Opto Channel Switching Question

[imaradiostar]

Some of the above schemes don't really address the combined noise of two channels feeding one preamp. Is there a compelling reason why you can't use a pair of relays?\

For the record, I like the OP's preamp design and I've long thought that such a design would be very musically useful.

jamie

[cbarrow7625]

With the scheme I posted I also always ground the last grid in the "off" channel(s) as well to avoid the noise of the unused preamp channel(s) all feeding the same common circuit downstream. I didn't explicitly add it because it just wasn't part of the discussion string at the time. Good point though.

[imaradiostar]

I meant to say two channels feeding one power amp but I guess that was evident.

jamie

[cbarrow7625]

That's what I thought you meant. By the way, what is "the OP's preamp design"?

[imaradiostar]

OP = original poster, in this case JoeK.

This is probably where I'll land with an upcoming build:



I'd start with dirty channel values from a Bogner Shiva or Top Hat Emplexador (both available online with a little searching) and go from there. I'd also try to use a clean channel EQ that allowed for a variable mid scoop and a post PI master brightness "cut" for the whole amp, possibly even switchable so it'd only work on the pentode channel.

Of course this is all bench racing- I haven't built such a thing yet but I'm planning to!

Jamie

[JoeK]

Just a quick update to say thanks for the help. I finally wired up the opto circuits in my test amp and got everything working after learning a bit more about leds working in parallel. I used the idea from cbarrow and have one feed from the input jack to two optos with 1meg across each. They feed the input grids which still have 33K resistors in series. The grids have a feed back to two more optos to ground either side and it all gets switched via a stereo footswitch jack or front panel DPST. The Marshall footswitch got rewired for the led in series on one side and even that comes on. Power is 6VAC rectified to 3.8VDC. I had to have individual dropping resistors on each led as I was mixing different types. I made my own optos. It seems to work very well but the ON is instant where the OFF takes a bit longer than I want. I need to experiment a bit more as I used just one sheaf of glued black heatshrink on the optos, ON is 100 ohms OFF is 20meg'ish. I'd post a schematic but my cameras flat so I'll post it later.
As far as the amp goes I've got another thread about EF86 drive and I'm still not a fan yet. The EF86 delivers 50 plus volts AC off the plate but it's still fairly clean and as it's close to the power amp it's bleeding into the signal via proximity (amps a rat nest) I was after some power tube distortion from the EF86 (I think) The other channel I am in love with! It's a AC30 top boost - Ken Fischer Rocket thing and it has some very nice tones to my ears at least. Cheers Joe.

[JoeK]

Finally got around to getting an image of my schematic/layout for the switching as per cbarrow's advise. Hope it makes sense for the people that are looking for this type of setup.