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Old 14th August 2012, 11:02 AM   #1
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Default Choosing a relay for loudspeaker switching

My amplifier (a modified Quad 405-2) produces a slight pop on power on, and a soft thump from the woofers when powered off. This isn't much of an issue, but I'd prefer to have totally silent operation.

My solution is to add a relay between the amplifier output and the speakers, which will turn on after the amp and off before the amp. I'm fine with the control side, and now just need to choose a relay.

I would prefer to use Farnell to source the parts from.


The amplifier is 100W max, so 5A max per channel, however it may make sense to get bigger relays so they're never close to their limit.

I'm looking for a DPDT relay in order to switch both channels, and a coil voltage of 48v DC.

So far the best two I have found seem to be:
- Omron 5A DPDT 48v
- Finder 15A DPDT 48v

Out of these, I'd be tempeted to go for the Omron as I've heard slightly more about them in audio - although would the 15A one be a better choice?

Or is there another relay I haven't considered? The list of possible relays Farnell sell (with the correct coil resistance, contact voltage, current and configuration) is this list - showing nine options.

Many thanks for any opinions!
Andy
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Old 14th August 2012, 11:19 AM   #2
Mooly is offline Mooly  United Kingdom
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The 15A would be a better choice.

(Would you consider a solid state relay (to diy it ?). I'm a total convert to these now. Dead easy and with same footprint as those in your link)

If you want mechanical though I would perhaps look at more substantial relays. Big issues are degradation of contacts if switching when there is significant signal present ie its turned up load and audible degradation at low levels which is what I have experienced.
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Old 14th August 2012, 11:27 AM   #3
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As an electronic engineer, my instinct tells me to use mechanical relays rather than solid state. With solid state, the signal has to travel through active devices, which are rarely linear devices. With a relay, you've just got a few bits of metal pressed together - it seems a much simpler and reliable solution.

The 15A relay (which is actually 15A peak, 8A continuous) has a minimum switching current Edit: switching load of 5mA. That seems pretty low - is that going to affect the audio at low volumes?

Thanks,
Andy

Last edited by cowana; 14th August 2012 at 11:56 AM.
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Old 14th August 2012, 11:48 AM   #4
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Location: Not where whatever I am looking for is at.
Isn't the switching current what the coil needs to close the contacts? I would not expect any influence on audio, as long as the DC supplied to the coil is clean.
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Old 14th August 2012, 11:57 AM   #5
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Sorry, that was a mistake - I should have said minimum switching load.
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Old 14th August 2012, 12:13 PM   #6
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Location: Not where whatever I am looking for is at.
That is just to keep the contacts clean. Switch the amp on/off with some signal, and there won't be a problem.
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Old 14th August 2012, 01:42 PM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by cowana View Post
As an electronic engineer, my instinct tells me to use mechanical relays rather than solid state. With solid state, the signal has to travel through active devices, which are rarely linear devices. With a relay, you've just got a few bits of metal pressed together - it seems a much simpler and reliable solution.

The 15A relay (which is actually 15A peak, 8A continuous) has a minimum switching current Edit: switching load of 5mA. That seems pretty low - is that going to affect the audio at low volumes?

Thanks,
Andy
As an electronics engineer that was always my view too... that you can't beat a good mechanical contact.

But think small as well as large signal. That tape hiss on a recording ! the faintest sounds ! What amplitude might they be voltage wise... how much current do they cause to pass through the relay contact. It's a lot less than 5 ma. A typical symptom of mechanical relays (as witnessed by many threads on here) is someone complains of amplifier distortion but finds it "goes" when they turn the amp up loud. And the caue is always the same... a slightly tarnished relay contact.

(so last mention of SS relays then, and a link to this rather long thread,
Output Relays

particular interest are the posts around the 200 to 300 mark)

Pull some data sheets on the latest FETs and see how low the RDs value can be. And it's consistent and guaranteed.
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Old 14th August 2012, 01:50 PM   #8
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by Mooly View Post
................. SS relays ....................FETs and see how low the RDs value can be. And it's consistent and guaranteed.
guaranteed for 1000's, or 10000's, or 100000s, or 1000000s, or 10000000s of operations?

A mechanical relay won't be guaranteed to switch DC currents above it's DC rating for many operations !!!!

Now I know why we (I) prefer to use the k & M multipliers.
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Old 14th August 2012, 06:54 PM   #9
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If this is for PA use with a lot of volume (voltage) copper contacts might be okay. Down to about 1 V signal, silver plated contacts are in order. Below 1 V, you want precious metal contacts (gold, platinum, rhodium, osmium).
paralleling a precious metal contact telephone relay with a big high rated copper contact might be okay for variable service.
I'm monkeying with 40A rated copper contact motor contactors to disconect the PS rails on my amp on detection of a speaker or output transistor (DC) fault, but since they are repairable, I'm going to take the contacts out and put silver solder on them.
You might find is cheaper to re-engineer your amp to get rid of the thumps and pops. I'd start buying CL-60 or bigger NTC resistor (GE) in series with the power transformer if it is a transformer type power supply. Final numbers depend on the rating of the amp. My PV 1.3k amps is a bear of a driver, but it doesn't pop or thump.
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Last edited by indianajo; 14th August 2012 at 06:57 PM.
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Old 15th August 2012, 09:27 AM   #10
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Working on the idea that levels under 5mA may be distorted:
- 5mA into 8 ohms is 40mV
- 5mA at 40mV is 0.0001W

From the above back-of-an-envelope sums, it seems there will usually be more than 5mA flowing so distortion shouldn't be too much of an issue. I'll give it a go with the 15A relay and report back how it sounds.

Andy
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