[BradJudy]

oh one thing i wanted to ask you. the big problem with finding a switch was they were all rated for only about 100w per channel and my SS amps put out 300w. this switch can handle up to 350w per channel. any idea what yours is rated for or is it only for your SET low powered amps?

I'm using power relays rated for 10A at 120VAC.  It's pretty easy to get ones rated up to 30A, but that seemed like overkill.  IIRC my circuits properly, a 10A switch will allow me 400W into 4 ohms or 800W into 8 ohms (P=I^2*R).  Given that the dummy load can only dissipate 100W average, the switch should be good for less than 100W averages with higher peaks. 

[BradJudy]

Brad your project is timely for me. I was just talking with some friends over the weekend about building such a beast. We were comparing a 30W ss integrated with 100W tube monoblocks. In the time it took to shut down, change over all the wires and repower, the mental reference evaporates. Not to mention cooling down the tubes, and adding various amounts of emotional distraction during the swap. You get out of the mood when the music stops, and sometimes differences are so subtle that it's the "being in the mood" part that you are evaluating...

Yes, it makes a big difference in comparing amps - none of that "buzz kill" from getting up and swapping things around.  Not to mention you reduce the risk of making a dumb mistake that sometimes comes with trying to swap equipment quickly like reversing +/- or accidentally shorting something. 

I was gonna build Rod Elliot's version, it is versimilar to yours. But you have some great ideas here and thanks for the part list too. Once the basic switcher is made, lots of options can be added, like processor to control relays for blind testing, etc, input opamps, switchable inputs, ad nauseum... like Rod Elliot's crazy fancy version.

I read both of these while making mine.  As you mentioned, the first article is very similar to what I built, but I added the switching of the negative lead (the first thing I was testing was T-amps, which can't have their negative leads tied together) and the dummy load.  I don't really need the dummy load for my immediate use, but I wanted something that was flexible and safe.

I guess it could be used to switch low level signals between sources/preamps too, right?

It could be used to switch a lot of things, assuming the non-active item didn't mind being attached to an 8ohm load.  Since pre-amps generally have a much higher input impedance (20,000+ ohms), I don't know if that would damage the source.  Perhaps someone else here can answer that.  Even headphone amps (which are much like pre-amps and are designed to handle loads down to ~30 ohms) can get unstable with this low of a load. 

One thing it can definitely be used for is switching between two pairs of speakers on one amp.  The non-active speaker certainly won't care if it's hooked to a dummy load. 

[BradJudy]

Pretty much any electronics supplier. Jameco has 12 position rotary switches for $2.89 each. I have an article in audioxpress on a nifty cascaded attenuator so you don't need 24 position switches. I'm sure DigiKey or Mouser would have even more options for pretty cheap.

I found the Jameco 12 positions for $2.89 - it looks like they are single pole, single deck.  That would be a lot of knobs for a four-channel, two tiered device, but an option.  I did some poking around Digikey/Mouser/Allied and the best I could find for 12 position 2 deck is $15 for Electroswitch C4 series at Allied. 

A 12 position would allow for a full on, full off and 10 steps.  That could be configured for two stages with 1db and 0.1db steps. 

That AudioXpress article would be cool if you could send me a scan, or at least the issue number.