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Author Topic: Thermal limiter: why not based on RMS current?  (Read 2513 times)

Andre Vergison

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Thermal limiter: why not based on RMS current?
« on: January 06, 2021, 03:20:57 pm »

Having read a lot about thermal limiters and RMS limiters , I've always wondered why manufacturers base their designs on the signal VOLTAGE?

The (simplified) electrical equivalent circuit of a speaker driver is a series of a resistive part and an inductive part (and an EMF). The resistive part is the voice coil resistance. So the voice coil heating will be proportional to the square of the CURRENT thru that circuit, not the voltage.
Yes - the voltage is okay as long as the speaker is resistive, but it never is.

For instance, the power fed to a speaker near the resonance frequency (or near the two impedance peaks of a bass reflex) is way lower than elsewhere, even on high voltages.

IMHO the CURRENT, and in particular the RMS value of it, would be better data to let the limiter work on.

Agree / don't agree?
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David Sturzenbecher

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Re: Thermal limiter: why not based on RMS current?
« Reply #1 on: January 06, 2021, 04:50:10 pm »

Having read a lot about thermal limiters and RMS limiters , I've always wondered why manufacturers base their designs on the signal VOLTAGE?

The (simplified) electrical equivalent circuit of a speaker driver is a series of a resistive part and an inductive part (and an EMF). The resistive part is the voice coil resistance. So the voice coil heating will be proportional to the square of the CURRENT thru that circuit, not the voltage.
Yes - the voltage is okay as long as the speaker is resistive, but it never is.

For instance, the power fed to a speaker near the resonance frequency (or near the two impedance peaks of a bass reflex) is way lower than elsewhere, even on high voltages.

IMHO the CURRENT, and in particular the RMS value of it, would be better data to let the limiter work on.

Agree / don't agree?

This is a limiter type available to powersoft users...
The TruePower limiting is a Powersoft technology useful to avoid overheating of the voice coil and can be used to avoid power compression. The DSP provides the measurement of the real power delivered (and then dissipated) to the coil, not the apparent power handled by the line.
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John Roberts {JR}

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Re: Thermal limiter: why not based on RMS current?
« Reply #2 on: January 06, 2021, 06:01:42 pm »

Having read a lot about thermal limiters and RMS limiters , I've always wondered why manufacturers base their designs on the signal VOLTAGE?
"manufacturers"?

Generally voltage is all the aftermarket has access to without placing thermal sensors inside the drivers.

That said there are smart driver protection circuits that can directly impute voice coil temperature from sensing voltage and current. I have been out of the trenches for 20 years but even back then we sold big dog amps for professional fixed install market that sensed both voltage and current at the amplifier speaker feed. Since the resistance of the voice coil increases with temperature you could predict temperature from comparing voltage and current draw to a cold reference resistance. Of course you have to learn the behavior for specific drivers. This amplifier current sensing is probably more commonly used to detect blown drivers in a large entertainment venue with tens of speakers (or more), so they can tell maintenance which boxes to repair.

I can also imagine a smart powered speaker that is made very hard to kill with DSP inside and some specific knowledge about the drivers. 

IMO impractical for stand alone aftermarket protection modules because the end users would need to know more about their speakers than most do. I though about this for a product but dismissed it because it would require so much hand holding (and claims for expensive replacement speakers if the customer didn't set up the protection properly).

I seem to recall one dsp based driver protection card that mounts right on the driver ( sold by eminence? IIRC), but probably cheaper to do that in software these days.

Quote
The (simplified) electrical equivalent circuit of a speaker driver is a series of a resistive part and an inductive part (and an EMF). The resistive part is the voice coil resistance. So the voice coil heating will be proportional to the square of the CURRENT thru that circuit, not the voltage.
Yes - the voltage is okay as long as the speaker is resistive, but it never is.

For instance, the power fed to a speaker near the resonance frequency (or near the two impedance peaks of a bass reflex) is way lower than elsewhere, even on high voltages.

IMHO the CURRENT, and in particular the RMS value of it, would be better data to let the limiter work on.

Agree / don't agree?
You are under thinking this... This is more complex than just performing RMS math, but not a lot more. Without knowing exactly how fast the driver dissipates heat you are still left guessing even if you perfectly compute the power going into the drivers. That is only one piece of the puzzle.

JR
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Brian Jojade

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Re: Thermal limiter: why not based on RMS current?
« Reply #3 on: January 06, 2021, 06:11:43 pm »

While it may be better data, voltage data is usually close enough to get the job done and far less complex and expensive to implement.
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Scott Holtzman

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Re: Thermal limiter: why not based on RMS current?
« Reply #4 on: January 07, 2021, 02:59:52 am »

"manufacturers"?

Generally voltage is all the aftermarket has access to without placing thermal sensors inside the drivers.

That said there are smart driver protection circuits that can directly impute voice coil temperature from sensing voltage and current. I have been out of the trenches for 20 years but even back then we sold big dog amps for professional fixed install market that sensed both voltage and current at the amplifier speaker feed. Since the resistance of the voice coil increases with temperature you could predict temperature from comparing voltage and current draw to a cold reference resistance. Of course you have to learn the behavior for specific drivers. This amplifier current sensing is probably more commonly used to detect blown drivers in a large entertainment venue with tens of speakers (or more), so they can tell maintenance which boxes to repair.

I can also imagine a smart powered speaker that is made very hard to kill with DSP inside and some specific knowledge about the drivers. 

IMO impractical for stand alone aftermarket protection modules because the end users would need to know more about their speakers than most do. I though about this for a product but dismissed it because it would require so much hand holding (and claims for expensive replacement speakers if the customer didn't set up the protection properly).

I seem to recall one dsp based driver protection card that mounts right on the driver ( sold by eminence? IIRC), but probably cheaper to do that in software these days.
You are under thinking this... This is more complex than just performing RMS math, but not a lot more. Without knowing exactly how fast the driver dissipates heat you are still left guessing even if you perfectly compute the power going into the drivers. That is only one piece of the puzzle.

JR


John do you know how the Meyer processors worked that had an input to the speaker?  I know it did more than limit it changed the crossover frequency too IIRC.
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Doug Fowler

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Re: Thermal limiter: why not based on RMS current?
« Reply #5 on: January 07, 2021, 03:52:53 am »


John do you know how the Meyer processors worked that had an input to the speaker?  I know it did more than limit it changed the crossover frequency too IIRC.

I was told by a "guy" in a position to know it's actually Apogee controllers that used the sliding xover, not Meyer. I used to have UPAs and USWs pre-measurement days.

I would love to see this settled. Anyone with a biamp Meyer analog controller could easily verify this with a simple measurement.
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Helge A Bentsen

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Re: Thermal limiter: why not based on RMS current?
« Reply #6 on: January 07, 2021, 05:16:11 am »

I was told by a "guy" in a position to know it's actually Apogee controllers that used the sliding xover, not Meyer. I used to have UPAs and USWs pre-measurement days.

I would love to see this settled. Anyone with a biamp Meyer analog controller could easily verify this with a simple measurement.

According to a measurement friend, Meyer didnít use sliding xovers, but once the controller started limiting in one band, the audible effect was similar to sliding crossovers. When measured with one band in limit, one of the bands would stay at itís level while the other would increase level leading to the audible xover moving. The electronic crossover stayed put.
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John Roberts {JR}

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Re: Thermal limiter: why not based on RMS current?
« Reply #7 on: January 07, 2021, 09:42:33 am »


John do you know how the Meyer processors worked that had an input to the speaker?  I know it did more than limit it changed the crossover frequency too IIRC.
I don't have any specific knowledge about that but vaguely recall some pre-DSP speaker processors that used a sliding HPF to reduce LF bass energy at high output levels.

I also suspect some audiophools played with 4 wire Kelvin sensing (2 speaker wires and 2 sense wires) to provide numerically high damping factor.

I momentarily considered an after market SKU that performed the the current sensing between amp and loudspeaker to provide the data to impute voice coil temperature, but almost as quickly abandoned it after thinking about the nightmare of customer service for users that in fact need such a product, and speaker specific set-up assistance. 

By now, decades later this should be mature technology and premium brands should be pretty well managed. Smart power amps with inboard current sensing, and modest DSP "can" do this but must be programmed for specific loudspeakers. 

Caveat, I have been out of the trenches for a couple decades so not up on latest technology.

JR
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Lee Douglas

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Re: Thermal limiter: why not based on RMS current?
« Reply #8 on: January 07, 2021, 11:13:11 am »


I also suspect some audiophools played with 4 wire Kelvin sensing (2 speaker wires and 2 sense wires) to provide numerically high damping factor.


I had a Peavey product called Dynamic System Controller that did that.  I used it on a 3020ht/415SUB system.  Output to amplifiers and low/mid/high sense wires from the speaker output of each amplifier.  Or was that something different?
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Jim McKeveny

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Re: Thermal limiter: why not based on RMS current?
« Reply #9 on: January 07, 2021, 12:45:19 pm »

Apogee controllers ... used the sliding xover, not Meyer. 

Renkus-Heinz had "Smart" loudspeaker line that moved crossover points upward for driver protection. Boxes were 2-15's handing off to a 2 inch HF. The whole crossover region polars adjusted dynamically with material. I gigged outdoors in San Jose on one of these rigs, way undersized for the crowd. The sonic effect was, umm, novel.

« Last Edit: January 07, 2021, 01:10:02 pm by Jim McKeveny »
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Re: Thermal limiter: why not based on RMS current?
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