Help with theory

johnheath · Dec 1, 2015

Hi all...

I would like some expert opinions on the use of a potentiometer as a level control right before output.

From my drawings you can see that A is a common way to use a pot (R1) but I have also seen it used as in B (R6).

As used in B there is a possibility to use R1 as well (or as a atepped attenuator) together with R6.

I have seen a R6 in different schematics but I cannot say thet I find it common.

So... thoughts or opinions?

Best regards

/John

gyraf · Dec 1, 2015

..you'll want to attenuate at the input to the amplifier in order to maintain a steady (and low) output impedance...

Jakob E.

ruffrecords · Dec 1, 2015

The advantage of the control at the input is that you can always ensure the output stage is not overloaded and, as Jakob says, the (low) output impedance is maintained. The disadvantage is that the , even with the control off, you have the noise of the output stage at the output. The advantage of the control at the output is that when it is fully off the noise is minimised. The disadvantage is that you get a higher and variable output impedance and there is a possibility of overlaoding the output stage.

Cheers

Ian

johnheath · Dec 1, 2015

Thanks both of you.

But… you both say that a pot before the output will result in a variable (and) higher impedance. So I guess that it also depends on the value resistance of the pot? Which gives me into the question of the recommended value of the load resistor like in A, R6? Not the variation that is

Regards

/John

PRR · Dec 2, 2015

Put the pot where the post-pot nominal signal level is 50mV-100mV.

Middle of the audio chain. Not at the mike, not at the speaker.

*IF* you pot at an output (throwing-away headroom when you may need most), the pot ought to be 5X-10X the amplifier impedance, the load 2X-5X the pot impedance. While that tube-stack you show may-or-may-not have low output impedance for small signals, any small tube is really >5K for large signal. This gives 25K pot and 50K load. Yes, this level may be fine for short lines as in hi-fi, but is far from best practice in modern studio work. If we assume the tube-stack is 1K impedance, a 25K pot will show from 1K to 6K to zero impedance at the wiper (why we pick 50K minimum load, 10:1 difference). 6K won't drive 30 feet of cable with good treble extension.