Switching scheme

[hylaphone]

Hi folks, I'm having trouble figuring out a signal switching scheme -- can anyone help?
Can draw a schematic if need be but the idea is that I have two outputs on a preamp and would like to switch a volume pot between the two.
When one output goes through the pot, the other needs to go directly out, and vice versa.

Can it be done?
Thank you --

[Pedro Vecino]

Directed towards the same output or towards two different ones?
In the first case, with both channels active always?

[hylaphone]

Two different outputs.

[eschertron]

Interesting problem. Thanks for bringing it up.

Is it important to have the preamp channels always assigned to a specific output, with the attenuator switching between channels? Or can the attenuated output always be assigned, and the preamp signals switched between outputs?

In addition, do you have a specific application in mind? What kind of impedance sources and loads can we expect? Will this be strictly made from passive components?

edit: I've had good luck with these alpha rotary switches www.mouser.com/ProductDetail/Alpha

a 4-pole 2 or 3 way switch would get you started. This way the pot could be handed 'back and forth' between the two channels. With a 3 way switch, the middle position could be straight through for both channels (no attenuator) and you could use a make-before-break switch (shorting type).

[hylaphone]

The preamp channels cannot be switched.
Here is the topology if it helps:

A mosfet source follower with say 250r output impedance splits to a 1/4" output, and to the primary of a 4:1 DI transformer. I would like to use one 10K pot so that either the 1/4" out or the DI out can be attenuated.

[eschertron]

with the 3 way switch above, I'd do this:
each channel uses 2 of the 4 poles on the switch, one common to the preamp output, one common to the interface (jack or DI). attached to each of these on/on/on switches are 1) a straight wire, 2) a straight wire, and 3) the attenuator. For the second channel, the attenuator is on the contact pair 1).

This allows shorting contacts as in position 2) the attenuator is out of circuit and cannot short the two channels together. Although if the two channels are actually one source, this is immaterial. So for the switch, position 1) Channel 1 'hot', channel 2 attenuated. 2) both channels 'hot' 3) channel 1 attenuated, channel 2 'hot'.

I'm not sure what the 10k pot will do to load down your signal. Have you breadboarded this circuit?

[Pedro Vecino]

This could work. A switch with two positions and four circuits. Or two relays doing the same.

[hylaphone]

Thanks guys! I'll try both of those ideas --

[eschertron]

A mosfet source follower with say 250r output impedance splits...

If both the inputs are actually the same source, then Pedro's drawing is right on. I'd still recommend a make-before-break switch to minimize popping noises when switching.

edit: just to play el diablo's advocate, why not simply have a 10k (or 25k) pot in each split? It would probably end up cheaper than the switched version.

[hylaphone]

I'm not sure what the 10k pot will do to load down your signal. Have you breadboarded this circuit?

I had a think about this and took some measurements.
The source follower actually has an output Z of 100 ohms.
Therefore I should be able to use a 1K pot, and retain low enough Z even at lowest settings to drive 10K loads - be it the 10K:600 DI tranny, or a typical outboard power amp.

Using two pots, perhaps -- but it makes 1:10 bridging a little tricky

I'm open to a sanity check if anyone is so inclined