Combining preamps via a transformer

[Xander8280]

To isolate preamps is a buffer enough? I've been told to combine two preamps that have tonestacks I would need a couple phase inverters and they would meets at a shared transformer.

The problem was that the tone controls would effect one another, but if there is a buffered mixer stage would that isolate them?

If you need a schematic I could draw something up, it's basically twin preamps mirrored except that one side is cathode bypassed for a lot more gain.

Thanks, it's a general inquiry. An idea I've been throwing around about having two preamps for one power amp, except the preamps can run together with seperate tone controls. If you wanted to have that biamp sound mono, or attenuate mids on your distorted preamp, or get some more sustain, etc etc.

[Leo_Gnardo]

A post-EQ buffer for each channel and a pair of mixing resistors ought to be enough. Mixing via transformers went out with poodle skirts, somewhere around 1959, and I've only seen it in very old studio gear, never a guitar amp. Well maybe just once in a fancy Magnatone and that was sort of a half ass design. Doesn't mean you couldn't do it but what transformer would you use?

[Phil_S]

+1 on what Leo says.

What exactly do you have in mind here? Are you intending to feed guitar signal to both preamps at the same time and then mix them for the power stage?

If you are simply building a two (or more) channel preamp, look at any of a number of production amps that use a pair of preamp circuits (each with its own input jack) and mix them before the PI. You'll find this in a number of Fender and Marshall amps. If you want to feed both preamps with one signal, you can install a switch that jumpers the inputs internally.

Maybe I missed the point. Sorry if I did.

[Xander8280]

+1 on what Leo says.

What exactly do you have in mind here? Are you intending to feed guitar signal to both preamps at the same time and then mix them for the power stage?

If you are simply building a two (or more) channel preamp, look at any of a number of production amps that use a pair of preamp circuits (each with its own input jack) and mix them before the PI. You'll find this in a number of Fender and Marshall amps. If you want to feed both preamps with one signal, you can install a switch that jumpers the inputs internally.

Maybe I missed the point. Sorry if I did.

I was just steered wrong by a local guy. Neglected the idea of buffered mixing. Or just gain stages after tone stacks.