Channel switching: what is the best way?

[vanslycken]

So I'm building a 5 channel tube amp. Each preamp section is completely independent. My first thought would be to connect all the preamp channel's inputs to the input signal, and then select which output is fed to the PI, with relays. I'm just afraid that this might cause popping sounds - haven't tried it yet.

Any thoughts on the best way to switch channels?

[guitician]

If you use mosfet's as switches, you can turn each one on/off without pop's. Or you can use Optoisolators.

[cbarrow7625]

I have a 5-channel amp with totally independent preamps that I built; sounds a lot like yours. I can tell you that you definitely do not want to connect the input to all 5 inputs at the same time. I thought this would be the best way to do it when I started as well. It turns out that running into all 5 preamps at the same time really muddied up the tone in a noticable way that I just couldn't live with. I don't know if it was just miller capacitance or leakage effects coming back through the power supply from all of the stages running at once or what but when I added input relays it came back to life in a dramatic way (like, it sounded right).

For the input relays I put a 5 Meg resistor in series with each grid (follow me here, I know that sounds like a bad thing to do). To switch between inputs I used a DPDT relay on each input tube (yes, Opto's would be better). On the 4 channels that I am not using I just grounded the grid wiht the relay. On the channel I am using I just shunt around the 5 Meg resistor. The 4 x 5 M resistors shunted to ground at the tube grids parallel up to give me a grid shunt resistor (for the working channel) of approximately 1.2Meg (right on target for a traditional guitar amp). Even with relays this method is not terribly pop prone but I'm sure it could be improved with Opto's.

My design has 3 high gain channels. I found that I also needed to shut off the high gain channels somewhere in the middle of the chain to reduce excess noise from bleeding into the rest of the circuit. I shunt all of those to ground right after the gain control (for 3 & 4 stage stage designs, you might have to do it in two places if you use more stages than that or for a noisy 4 stage design).

Right before the PI I have a 5 to 1 relay multiplexer to choose which channel gets fed to the PI (as you were thinking of doing). It works great, optos would make it better. I put a single coupling cap on the PI side of the relays so that I don't have to deal with 5 different caps charging up & popping as I selected them. For a Fender-type stage you will have to block the DC and use a bleed resistor to keep the popping at bay (I have one of those built in there too).

Good luck.