On Tue, 10 Nov 2009 12:22:32 -0000, "David Looser"
wrote:

"Don Pearce" wrote in message
...
On Tue, 10 Nov 2009 12:04:43 -0000, "David Looser"
wrote:

"Dave Plowman (News)" wrote

Swap 2&3 and you will likely get strange frequency response errors plus
possibly distortion near zero.

Using pots like that: ie. with the signal applied to the slider and
extracted from the non-earthy end of the track, is not unknown. It was
faily
common in early "transistorised" radios and record players etc. And it
also
has advantages for simple audio mixers which don't use a virtual-earth
summing amp.

David.


The mixing solution I used before I discovered the virtual earth
amplifier was to connect the pots the usual way, but add a resistor in
series with the slider so it would not impose a short when turned all
the way down.

I've never seen a circuit (except inside some guitars for various odd
tonal reasons) that used a configuration that put the signal into the
slider.


Early semiconductor small-signal amplifier stages often used a single
germanium transistor in a grounded emitter configuration, with a low
resistance bias network connected to the base. Such a stage has a lower
input than output impedance. In those circumstances it makes sense to run
the volume control "backwards", as the control is turned down the pot loads
the relatively high output impedance of the preceding stage. And simple
mixers often work run that way, each input is applied, via a resistor, to
the slider of a pot. And the output is taken from the tracks of all the pots
wired in parallel. It has some advantages compared to the way you did it.

David.


Still a strange topology. What normally happens when you load down a
transistor stage with too low an impedance is massive distortion
because of the asymmetric sink/source current drive capacity.

d