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Thermistor removal from crossover rebuild?
Thermistor removal from crossover rebuild?
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Old 15th May 2020, 06:21 AM   #1
CliffR52 is offline CliffR52  Australia
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Default Thermistor removal from crossover rebuild?

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
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Old 15th May 2020, 06:29 AM   #2
TBTL is offline TBTL  Germany
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Why do you want to remove the thermistor?
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Old 15th May 2020, 06:34 AM   #3
CliffR52 is offline CliffR52  Australia
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Quote:
Originally Posted by TBTL View Post
Why do you want to remove the thermistor?
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.
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Old 15th May 2020, 06:40 AM   #4
Kay Pirinha is offline Kay Pirinha  Germany
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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!
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Old 15th May 2020, 06:47 AM   #5
AllenB is offline AllenB  Australia
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Thermistor removal from crossover rebuild?
Quote:
Originally Posted by CliffR52 View Post
After removal of the Thermistor do I maintain that second path with a wire, or is the Resistor path enough to complete the circuit?
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.
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Old 15th May 2020, 07:38 AM   #6
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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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.
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Old 15th May 2020, 08:06 AM   #7
CliffR52 is offline CliffR52  Australia
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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
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Old 15th May 2020, 01:30 PM   #8
Galu is online now Galu  Scotland
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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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Old 15th May 2020, 01:36 PM   #9
Galu is online now Galu  Scotland
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Quote:
Originally Posted by JonSnell Electronic View Post
We use multiple amplifiers and electronic crossovers now.
Is that 'the royal we'?
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Old 15th May 2020, 02:27 PM   #10
Galu is online now Galu  Scotland
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Here are a couple of quotes from Chief Moderator Salas regarding 'Positec' or 'polyswitch' devices:

Quote:
"Recently I had an MS speaker to see for fix, had an issue of volume drop as it was fed more power. It was the polyswitches."
Quote:
"When they age, especially after when they had been triggered before, they tend to duck power early."
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.
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