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 Post subject: Speaker switching relay specs?
PostPosted: Feb Tue 26, 2019 10:17 pm 
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I'm intending to use one set of speakers to be fed by two different amps (a Pilot 6V6 P/P tube amp, and a Sony solid state receiver). I was thinking of using a 4PDT relay with a 6V. coil connected in parallel with the Pilot amp heater chain, so it would see a load whenever it was turned on, and have "priority". (The Sony has a second pair of speakers, on separate terminals, so it always sees a load.)

The Pilot puts out somewhere around 12 watts or so, and the Sony is rated at 50/channel. I'm thinking 5 amp contacts should easily handle this load, but self doubt has once again reared its ugly head. Comments welcomed!

Dan


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 Post subject: Re: Speaker switching relay specs?
PostPosted: Feb Tue 26, 2019 10:42 pm 
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Location: Austin, Texas
50 watts into an 8 ohm speaker is 2.5A so 5A contacts would seem to be OK. I would look for a relay with silver or gold contacts.

Jay


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 Post subject: Re: Speaker switching relay specs?
PostPosted: Feb Tue 26, 2019 10:59 pm 
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Yea I would check/clean the relay contacts if open frame as I would assume you may not be using a brand new factory fresh relay, but probably used or NOS. I assume you can also wire contacts in Parallel to double the current capacity in the instance all you have is a smaller sealed type. And an encapsulated relay to keep dust out of the contacts seems like a good idea, as well as not switching with the volume cranked up on music.

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 Post subject: Re: Speaker switching relay specs?
PostPosted: Feb Tue 26, 2019 11:48 pm 
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The relay circuit should be designed so a failed coil can leave neither amplifier
inloaded.

A 68 ohm 2 watt resistor permanently each amplifier should do.

Also all relays can chatter.

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 Post subject: Re: Speaker switching relay specs?
PostPosted: Feb Wed 27, 2019 3:11 am 
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The solid state amp does not care much if you have a load probably. When you plug in headphones the speakers are muted in most cases and the headphones are not direct to the amp, which could burn up the phones or your ears. They have a high resistance in series or a divider that attenuates the signal and presents a pretty high resistance to the amp output. Thus also attenuating any hum and hiss you would hear with phones direct to the amp. The ideal is to get the headphone level down to about the same apparent loudness as the speakers. Just tube amps seem to be unhappy with no load. You could wire so the tube amp is connected to the speakers in the de- energized position if you were worried about relay control voltage failure.

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 Post subject: Re: Speaker switching relay specs?
PostPosted: Feb Wed 27, 2019 4:49 pm 
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Using a relay would seem to be a unnecessary complication- why not just use a manually-operated mode switch?


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 Post subject: Re: Speaker switching relay specs?
PostPosted: Feb Wed 27, 2019 7:45 pm 
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Location: Boston, MA USA
I do this to share speakers between a stereo amplifier and a movie projector. A manual switch works fine but a relay is much more convenient. I use a relay with 10A contacts (the stereo amplifier is powerful) and a 120VAC coil. I plug the relay coil into one of the amplifier's switched outlets -- works great.

-David


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 Post subject: Re: Speaker switching relay specs?
PostPosted: Mar Fri 01, 2019 5:11 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
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Location: Fayette County, Pa
I agree regarding not running any amp unloaded. While this is usually OK for SS amps, the reason is they have no output transformer. But there are a few that do! These can be damaged if run unloaded. So as a matter of safety I would make sure both have provisions to prevent them from running open. Better safe than sorry!


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 Post subject: Re: Speaker switching relay specs?
PostPosted: Mar Tue 05, 2019 1:22 pm 
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Location: Sunbury, Ohio 43074
Use a high quality relay. I did a lot of museum displays back in the day that had switching I built for the speakers, using relays. Occasionally we'd burn up an output transformer due to relays ... never did figure out exactly why, but eventually switched over to plug in P&B sealed relays and had no further problems. There were a variety of loading conditions from 8 overhead speakers, to multiple "HearPhone" museum type 'hold it to your ear' listening devices, to various others for some displays. So I was also switching in resistors for the changing loads.

Back then, output transformers were relatively cheap and easy to get.... today............. not so much.

find the best relay you can. Don't use anything that has an overly long contact spring. Short throw is best. Even if enclosed, if it's an old one, take it apart and clean the contacts before using. New ones should be OK.

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