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Re: dodgy mordaunt short speakers



"Terry Given"
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  **  1 amp rms continuous.



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** You are looking at the ****wrong****  devices .

 The PTCs that are used in speakers and battery packs are called
"PolySwitch".

 See Farnell cat:  book 2 pages  345  to  347.

 See the RXE series,  60 volt rated.






..........   Phil







Re: dodgy mordaunt short speakers


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Bingo, thanks Phil & Trevor.

070 and 135 are the device markings - so an RXE075 and an RX135 ought to
do the trick.

I've never used a polyswitch before, but did test some about 9 years ago
- no good for that application, a +24Vdc supply that had to survive
idiots connecting 400Vac to it, they were too slow.

Cheers
Terry

Re: dodgy mordaunt short speakers



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**MS was the first audio company (AFAIK) to use Polyswitches for protection
of their drivers. Normally, the system is pretty foolproof, though
Polyswitches can and do fail, usually due to over-Voltage. Naturally, you
would expect to measure a DC resistance of around 5 Ohms (not 50 Ohms) for
any driver which is rated at 8 Ohms. Polyswitches maintain a low resistance,
until they hit their critical temperature. Then they 'switch' to a high
resistance mode (sort of), whereby current is severely limited. Internal
heating will keep the Polyswitch at this point, until the overload is
removed.


--
Trevor Wilson
www.rageaudio.com.au



Re: dodgy mordaunt short speakers


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Hi Trevor,

it was the tweeter polyswitch that measured 50R, I never measured the
speaker impedance.

Thanks,
Terry

Re: dodgy mordaunt short speakers


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I know you had fun out there on the deck, and you may not thank me for
saying this, but pumping loud continuous tones into hifi speakers is evil.
This is because domestic speakers are designed for real music which has very
high peak values but and relatively low RMS (heating) values.

To test your amplifier, I suggest a CRO and a resistor dummy load.  Many
hifi amplifiers will also overheat when supplying high powers for long
periods, so watch that.  Otherwise, listen to real music.

Once your amplifier checked out OK, you could than have checked out the
speakers.  Another trick is to swap amplifier, swap speakers etc and
eliminate the culprit.

By the same token don't plug a microphone into your hifi, because a single
feedback squeal can toast something.

Things are a lot safer these days, thanks to polyswitches, but I would not
want to bet my nice speakers on a polyswitch.

Roger Lascelles



Re: dodgy mordaunt short speakers


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real music made it harder to pinpoint the problem.

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Hi Roger,

point taken. Although it was LF, and I suspect modern music has fairly
high peak-to-average LF content (I suspect most stereo buyers would be
happy sans tweeters nowadays). The amp ran cool, and the actual power
level was 1/3 the speaker rating.

Cheers
Terry

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