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Question on Relays for Switching One Pair of speakers betwee

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Question on Relays for Switching One Pair of speakers betwee

Postby woodnut » Thu Sep 28, 2017 7:42 pm

Hello folks,

I'm looking at building a switch box to switch two amps (a valve amp and a class D) into one pair of speakers. I was going to use a simple toggle switch but after reading a few threads on this I am looking at relays now.

I was thinking of two 4PDT relays so that they could switch between routing an amp to the speaker or to a dummy load just in case the valve amp is on when it is switched.

I had my eye on this relay (2 of them) then a simple toggle switch to route a DC voltage to the relay coils.
http://uk.rs-online.com/web/p/non-latching-relays/7943819/

Datasheet:
http://docs-europe.electrocomponents...6b81290768.pdf

It seems that the advice is to strictly use a break before make type so that the two amps would not feed into each other even for a moment, but I can't tell what type this relay is. Are these types of relays always one type or the other?

Secondly if the advice is to use a break before make, does this put the valve amp at risk even if for a fraction of a second, since they don't like to have zero load, or is it such a small amount of time that it is not an issue? I would expect that it is not an issue.

Many thanks!
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby Folderol » Thu Sep 28, 2017 9:05 pm

It's rather a case of "If I was going there, I wouldn't start here".
Switching out a valve amp even briefly while it's handling any signal is asking for a fried O/P transformer. Whereas the semiconductor one is exactly the opposite!

If it was two valve amps you'd want make before break (quite rare in relays) and two semiconductor ones would want break before make - the usual configuration.

The question is what exactly are you trying to achieve?
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby woodnut » Thu Sep 28, 2017 10:09 pm

Thanks for the advice.

I am looking to make a switch box for quick switching between two amps connected to the same set of speakers.

I actually don't intend for both amps to be on at the same time but wish to have protection in place just in case, as I would not be the only person using the system.

It might even be possible to trigger the switching system from the digital amp as this has an aux PSU on board of 12volts which is activated when the amp is brought out of standby. I figured this could feed the coils of one or more relays.

I will see if I can track down a MBB relay. That way, as you say, the dummy load would be connected to the valve amp before it was disconnected from the speaker in the event that it was on while switching.

Is there a much better / safer way than using relays for such a switch box?
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby Folderol » Thu Sep 28, 2017 11:03 pm

My suggestion would be a multi-function timer, and two normal relays. R1, R2.
In the all off state, R1 normally closed contact would put the dummy load across the Valve amp (A1) and R2 normally closed contact would connect semiconductor amp (A2) to the speaker. This is the fail-safe state.

On triggering the timer (via a simple switch) one of it's outputs would energise R2 and immediately switch A1 to the speaker via its normally open contact (disconnecting A2), and after about 100mS delay a second output from the timer would energise R1 and disconnect the dummy load (power relays typically take 50mS to switch and settle.)

When removing the timer input, R1 would be immediately de-energised and connect the dummy load to A1 and after 100mS R2 would be de-energise and switch back to A2.
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby woodnut » Thu Sep 28, 2017 11:26 pm

Many thanks, that's great. I hadn't thought of using a timer. There appears to be a few ready built adjustable timer switches on ebay. Perhaps I could use one as you described to trigger the relays.
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby ef37a » Fri Sep 29, 2017 10:51 am

I am sorry Will but I am going to disagree with you on this occasion.

Back at Blackstar we had a big relay switch box to A/B amplifiers and that was a simple Bb4M that switched the test amps.

The box must have switched a score or more of different amps thousands of times and under serious overdrive conditions, never a problem that I recall.

I also built a very simple DPDT footswitch box for quick and dirty tests, again, no smoke!
I even built a switcher that A/B'ed the whole OP transformer to do audio tests on other brands switching primary AND secondary (and feedback loop where fitted!) Fine.

I theorize that for the time (mSecs?) that the load is disconnected the interwinding traff capacitance gives enough load to keep things in check* . For a transistor OP stage you need to avoid a short, obviously but connecting a valve OP traff to a transistor stage momentarily is NOT a short and should not bother any decent amp valve OR Silicon.

I take the view that if the amp maker allows an external speaker connection said amp should be protected from reasonable abuse.

*B's valve amps have a 'secret' that affords SOME no load protection. NOT fool proof but I never had a traff fail. (and believe me a few bods tried!)

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Re: Question on Relays for Switching One Pair of speakers betwee

Postby woodnut » Fri Sep 29, 2017 2:00 pm

Thanks for the reply, that is reassuring as I am struggling to find / work out a simple way to delay the switch off of the relay R1. There are small drop in circuits that delay switch on which would work for R2.

It would be good to have the protection delay in there just in case if i can work it out without too much extra complication.

Is there a simple way to keep a relay energised an extra 50ms or so after the supply to the coil has been removed?

I just looked at the spec sheet of the relay I was looking at, the" typical response time" is 13ms and the " typical release time" is 14ms. Do these specs mean 13ms to energise and 14ms to de-energise and close?

Thanks.
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby ef37a » Fri Sep 29, 2017 3:21 pm

If you were only switching valve amps I would suggest putting 100nf caps across each contact then neither amp is truly ever OC load. The problem is the class D jobbie. Amps SHOULD be stable on capacitive loads but don't bet yer wife on it!

I really don't think you have anything to worry about. I doubt you will be switching when an amp is being seriously over driven?

Re relays: The 'response times are for energize and dropout. Dropout is a pretty fixed quantity but tEng' is a bit dependant on the supply voltage. You can speed things up a bit by having a higher than usual voltage across a big cap fed from a resistor. The cap dumps into the coil and then the resistor ensues the holding voltage is within rating. N.B. if you use any transistors around relays be sure to shunt the coils with a rev biased diode. The magnetic pulse is akin to a car ignition coil and will fork most Silicon devices! 1N4007.
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby Folderol » Fri Sep 29, 2017 4:53 pm

Hi Dave,
I bow to your greater experience with switching live valve amps (still wouldn't do it myself) but would advise against speeding up relays. While this is quite acceptable and standard practice for solenoids, relay contact making is a balance between armature speed, and de-bounce. Make it go faster and you're more likely to get a bounce, with unpredictable consequences.

If it is indeed safe to switch over directly then rather than a relay I'd be inclined to use a solidly constructed toggle switch - with a proper sprung action that goes over with a thump.
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby woodnut » Fri Sep 29, 2017 6:55 pm

Well I just found and ordered a couple of these cheap programmable timer relay switches
http://www.ebay.co.uk/itm/132297113012

They appear to be ideal with various settings for a delaying the switch on or off of the onboard relay. I could put these ahead of the relays I linked to in my earlier post.

I may end up not using them but they would sure come handy for some other projects I have in mind.
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby Wonks » Fri Sep 29, 2017 10:25 pm

Woodnut, I really don't think you know exactly what you're doing. I certainly wouldn't use those boards for relay logic. They don't even state whether they are AC or DC powered - which show how poor the documentation is (I'd assume DC but you never know). I'd be looking at a proper multi-function timer relay from Omron or similar where you can set the delay time in milliseconds, not tenths of seconds.

If you want to ignore ef37a, fine, it's your own amps you'll damage, but I'd certainly draw out your relay circuit first, including a power supply. I'd suggest using 24V or 240V AC coil relays, not DC coil relays. Draw out the circuit then post it here, so that it can be checked before you build anything.
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby Folderol » Fri Sep 29, 2017 11:08 pm

Boards are definitely DC, and appear to give out isolated relay contacts, which is nice.
Just curious. Why would you advise AC relays Wonks?
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Re: Question on Relays for Switching One Pair of speakers betwee

Postby ef37a » Fri Sep 29, 2017 11:32 pm

Folderol wrote:Boards are definitely DC, and appear to give out isolated relay contacts, which is nice.
Just curious. Why would you advise AC relays Wonks?

I am also curious.

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Re: Question on Relays for Switching One Pair of speakers betwee

Postby Jumpeyspyder » Sat Sep 30, 2017 1:33 am

interesting thread,

Found a circuit diagram for radial Headbone TS that you might find useful ?

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Re: Question on Relays for Switching One Pair of speakers betwee

Postby ef37a » Sat Sep 30, 2017 6:05 am

Yes J.S. I was going to mention the Radial box as evidence that you could switch amp outputs but of course, they mute the input on the 'non-speakered' amplifier.

OP could still crib the output relay circuit? Just make the loads big Mothers*. If 50W ally clads were used and bolted to a metal box they could easily handle 100W for a few seconds. The resistors could also be higher than optimum for a valve amp. 10R for an 8R tap and 20R for 15. Those values would also suit a transistor amplifier very well.

*The Radial uses wee rated Rs because the inputs are muted and in theory there will never be any significant power into them but boy! If the mute circuit ever failed, would it smoke!

I note that the Radial box switches just one pole. I strongly suggest you switch both hot and earth wires and keep the amplifier's circuitry totally isolated.

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