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Old 30th March 2015, 09:37 PM   #1
peraho is offline peraho  United States
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Join Date: Jan 2013
Location: Brooklyn, NY
Default Amp Selector Switch - multiple amps, rotary switch

Hi everyone - longtime lurker, first time poster...

I have a couple questions regarding an amp selector switch.

I'd like to make a box that will allow me to switch between various amps, using one set of speakers (or maybe including an additional A/B speaker switch, as that'd be easy enough, and I can handle that...) - so that as I build new amps / bring home more vintage gear / want to show my friends what I'm talking about I can switch from A>B>C>D and see what effect, if any. (I know it'd be ideal to just switch the wires, but this is for convenience - for any critical listening I'd hook it all up directly.)

Basically, something like this - Niles DPS-1 - DPS-1 | | Niles Audio - but with at least amp 4 inputs, and thinking of putting it in a 1U rack mount box. Multiple inputs are key.

Here's the setup I have in mind:

I'm imagining a box with a pair of L/R RCA inputs coming in from the preamp, and a pair of L/R banana outputs going to the main speakers.

Then there are then 4 x L/R RCA outputs going to each amp, and 4 x L/R banana inputs coming from the speaker terminals of each amp - and each RCA input / speaker output pair would be selected according to switch position. As an amp is selected by the switch, the signal is routed to that amp, and then the signal from that amp's speaker outputs is routed to the main speakers.

My questions are as follows...

*Can I use a rotary switch? What type?

I think I can do it with an array of 4PDT switches (and have seen those diy plans discussed here) but then it gets complicated with too many switches to flip - ideally in my head I have a single elegant rotary switch, and I turn that to select the amp and rout that to the speakers. (I started researching rotary switches and must admit got a bit overwhelmed with the options / specs...)

*As I'd have L/R line inputs coming in, as well as L/R speaker outputs going out through the same switch - would that work? Could I use one rotary switch and have signals going "both ways"? Inputs coming "in" and the signal to the speakers going "out"?

*How can I / should I incorporate dummy loads if an amp is not selected so if I am using a tube amp, then switch to the solid state, the tube amp OTs don't fry (can't power the tube amp without a load, right? Would it need to be MBB?). It's not essential, but I'd like to make the selection on the fly.

*Could I incorporate some sort of clipping indicator / comparator / VU meter on the front of this box that would work for the selected amp? e.g. just noticed some clipping on the 8W tube amp, but then flip to the 100W solid state and no longer an issue, comparing the input to the output.

If too complicated, I guess a couple Niles boxes could do it, but I looking for some elegant way to create one of those with more inputs...

Many thanks for any help!

Pete
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Old 31st March 2015, 10:10 AM   #2
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Join Date: Jul 2003
Location: Dona paula, Goa
Using appropriate relays will make things tidier and with less pickup from long internal wirings. The selector switch for input signal and output of amps can be a rotary switch of the number of positions. Having changeover relay, you can wire the needed load resistance to the normally closed contact on the relay.

You can also decide to switch the grounds if needed with extra contacts on the relay(s) in both(preamp & power amp).

Gajanan Phadte
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Old 1st April 2015, 09:34 AM   #3
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Join Date: Jul 2003
Location: Dona paula, Goa
The clipping indicator...

Yes you can connect one at the speaker terminals, but you will have to use a switch to select the wattage of the amp. This can be done by changing an attenuator resistor. If you want to make a clip indicator, look for the datasheet of LM3915, VU meter IC.

Or you can use a comparator ic (LM393) to switch on an LED.


Gajanan Phadte
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Old 1st April 2015, 10:24 PM   #4
peraho is offline peraho  United States
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Join Date: Jan 2013
Location: Brooklyn, NY
Gajanan - thanks so much for the info - this is incredibly useful - I really appreciate it.

I realize I'm going need to read up on relays and get my ideas together, then post an update.

Thanks again-

Pete
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