Speaker Protection Question.

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Hi. I've recently bought some second-hand speakers that are a lot more  
power-hungry than my previous speakers so am in the process of refurbishing  
and tuning a 'Playmster Pro Series Three Power Amplifier', a kitset amp  
designed by Electronics Australia with kits put together by DSE and Jaycar.  
The guy who built it was a bit... agricultural and I can see how to make it  
sound better. I'm also upgrading and replacing the filter capacitors in the  
power supplies (it's dual mono construction with two 300VA toroidial  
trannies).

The issue I have is that it has no speaker protection and my speakers are  
irreplaceable (rarity and my budget considered). I have two speaker  
protection PCBs sourced from AliExpress that look up to the task, well  
designed with heavy duty relays and thick signal traces made thicker by  
having the solder mask removed and solder added. My problem is that I can't  
fit them and their power supply into the amplifier case.

So I'm planning on putting them in a box between the amp and speakers with  
speakers in and out connectors on the back. Just checking to see if anybody  
thinks this isn't worth doing for any reason? From what little I know they  
should work fine like this, it's just a little fiddly.

Thoughts?

Cheers,
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Speaker Protection Question.
~misfit~ wrote:

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  ** Having the speaker relay and drive circuit outboard and with its own PSU is a good idea - then it can be used with any amp you like.  

But a word of warning, the Playmaster amp has DC rails of about 60V right?  

Even relays rated for 10amps at 240VAC cannot switch off that much DC - an arc forms across the contacts and will burn the relay and you speakers to destruction.

There are a couple of ways to fix this but the best is to use the normally closed contacts of the relay the SHORT the speaker line rather than opening the connection.

In this scheme the speaker connects to the moving contact and the normally closed one is grounded. The amp output connects to the normally open contact.

If an arc ever does develop when the relay opens, the speaker is protected by being shorted.


.....  Phil  
  


Re: Speaker Protection Question.
Once upon a time on usenet Phil Allison wrote:
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Cool - and thanks for the reply. :)

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This is the "III" model which uses TO-3P trasistors and  has 75V rails.

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These are apparently 30A but that can be taken with a pinch of salt;
https://www.aliexpress.com/item/upc1237-large-current-speaker-protection-board/32809522760.html
(I purchased what look like identical items from a different seller who no  
longer has them for sale. I see that listing says maximum amp 20w - that  
seems as unlikey as the previous listings 600w.)

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Ahh this is the 'crowbar' method of speaker protection?

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... which usually blows the output devices of the amp correct?

The speakers cost me more than the amp (not counting 8 x 10,000uF / 100V  
filter caps and time sepnt on it since then) and I *would* prefer to have  
the amp blow rather than the speakers. I dare say I can source some 2SJ162  
and 2SK1058 MOSFETs if I need to whereas it's unlikely I could source  
replacement speakers (even if I could afford to).

However I have these protection devices already and surely they're better  
than running the amp with no speaker protection at all (as it has been for  
the last 30 odd years)?

Thanks again for the reply, I appreciate it. I have a lot to learn.
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Speaker Protection Question.
~misfit~ wrote:

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 ** Worse than I thought then.  


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** But rated at only 30VDC.  

These relays come in single pole and changeover versions.  

Single pole types have 4 pins on the bottom, changeover ones have 5.


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** Not a crowbar, the speaker is shorted but NOT the amplifier.  


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 ** Not normally cos is  NOT  a crowbar.  


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** If those relays are single pole it's good as worthless.  

If changeover types, you need to wire them the way I described or it is still worthless.  

People have been selling WORTHLESS relay protection systems for decades.  


BTW:

the relay PCBs must NOT power up before the amp does or it is worthless.  

That's why installing them in the case is actually better.  


....  Phil  

Re: Speaker Protection Question.
Once upon a time on usenet Phil Allison wrote:
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It has 4.

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Ok got ya.

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Understood.


Ok I won't bother then, just run the amp straight into the speakers.

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Thanks but as they're worthless I won't bother going to the trouble of  
mounting them in a box, hooking up a power supply and connectors.

Cheers,
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Speaker Protection Question.
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It shouldn't be hard to replace the relays.

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Re: Speaker Protection Question.
Once upon a time on usenet Computer Nerd Kev wrote:
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It doesn't seem easy. I have no idea how to source relays like these and the  
PCBs are made for them - there are no other holes for any other type. It's  
bloody annoying frankly, I thought I was doing the right thing buying these  
as the amp doesn't have any speaker protection, not even fuses. I'll add  
fuses (not sure of values to use) and throw these things in a drawer I  
guess.

Cheers,
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Speaker Protection Question.
 ~misfit~ wrote:

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** So they are 4 pin types ???  



.....   Phil  

Re: Speaker Protection Question.
Once upon a time on usenet Phil Allison wrote:
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Yes, as I said in my reply to you when you asked before.

The negative connection to the speaker passes through unswitched, the  
positive goes to pins on the relay and there are two other pins at the other  
end of the relay which appear to be for the coil.

They aren't powered up and there is no continuity between the pins connected  
to the positive speaker rail.
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Speaker Protection Question.
 ~misfit~ wrote:

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** I somehow missed that line.  

 Similar relays ( with 5 pins) are available from Element 14 and RS Components.  



.....  Phil  


....  Phil  

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Re: Speaker Protection Question.
Once upon a time on usenet Phil Allison wrote:
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Thanks Phil. So exactly what type of relay do you suggest using? Do you mean  
to use a SPDT relay and wire it so that in it's non-energised position the  
speaker is shorted?

Cheers,
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Speaker Protection Question.
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They don't need to be exact matches. Key parameters are operating
voltage, maximum switching current/voltage, number of poles (greater
than two apparantly) and, in this case, switching time. With these
conditions met, most normal relays will be suitable substitutes for
the ones you have. You should be able to find some suitable ones
on Ebay/Aliexpress, as well as the major suppliers like RS.

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The board has PCB mounted relays, but you can desolder them and just
solder wires to a relay glued beside the board in the case. Many
relays have large contacts suitable for soldering wires to, and even
a PCB relay can be connected in this way if you're careful.

I rarely have relays mounted on-board for hobby projects.

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I've never had anything to do with speaker protection, but I doubt a fuse
would be quick enough - especially if the arcing in a relay goes on long
enough to cause trouble.

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Re: Speaker Protection Question.
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Actually I might have misunderstood that. Thinking about it, I think
Phill must mean you need Double Throw relays instead of Single Throw
relays.

So you want an arrangement like this where when the relay (just shown
as a switch) switches, the two speaker lines are shorted together and
any arc just flows straight back to the amplifier:

                |
  _*      /*----|AMP OUT 1
 |       /      |
 |      /    |
 |     *-----|SPK IN 1
 |           |
 |              |
 |--------------|AMP OUT 2
 |              |
--------
SPK IN 2

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Re: Speaker Protection Question.
Once upon a time on usenet Computer Nerd Kev wrote:
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That could be what he means, using a SPDT relay (instead of the SPST type  
fitted) with the NC contacts shorting the speaker - I'll ask him (then try  
to get my head around it and work out how to wire it up using the existing  
uPC1237 based protection boards).

FWIW here's a blog of a guy making his own speaker protection boards using  
the uPC1237 and he's just using SPST relays...
http://blog.audioworkshop.org/upc1237-amplifierspeaker-protection-module/
I wanted to avoid having to make my own so, after reading up on them on  
several DIY audo sites decided these modules were the answer. However if I  
have to modify them to improve them further I will.

The uPC1237 is quite nifty, it has a built-in delay so that when it's  
powered up it doesn't connect the speakers for a couple seconds to stop  
'thump'.
Cheers,
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Speaker Protection Question.
Once upon a time on usenet Computer Nerd Kev wrote:
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Thanks for this Kev. I'm considering my options and appreciate all info.

Running relays off-board might be the answer. I'm in the process of  
rebuilding the power amp at the moment but am thinking about the speaker  
protection issue and will decide what to do when I've finished the amp.

Cheers,
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Speaker Protection Question.
On 19/06/2017 16:51, ~misfit~ wrote:
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IIRC Phil posted something about finding that a capacitor across the  
relay contacts generally stopped them from arcing. Phil, can you remind  
us of the result of your experiment? Perhaps it would not be ideal (e.g.  
if the amp went into high frequency oscillation), but may be better than  
nothing at all.

Re: Speaker Protection Question.
Once upon a time on usenet Chris Jones wrote:
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That could be good if it works. A lot easier than running outboard relays on  
these modules.

I'm bloody annoyed actually, should have done this 'research' first as these  
speaker protection circuits were far more expensive than others using the  
same uPC1237 IC and from what I could see the added expense was mainly  
because of the heavy-duty relays used.
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
We've slightly trimmed the long signature. Click to see the full one.
Re: Speaker Protection Question.
On 29/06/2017 12:11, ~misfit~ wrote:
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FYI a link to Phil's post:
https://groups.google.com/d/msg/sci.electronics.repair/nwWqmrjKEsU/A1SGGVcCbj8J

Full Text:
** I have just been experimenting with a PCB mount, 240VAC/12amp relay  
made by "Schrack" and a variable PSU - consisting of a 300VA tranny,  
bridge rectifier and 10,000uf cap. The tranny was fed from a Variac.

The relay, when energised, connected a 4ohm high power load to the PSU  
and tried to disconnect it when de-energised. With no cap across the  
contacts, serious flash arcing occurred at 30VDC. With 50VDC, you can  
normally expect the arc to become continuous, first shot. The maker's  
rating for DC switching is 24V at 10 amps max.

However, with a 20uF film cap across the contacts, all signs of arcing  
at switch off disappeared. Amazingly, this was still the case when tried  
with 6, 3 & 1uF instead. When I tried 0.22uF, flash arcs appeared on  
about 1 out of 3 tries.

To simulate a *bad* inductive speaker load, I added a 5mH air core choke  
in series with the 4 ohm load and saw slight flash arcing with a 1uF cap  
but none with 3uF.

With a 8 ohm load and 100VDC, 6uF was enough to reduce arcing to minor  
flashes.

So, a film cap across the relay contacts made a huge difference when  
breaking DC current at voltages well above the relay's ratings.

FYI: the film cap delays the voltage rise across the relay's contacts  
for the first 10 to 50 *microseconds* after opening - which is when the  
arc forms. Delay that rise enough and there is no arc.



Re: Speaker Protection Question.
Chris Jones wrote:


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** Yep, that was posted my me some time back.  

A 10uF, 200V or better film cap would do the job very nicely.
  
But there is a problem:  

Speaker relays are mainly used to STOP switch on noises - like loud cracks, squeals and thuds.  

Having a 10uF cap across the contacts lets a lot of it through - certainly to the mids and tweeters in a system.  



..... Phil  


Re: Speaker Protection Question.
On 29/06/2017 17:17, Phil Allison wrote:
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Yes, though in the present situation with no speaker protection at all,  
it is just as bad, and adding the relay with capacitor might help the  
speaker to survive an amplifier failure without burning out. Perhaps  
this particular amplifier does not suffer from those switch on noises  
anyway, e.g. due to lucky supply sequencing.

If the relay has two sets of contacts that are able to be connected in  
series, then a capacitor could be connected across just one set of  
contacts. The contacts with the capacitor would be able to switch off  
the speaker hopefully without arcing in the event of an amplifier fault,  
and the contacts with no capacitor would stop the clicks and pops if  
they occur when the relay coil is not energised.


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