Its an active (buffered) control. Shouldn't this thread have a different title?
"What could be better? Hardly any noise or distortion added by these
simple passive parts. No feedback, no worrying about what type of
capacitors – just musical perfection.
And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”,
is a commonly heard refrain (really - I’m being serious here!). Maybe
they are reacting psychologically to the need to turn the volume control
up compared to an active preamp."
George always leaves out the rest of Nelson's words, as if they don't exist. Here they are, as you can see Nelson does not think passives are the last word by any means:
I suppose if I had to floor the accelerator to drive 55 mph, maybe I’d
think the life was being sucked out of my driving. Then again, maybe I
like 55. Nice and safe, good gas mileage…
Is impedance matching an issue? Passive volume controls do have to make
a trade-off between input impedance and output impedance. If the input
impedance is high, making the input to the volume control easy for the
source to drive, then the output impedance is also high, possibly
creating difficulty with the input impedance of the power amplifier. And
vice versa: If your amplifier prefers low source impedance, then your
signal source might have to look at low impedance in the volume control.
This suggests the possibility of using a high quality buffer in
conjunction with a volume control. A buffer is still an active circuit
using tubes or transistors, but it has no voltage gain – it only interposes itself to
make a low impedance into a high impedance, or vice versa.
If you put a buffer in front of a volume control, the control’s low
impedance looks like high impedance. If you put a buffer after a volume
control, it makes the output impedance much lower. You can put buffers
before and after a volume control if you want.
The thing here is to try to make a buffer that is very neutral. Given
the simple task, it’s pretty easy to construct simple buffers with very
low distortion and noise and very wide bandwidth, all without negative
feedback.
There are lots of different possibilities for buffers, but we are going to pick my favorite: