Master volume vs Atenuator

[Chorch Cluni]

Hi all.
I guess I know how an atenuator works, so my question is how in real life does the master volume compare?.

I think a master volume pot works like, say, an EQ through the FX loop, so it can lower the level of a maxed out preamp into the power section, right?.

[J M Fahey]

The attenuator is the real thing, it is flat, sends to speaker an attenuated but accurate version of amp output but most important, it preserves power tube distortion which is warm and full.

Master volume does not, power amp works clean contributing little, all distortion you have is preamp distortion, thin and buzzy.

Besides, attenuator keeps power pentode distortion, MV keeps Triode distortion, very different.

[joecool85]

The attenuator is the real thing, it is flat, sends to speaker an attenuated but accurate version of amp output but most important, it preserves power tube distortion which is warm and full.

Master volume does not, power amp works clean contributing little, all distortion you have is preamp distortion, thin and buzzy.

Besides, attenuator keeps power pentode distortion, MV keeps Triode distortion, very different.

I would argue that on most solid state amps, the end result is very similar when comparing master volume to attenuator.  Like you said though, tubes are a different story where you can get significant (and pleasant) power amp distortion which you lose when you turn down a master volume.

The biggest difference conceptually is location in the circuit.  MV sits between preamp and power amp.  Attenuator sits between power amp and speaker.

[J M Fahey]

Yes, in SS aamps power  amp is just a BIG Op Amp, and clips as such, boring.

A few SS buikders such as Peavey Transtube, Pritchard, Quilter, Roland Blues Cube have power amps emulating tubes, but they are the exception.

[Kaz Kylheku]

The MosValve 500 has an interesting take on it. There is a voltage amplification stage which runs on significantly wider power rails than the output stage. Negative feedback is taken from the voltage amplification stage, not the speaker output.

That should have two effects: one is that the impedance of the MOSFET output devices is not hidden from the speakers; i.e. it raises the output impedance a bit ("tube like").

Secondly, the MOSFETS will clip long before the VAS does and their clipping is outside of the NFB loop. Clipping being out of the NFB loop means that we don't get the hard clipping where the output is forced to stay linear until the very limit; the output stage is "allowed" to produce nonlinearities before clipping. This is again like a tube output stage without NFB.