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Old 15th August 2012, 10:31 AM   #11
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by cowana View Post
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
It will sound fine. It's at some time later (weeks/months/years) that it may play up.

You see its happened to me.
Amp in a clean environment and clean air so this is the kind of problem affects other folk not me.

Right... err... wrong

I wasn't aware audibly (on music) that there was any issue but playing a sine wave of around 400hz (from memory) and increasing the volume and one channel was horribly distorted. I went back to music and coudn't fault it. Weird ! I really thought my speaker was faulty. I turned the volume up more and more and it suddenly cleared. It was the relay.

More of the same here. Just two of many that I could remember.

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Old 15th August 2012, 12:31 PM   #12
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Quote:
Originally Posted by cowana View Post
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

Andy,

It is not so much that under 5mA there will be distortion, but rather that the relay wants at least 5mA of current while switching in order to keep the contacts clean.

vac
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Old 15th August 2012, 03:29 PM   #13
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I can only echo what Mooly has said. I never expected my relays to start acting up, but that's what they did and after only a couple of years. The main issue with standard relays though is that they aren't particularly good at switching high DC currents and it's DC that your amplifier will produce under a fault condition. This DC is what will fry your loudspeakers and if the relay fails to open, because the contacts have welded shut due to the DC arcing, then your relay didn't do it's just properly and you're down a pair of speakers AND a relay.

The solid state relay on the other hand is perfectly suited to switching large DC currently, it is after all what lots of MOSFETs are designed to do in the first place anyway. These will ensure that your loudspeakers are protected and they will also switch faster then a mechanical relay too.

I posted measurements in the thread that Mooly linked to showing the performance of the amplifier with the bad relay and then measurements after with the solid state relay in place, with the problem eliminated.
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Old 15th August 2012, 03:39 PM   #14
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My 2 cents worth:

Use paralleled contacts - as many as you can. 4PST/DT can be had,
more is better. Assuming it is in series with the amp output, not shunting to ground. Although more there is not a bad idea either.

Can't speak to the SS relay shunting to ground.

Single contacts? For me, never.

Ymmv.

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Old 15th August 2012, 05:21 PM   #15
AndrewT is offline AndrewT  Scotland
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Bear,
how do you get the multi-parallel contacts to open at the same time?
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Old 15th August 2012, 05:35 PM   #16
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I suppose the point would be, that if you've got multiple contacts with dual pole relays, with the 'open' state being the output of the amplifier shorted to ground, then you only need one of the relays to successfully open, because even if the other ones weld shut, one has at least shorted the amps output and protected the loudspeakers.

I still don't see why this is superior to a solid state affair, where you can connect them so they are either floating, or ground referenced. Heck you could use a mechanical relay + solid state where the SSR, with it's faster switching time, is used solely to connect the output of the amplifier to ground during the presence of a fault.
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Old 16th August 2012, 08:58 AM   #17
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Quote:
Originally Posted by 5th element View Post
...The main issue with standard relays though is that they aren't particularly good at switching high DC currents and it's DC that your amplifier will produce under a fault condition. This DC is what will fry your loudspeakers and if the relay fails to open, because the contacts have welded shut due to the DC arcing, then your relay didn't do it's just properly and you're down a pair of speakers AND a relay....
My amplifier already has DC protection, so the relays are just to connect the speakers just before power on and disconnect them just after power off.

I've installed the 15A relay into the amp and it currently seems excellent, although it will be interesting to see how it changes over time. I've used crimp on bolt terminals to connect it, so swapping it out for a different relay should be very easy if I wish to try that.

Andy
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Old 16th August 2012, 12:28 PM   #18
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..and that should have read "connect just after power on and then disconnect them just after power off".

The amps capacitors take about 10 seconds to discharge before there is the thump, so having the relay turn off 2 seconds after the power is removed works fine.

Andy
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Old 16th August 2012, 01:31 PM   #19
Mooly is offline Mooly  United Kingdom
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Thanks for updating. Good to hear you've got the relays working OK
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Old 16th August 2012, 01:58 PM   #20
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I'm using a simple RC circuit wired as a low pass filter (connected to 50v ac via a single rectifying diode) to provide the delays. The maths, if anyone's interested:
- Series resistor: 270R
- Coil resistance: 3.4k
- Thevenin resistance: 250R
- Capacitance: 470uF

Which leads to:
- Power on delay = ~0.13 seconds (250*470E-6)
- Power off delay = ~1.6 seconds (3400*470E-6)

Which is ideal to avoid both power on (which happens immediately) and power off noise (which happens when the PSU caps discharge below 15v and the opamp switches off, so 8 to 10 seconds after pulling the plug).

Last edited by cowana; 16th August 2012 at 02:04 PM.
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