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Old 28th June 2009, 07:02 PM   #31
njepitt is offline njepitt  United Kingdom
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Join Date: Aug 2008
Location: Maine
Default Closing the stable door...

For those of you who don't have English as a first language, there's a saying about closing the stable door after the horse has bolted - meaning taking precautions too late.

Yesterday I was listening to the F4 in the pictures above, when there was a pop in the left channel and an immediate drop in volume and sound quality. It turns out that one of the mosfets had failed (although I didn't realise this at once) creating a short, and about 15 VDC across the speaker..... Of cours,e being a dummy, I tried switching speakers to see if the problem was in the amp or in the speaker, and thus managed to blow the other speaker also.... (I've told this sob story on the full range forum, hoping for help in locating replacement drivers...) So I managed to switch out the mosfet and get the amp running, and I have reduced the bias current to about 0.43A instead of the 0.53 recommended, in the hopes of keeping temperatures a little lower and reducing the chance of this repeating, but this isn't an ideal solution, of course. Temporarily I am using speakers "borrowed" from an Aiwa mini system, which are far from ideal also (although rather better than I had expected).

So my question is what are people using for speaker protection? There are circuits on the ESP site which look quite good, but Rod Elliott suggests one of them isn't appropriate if input voltage is less than about 70V, which would rule out the F4. Any suggestions? I'm not worried about small DC offests, I just want to protect against this sort of catastrophic problem.

Cheers
Nigel
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Old 28th June 2009, 09:17 PM   #32
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Join Date: Nov 2006
Quote:
So my question is what are people using for speaker protection? There are circuits on the ESP site which look quite good, but Rod Elliott suggests one of them isn't appropriate if input voltage is less than about 70V, which would rule out the F4. Any suggestions? I'm not worried about small DC offests, I just want to protect against this sort of catastrophic problem.
I build my own speakers -- so if something like this happens, I will just have to replace the woofers.

JJ
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Old 28th June 2009, 09:55 PM   #33
njepitt is offline njepitt  United Kingdom
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Join Date: Aug 2008
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Quote:
Originally posted by jupiterjune


I build my own speakers -- so if something like this happens, I will just have to replace the woofers.

JJ
Well, in this case I built my own too (they're a version of the monopoles from the diyaudio reference thread on the Full Range forum).... All joking aside I realise what you are suggesting is a sensible option in many cases, since the cost of the Fostex drivers isn't so high in the US. But they aren't available here in Brazil at any price, and importing is difficult et cetera.... The FE127e's I blew up were brought down from the US for me by a friend, so before risking a repeat experience I was thinking of looking into protection circuits.... (If I could just order a pair online at a normal price I'd likely just do it.)

Anyone have suggestions? (Please don't say I'm stuck with the Aiwa's...

Cheers

Nigel
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