Speaker Protection Question.

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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~misfit~
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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

Reply to
Phil Allison

Cool - and thanks for the reply. :)

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

These are apparently 30A but that can be taken with a pinch of salt;

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(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.)

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

... 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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~misfit~

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** Worse than I thought then.

** 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.

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

** Not normally cos is NOT a crowbar.

** 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

Reply to
Phil Allison

It has 4.

Ok got ya.

Understood.

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

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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~misfit~

It shouldn't be hard to replace the relays.

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Reply to
Computer Nerd Kev

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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~misfit~

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** So they are 4 pin types ???

..... Phil

Reply to
Phil Allison

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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~misfit~

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** I somehow missed that line.

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

..... Phil

.... Phil

Reply to
Phil Allison

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.

Reply to
Chris Jones

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.

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.

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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Reply to
Computer Nerd Kev

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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Reply to
Computer Nerd Kev

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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Reply to
~misfit~

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...

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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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Reply to
~misfit~

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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Reply to
~misfit~

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  
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Reply to
~misfit~

I had some Jamo speakers that never sounded very good despite having what appeared to be high quality drivers (and of course the Jamo name). After a while I decided to check the crossovers with an eye to re-capping them if they didn't use good quality caps. That's when I saw the polyswitches. I did some research at the time and found these is almost universal condemnation for having polyswitches in crossover circuits - unless you're happy with 'mid-fi'.

I wasn't (and am not) and so sold the Jamos.

--
Shaun. 

"Humans will have advanced a long, long way when religious belief has a cozy  
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~misfit~

FYI a link to Phil's post:

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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.

Reply to
Chris Jones

** 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

Reply to
Phil Allison

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