Hi.
Having a closer look at my Jamo Crossover.
The negative wires coming from the midrange and tweeter drivers meet together and then pass through 2 circuits on their way to the negative speaker terminal.
The first circuit contains an 18 ohm 5 watt Resistor.
The second circuit (in parallel) contains a Thermistor (PTC C995).
As they can be problematic, I'll be removing the Thermistor.
After removal of the Thermistor do I maintain that second path with a wire, or is the Resistor path enough to complete the circuit?
Thanks
Cliff
Having a closer look at my Jamo Crossover.
The negative wires coming from the midrange and tweeter drivers meet together and then pass through 2 circuits on their way to the negative speaker terminal.
The first circuit contains an 18 ohm 5 watt Resistor.
The second circuit (in parallel) contains a Thermistor (PTC C995).
As they can be problematic, I'll be removing the Thermistor.
After removal of the Thermistor do I maintain that second path with a wire, or is the Resistor path enough to complete the circuit?
Thanks
Cliff
I'm rebuilding several crossovers from a number of vintage speakers.
Members have indicator older thermistors are prone to failure and can be problematic leading to issues.
So, as part of the rebuild I'm removing them.
Out of 5 sets of speakers, two designs have Thermistors in the crossovers.
If you're really afraid that they might blow, I'd lift one leg of an intact one, measure it's cold resistance and note the value somewhere next to the xover board. So it probably might be easier to be replaced in case of a failure.
Don't leave it out or short-cut it, both will impact the xover properties.
Best regards!
Don't leave it out or short-cut it, both will impact the xover properties.
Best regards!
It sounds as though the resistor is there so that when the thermistor cuts out, there is still a load on the amp. I wouldn't leave the 18 ohm on its own. Measure the thermistor and replace like with like even if it is a resistor.
The "Thermistor" is a PTC thermistor. As the voltage across it increases, 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.
Large older PA speakers used a 24volt 15Watt light bulb in the same position. We use multiple amplifiers and electronic crossovers now.
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.
Large older PA speakers used a 24volt 15Watt light bulb in the same position. We use multiple amplifiers and electronic crossovers now.
Thanks all.
I'll leave it inplace.
Is it something that either works or fails?
I'd rather not tamper with it if it doesn't affect the Loudspeaker sonically.
Cliff
I'll leave it inplace.
Is it something that either works or fails?
I'd rather not tamper with it if it doesn't affect the Loudspeaker sonically.
Cliff
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.
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Here are a couple of quotes from Chief Moderator Salas regarding 'Positec' or 'polyswitch' devices:
A Positec has a resistance of only a fraction of an ohm when operating normally, so its removal from the circuit is highly unlikely to make an audible difference.
Just to stress, Cliff, if you remove the polyswitch you have to replace it with a length of wire to maintain the circuit path. Simplest thing to do is leave it in place and short ciruit it (bypass it) with a short length of wire.
A Positec has a resistance of only a fraction of an ohm when operating normally, so its removal from the circuit is highly unlikely to make an audible difference.
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
Loudspeaker Protection Components
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.
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.
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.
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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
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Sorry Jon, I understood that on re-reading your post.
Thanks for taking my remark in good humour. 🙂
Thanks for taking my remark in good humour. 🙂
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.
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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.
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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.
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.