Building an AMP Switch: Relay/Switch Requirements

[Riki]

I want to share a single set of speakers between my Yamaha CR-1000 and a Pioneer VSX-32. I want to build a switch box for them. I've noticed while shopping for switches and relays (I haven't quite decided which way I want to go) that the prices increase exponentially with the current rating. The Yamaha is rated at 70Watts RMS per channel (into 8ohms) and the Pioneer 110 watts per channel (their spec sheet does not specify, so I assume its peak into 8ohms). How would you spec a switch/relay that would handle the power of these amps? I've seen similar discussion in the forums, but recommendations were generally provided without the formulas/methodology behind them. If anyone would like to teach me to fish here, I'd really appreciate it.

 

[fernarias]

You have watts, you have resistance (8 ohm) and you can use this formula to calculate voltage: V=sqr(PR), so for the yamaha the voltage should be sqr(70 x 8) = 23.7 V and the pioneer sqr (110 x 8) = 29.7 V (sqr is square root). Get a switch that can handle higher voltage. Amperes for yamaha are V/I, 23.7/8 = 3A and for the pioneer it's 29.7/8 = 3.71A. Just get a switch rated at higher than 50 Volts and higher than 5 amperes.
Get a 4pdt knife switch.
Note: I would never connect two amps to one speaker.

 

[avionic]

Note: I would never connect two amps to one speaker.

Accident waiting to happen.

 

[Riki]

Fernarias, thanks for the formula! I appreciate the caution on sharing the same speakers between two amps. I don't intend on ever having both of them on at the same time. The pioneer doesn't have a phono section and the Yamaha doesn't do a/v. My current practice of re-cabling the speakers is a hassle. Increasing risk to reduce effort, what could possible go wrong

 

[sol7]

Make sure the switches are "break before make" so the amps stay isolated from each other.

 

[fernarias]

Fernarias, thanks for the formula! I appreciate the caution on sharing the same speakers between two amps. I don't intend on ever having both of them on at the same time. The pioneer doesn't have a phono section and the Yamaha doesn't do a/v. My current practice of re-cabling the speakers is a hassle. Increasing risk to reduce effort, what could possible go wrong

Run the preout of the front channels to the yamaha (has a much better amp) to aux (you can also use tape in) and connect the yamaha to the main front speakers. When listening to the turntable, only the yamaha would be on and when using surround, the yamaha would provide true 70 watts and it would also give your pioneer a rest (by powering less speakers).

 

[z-adamson]

Switches rated for 15amps at 120volts are readily available and inexpensive.

And I agree, not the greatest idea. The only time "switching" makes sense is when you are making a kill switch to turn speakers off completely or you are switching between multiple pairs of speakers with a single amp. Multiple amps to a single speaker pair, not wise.

I would move speaker wires around or get a second pair of speakers.

 

[z-adamson]

Run the preout of the front channels to the yamaha (has a much better amp) to aux (you can also use tape in) and connect the yamaha to the main front speakers. When listening to the turntable, only the yamaha would be on and when using surround, the yamaha would provide true 70 watts and it would also give your pioneer a rest (by powering less speakers).

As long as you have pre out and main in on the pioneer, this could work.

 

[drabina]

Few years ago, I built a DIY switchbox that I have use for couple of years without any issues. It would connect speaker wires to the box and one set of wires would go out to my one set of speakers. The switch was break before make as I was using solid state amps. Don't remember the ratings but it was pretty beefy. The solder connections would also accept spade connectors.

 

[avionic]

http://sound.whsites.net/absw.htm

 

[damacman]

Use one SPDT relay for each of the positive and negative of each channel - that way, you can connect any amps to it you may like, including those with speaker outputs which are not common to ground.

I built one years ago - four 30A Bosch style automotive relays. After humbling myself with it on multiple occasion, I put it in a desk drawer - where it still resides to this day.

 

[Bill Ferris]

Interesting-
Around 19 years ago, I purchased a Mac C22 preamp, and thought how nice it would be to be able to turn it on and by doing so it would energize 4(1 for each +/- speaker wire)30 amp 120 volt relays using one of the C22`s 120 volt switched outlets.
I desired to do this so to be able to switch out the normally connected to the sunrooms DQ-10`s Mac C27/MC 2205/2 Velodyne ULD 15 powered subwoofers setup, to switch over to what ever tube amp/s I might be rotating that would be driven by the C22 for a total tube stereo listening experience, when the mood struck me..

I used the octal type relays, and barrier strip/w screws style sockets for any future serviceability and the reason of the seemingly over kill of the relay`s contact current rating, was 2 fold: #1 for extreme reliability, and #2, was when a very high wattage SS amp would be hooked up to that system, that there would be little chance of stressing the contacts when cranked up(also the same consideration for the all day normally in use MC 2205, and when switched over to my tube amps.

Has worked flawlessly for these nearly 20 years, with no connected tubes ever energized without a speaker load, as the volume of the C22 is turned all the way down before switching in or out.

And of course, in this setup, I had to build a separate line level high isolation 12v. multi coaxial reed switches in it`s own metal box system to deal with the L/R signal`s in`s/out`s of the Velodyne`s power amps crossover to facilitate the switch between the two systems using a 12v non switching wall wart also plugged in to another of the C22`s back panel switched power outlets.

I just turn on the C22, and all switch over connections are automatically made, and after 20 seconds, or so I advance the preamp`s volume control to whatever level I wish.

So, IMO/E it can be done reliably, and safely unless the industrial grade speaker connection relay`s 120v coils fail,..
As a connected tube amp`s output transformer safety check, I always first mildly turn up the C22, and if I didn`t hear any sound out of the DQ 10`s, I would shut off the C22, and investigate the reason, though no problem/s/failures so far.

Just a thought folks.