdriven, peaks will be more or less symmetrical.

Then as you showed it, both versions will clip and sound about the same, but B is more practical to build.