Small valve mixing console

[C.B. - Boudio]

This looks like a very nice circuit.

Do you mean the EAB or my revision too?

That is a NFB tone control as used in many of the Mullard/ Philips in their project books. The low Z drive requirement does not apply.

Oh okay, that makes sense.

That is not feed forward, it is a totally standard NFB circuit.

But the signal at the anode of the second triode is in fase with the grid/cathode of the first triode, no?

You could do a lot worse than copy this schematic.

The reason I don't really want to copy it as-is, is because the output is so low, and if I'd use an SRPP or something like that as an output stage, the input won't be in fase with the output (well, I could wire the IPT accordingly, but I'll still run into the same problem for the instrument input), and I fear that even more gain stages will make it too noisy even for me. Otherwise, I would probably do it that way.

 

[Disco Volante]

The first cathode lacks a capacitor, which is effectively local feedback meaning lower gain. A pentode output stage might also help get you where you need to go?
I'm sure DaveP would know how to calculate all of that ;-)

 

[DaveP]

But the signal at the anode of the second triode is in fase with the grid/cathode of the first triode, no?

Yes, it is in phase and it makes the un by-passed cathode more positive, more positive than the grid which is the same as the grid becoming more negative, i.e. NFB.
best
DaveP

 

[ruffrecords]

View attachment 146104

Since you say a baxandall has to be driven from a low impedance source to a low impedance load, I'm a bit confused then by what they did here, with such high impedances.

It is all relative. The bass and treble pots are most likely 1Meg and the output impedance of the driving stage is about 50K which is low compared to 1Meg

Cheers

Ian

 

[DaveP]

Do you mean the EAB or my revision too?

I mean this VE 12 German amp. They are nearly always very well designed.
best
DaveP

 

[C.B. - Boudio]

Yes, it is in phase and it makes the un by-passed cathode more positive, more positive than the grid which is the same as the grid becoming more negative, i.e. NFB.
best
DaveP

Now I see! I thought NFB was always with one signal in the opposite phase.

It is all relative. The bass and treble pots are most likely 1Meg and the output impedance of the driving stage is about 50K which is low compared to 1Meg

Cheers

Ian

Ah, that makes sense. Bass and treble are actually 510 and 470k respectively (see the resistor ladders at the bottom).