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Multi-Way Conventional loudspeakers with crossovers

Thermistor removal from crossover rebuild?
Thermistor removal from crossover rebuild?
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Old 15th May 2020, 02:39 PM   #11
Galu is offline Galu  Scotland
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For those interested in the use of PTC resistors (polyswitches) for loudspeaker overload protection, there's a handy calculator on this site:

Loudspeaker Protection Components
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Old 15th May 2020, 02:57 PM   #12
AllenB is offline AllenB  Australia
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Thermistor removal from crossover rebuild?
This has made me curious as well. IIRC polyswitches change state when 'triggered' then reconstitute. There has been uncertainty around them always coming back to the same low resistance.

Additionally the PTC could cause a varying small change in response along with changes in resistance. I don't recall what the thermal inertia is like but if it is slow it stands to reason it could also cause compression, and if fast, harmonic distortion.
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Old 16th May 2020, 04:44 AM   #13
CliffR52 is offline CliffR52  Australia
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Quote:
Originally Posted by Galu View Post
As JonSnell explains, this is a 'Positec' overload protection device. I discussed such devices with you in relation to your Mordaunt Short speakers. Sometimes their operation can become erratic with age. Provided you are not going to be turning the volume up beyond what is tolerable, you can short out the Positec by soldering a short length of copper wire across it.
Thanks G.

The overload aspect isn't a worry to me.

I never run Amps to crazy volumes and rarely exceed 50% on the volume control, so clipping or power damage shouldn't be an issue.

My concern was Jon Snells comment:

"it warms up, as it warms up the resistance increases and reduces the power to the tweeter.
This is a feature used on drivers to reduce the failure rate of overloaded speech coils and to make the sound more pleasing to the ear.
Removal will make the tweeter respond as though it is not working
" .

I wouldn't want their removal altering the sound.

Last edited by CliffR52; 16th May 2020 at 04:47 AM.
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Old 16th May 2020, 04:52 AM   #14
Topping is offline Topping  United States
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Without reading the whole post , the common knowledge. Is remove thermister , if you dont have RUGRATS around to blow up your tweeters. hhhhhaaaahhhhhhaaaaaaa

Last edited by Topping; 16th May 2020 at 04:54 AM.
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Old 16th May 2020, 08:31 AM   #15
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Quote:
Originally Posted by Galu View Post
Is that 'the royal we'?
By we, I mean the PA industry.
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Old 16th May 2020, 10:15 AM   #16
Galu is offline Galu  Scotland
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Sorry Jon, I understood that on re-reading your post.

Thanks for taking my remark in good humour.
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Old 16th May 2020, 10:34 AM   #17
Galu is offline Galu  Scotland
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Quote:
Originally Posted by CliffR52 View Post
My concern was Jon Snells comment:

"it warms up, as it warms up the resistance increases and reduces the power to the tweeter.
This is a feature used on drivers to reduce the failure rate of overloaded speech coils and to make the sound more pleasing to the ear.
Removal will make the tweeter respond as though it is not working
" .

I wouldn't want their removal altering the sound.
If I may be so bold as to speak for Jon, then by 'more pleasing to the ear' I think he means 'compressed'. Compression is not desirable, but during an overload it is preferable to a distorted then ruined tweeter.

By 'Removal will make the tweeter respond as though it is not working', I think he is stressing that if you simply remove the polyswitch, you will break the circuit path and the tweeter will stop working. That is why I tell you to replace the polyswitch with a short length of wire.

Last edited by Galu; 16th May 2020 at 11:04 AM.
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Old 16th May 2020, 11:01 AM   #18
Galu is offline Galu  Scotland
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Quote:
Originally Posted by audiotechnica View Post
The parallel resistor is to allow continued and attenuated but not muted operation of the mid/highs during overload condition, ie idiot proofing.
Thanks for clarifying that. You appear to have interpreted Cliff's circuit description correctly.

@Cliff - Given audiotechnica's explanation, you may be advised to leave the protection circuit (the parallel combination of resistor and polyswitch) alone unless it is compromising the sound of the speaker. Failing that, it would have to be shorted out with a wire link.

If you can supply a crossover schematic then that would be helpful.

Last edited by Galu; 16th May 2020 at 11:04 AM.
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Old 16th May 2020, 11:17 AM   #19
Galu is offline Galu  Scotland
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For those interested in how polymeric positive temperature coefficient (PPTC) devices work, look here: Resettable fuse - Wikipedia

The article seems to suggest that allowing a polyswitch to operate frequently under overload conditions may lead to it no longer returning to its low resistance state and instead stabilising at a significantly higher resistance.
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Old 16th May 2020, 11:24 AM   #20
turk 182 is offline turk 182  Canada
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if the polyswitch is considered dubious why not consider a lamp limiter to protect the tweeter?
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