That is something worth considering, particularly if you are using independently fused rails as per the Maplin amp. I use these 12-24V Dual Relay 7812+UPC1237 Speaker Protection Board Module DIY Kit for HIFI | eBay
That module is so cheap!
UPC1237 datasheet
I am interested in the datasheet's circuit. I'd be inclined to put snubbers on the relay outputs to avoid arcing when disconnecting a speaker under a high dc offset condition.Eg: split the zobel in to two zobels of double impedance, one either side of the relay. But this is me being fussy.
UPC1237 datasheet
I am interested in the datasheet's circuit. I'd be inclined to put snubbers on the relay outputs to avoid arcing when disconnecting a speaker under a high dc offset condition.Eg: split the zobel in to two zobels of double impedance, one either side of the relay. But this is me being fussy.
Problems have been reported with relays where the instant the connection between amp and speaker is broken, a large voltage causes arcing that melts the relay contacts together. In effect, the relay contacts never open. Might not be too good for the speakers either.
I've never looked in to this so I can't comment on how likely this would be. I suppose the more inductive the speakers/cables and the lower the contact ratings of the relay the more likely it is to happen.
I've never looked in to this so I can't comment on how likely this would be. I suppose the more inductive the speakers/cables and the lower the contact ratings of the relay the more likely it is to happen.
Well, I would consider replacing those OMRON G2R-2 relays. If I'm looking at the correct datasheet they're rated at 2A ac inductive load. And rated to switch 5A dc. I can't see what the contacts are made of but the absence implies they aren't silver or something fancy.
G2R
But I'm being fussy again. I am very fussy.
G2R
But I'm being fussy again. I am very fussy.
there is only arcing on closing if the contacts bounce after closing. Damage during closing is minor.
On opening the arc caused by the inductor trying to maintain the same current really damages the contacts. It is during opening that melting and welding of the contacts occurs. Maybe again during the bounce period when the arc is still there and the molten metal is held in the arc during the re-closing phase.
To help prevent melting the contacts/points in the old fashioned automobile distributor were made from tungsten. Good for long life with a constant inductive load designed to spark every time the contacts opened.
I ordered a bk one that used switching fets in the o/p stage...it was unable to bais up to there booklet spec of 40mv.
Less impressed with it. I built two of them may years back using t03 fets and 2sk1058 and it's counter part my amp used 2 pair per side for better current sharing.
Less impressed with it. I built two of them may years back using t03 fets and 2sk1058 and it's counter part my amp used 2 pair per side for better current sharing.
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No I didn't, they are as is, although speaker protection is something I would like to see, Shame it was never built onto the board from the start.
Sorry I've been off the forum for a while, but came down with some bug or something, totally knocked me sideways for the last week or so, thus all projects underway at the moment have been put on hold.
Been reading through some of the posts concerning arcing of contacts, surely a good amp with a very tiny DC offset in the order of millivolts or less wouldn't be enough to cause the contacts to arc.
There was a thread TGM8 (something like that) It was a take on the Rod Elliot (again, possibly P3A amp) where improvements were suggested, but while also trying to keep the overall sound of the amp the same, solid state relays were suggested, but not in series with the outputs, more like to switch off the supplies in the event DC should get onto the speaker outputs, That's an idea I like
Looking at the ebay Kit, what size welding rods do they recommend for "Welding" the components in, could I use a MIG ....
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The problem comes if the amp puts out DC at rail volts.
The DC into the speaker will be a very high current and as the relay opens can arc and weld the contacts.
On my designs I have put a 100nf across the contacts to soak up the arcing.
The old points ignition system also had something like a 0.1uf cap in parallel with them to help avoid arcing, you could also tell if the capacitor was on it's way out, there would start to be a build up on one side of the points, something my Father taught me many years ago.
On some manufacturers points they deliberately started off with a pip on one side and pit on the other. As time went on the pip would transfer across into the pit on the other side. This made the points last longer.
Yes, Absolutely Correct, but what I was getting at was the fact that a build up of material on one side of the points indicating the capacitor was failing.
Thread going off topic again I think as mentioned, a snubber or Capacitor in parallel with the relay contacts would probably help avoid or at least reduce any arcing, much like the X class capacitors that that are fitted across mains switches to stop the contacts arcing and causing loud Cracks through the speakers at Turn On / Off.
Amplimo makes special speaker relays of the type described here: Loudspeaker relay 24V LR-24 Amplimo
As many here have found in recent years though, a solid state relay (based on 2 very low Rds power mosfets) is the only sensible and reliable way to fly.
As many here have found in recent years though, a solid state relay (based on 2 very low Rds power mosfets) is the only sensible and reliable way to fly.
the capacitor in conjunction with the inductor induced severe ringing.
It is the ringing waveform that creates many sparks over a much longer period than a single spark.
The capacitor did not quench the spark, it enhanced them for more effective ignition of the fuel.
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This logic is wrong.
A capacitor on it's own does not have a snubbering effect.
One needs resistance to damp/absorb energy to suppress the ringing.
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