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Author Topic: Thermal limiter: why not based on RMS current?  (Read 2521 times)

John Roberts {JR}

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Re: Thermal limiter: why not based on RMS current?
« Reply #10 on: January 07, 2021, 01:14:02 pm »

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?
That was perhaps a little before my time, and definitely from a different engineering group.

My guess is that it was a Peavey version, of some other companies respected product. The transducer group didn't do much of their own electronic circuit design so that was probably inspired by some other companies existing design. I don't know what was inside.

JR 

PS: Speaking of damping factor Jack Sondermeyer (RIP) invented a circuit to allow amplifiers to deliver negative output impedance. Not only could he drive the amplifier's output impedance to zero ohms, be he could go further to compensate for 100% of the speaker wire losses, and even inside the box to compensate for crossover and voice coil impedance. The ability to alter driver behavior is obvious, less obvious how to use it productively for Peavey customers. This technology was clearly too cool for the room (Peavey distribution) and the high damping factor capability was only built into one amp model (the CS800s). 
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Andre Vergison

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Re: Thermal limiter: why not based on RMS current?
« Reply #11 on: January 07, 2021, 04:06:17 pm »

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

My original simplified 'model' did not elaborate the EMF part, which, for a closed box or a driver in open air, can be modeled as a parallel LCR circuit. So the total load is 'complex', it has a resistive part and a reactive part. The reactive part cannot dissipate or produce any power.

The 'real' power is the power dissipated by the resistive part, which in case of a speaker is the sum of the voice coil resistance and the resistive part of the LCR circuit (EMF). We know that the total resistance varies by frequency between the voice coil resistance and a number of times that value.

The voice coil resistance is a constant for a given temperature. It rises as the coil temperature increases.

The point is that the heating of the voice coil is purely caused by the value of the current, and also by the temperature dependent value of the voice coil resistance (the product).

That means that an amplifier can safely feed a speaker with a voltage higher than normal at cone resonance frequency (leaving aside Xmax), simply because the resistive part of the EMF is many times higher than the voice coil resistance, e.g. 100 ohms for a 8 ohms speaker. Hence the current is low, and hence little voice coil heating will occur.

Take a typical bass reflex enclosure (see pic). Again leaving Xmax aside, the actual impedance for played bass notes will be way higher than 8 ohms. Why would we then thermal-limit the input voltage if the current, and heating, is lower?

That's exactly my point: RMS limiters don't take current into account, they work on voltage only, so they may limit needlessly (or not enough).

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.

Powersoft's TruePower seems to be the right thing indeed, according to https://www.soundpro.com/catalog/documents/22802_2.pdf , see 6:10.6.2:

TruePower™: the amplifier’s active output power is estimated by measuring the load current. The Truepower limiter is a powersoft patent technology useful to avoid overheating of the voice coil; it can however also be used to avoid power compression. The DSP provides the measurement of the real power delivered (and then dissipated) to the coil, ignoring the apparent power handled by the line.


Looking good. They also allow empirical parameters to be entered about type of speaker, thermal inertia, number of speakers etc.
Glad to see they did it this way.

More about complex (resistive and reactive) power (you can skip the formulas) :
https://electrical-engineering-portal.com/complex-power-analysis

Interesting analysis of speaker back-EMF (4 parts):
https://audioxpress.com/files/attachment/2701
https://audioxpress.com/files/attachment/2702
https://audioxpress.com/files/attachment/2703
https://audioxpress.com/files/attachment/2704
« Last Edit: January 07, 2021, 04:08:35 pm by Andre Vergison »
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Ivan Beaver

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Re: Thermal limiter: why not based on RMS current?
« Reply #12 on: January 07, 2021, 04:57:56 pm »

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.
In the first or second edition of sound system engineering, there was a mention specifically of Meyer and the sensing and sliding xover.  I loaned my copy out and have never seen it since :(
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Ivan Beaver
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Ivan Beaver

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Re: Thermal limiter: why not based on RMS current?
« Reply #13 on: January 07, 2021, 05:01:50 pm »



PS: Speaking of damping factor Jack Sondermeyer (RIP) invented a circuit to allow amplifiers to deliver negative output impedance. Not only could he drive the amplifier's output impedance to zero ohms, be he could go further to compensate for 100% of the speaker wire losses, and even inside the box to compensate for crossover and voice coil impedance. The ability to alter driver behavior is obvious, less obvious how to use it productively for Peavey customers. This technology was clearly too cool for the room (Peavey distribution) and the high damping factor capability was only built into one amp model (the CS800s).
That sounds a lot like what powersoft is doing in their amps.

I believe an old Crown (Delta ohmega?) that would do the same sort of thing, or so I remember, but it has been a few decades.
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Andre Vergison

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Re: Thermal limiter: why not based on RMS current?
« Reply #14 on: January 08, 2021, 12:27:36 pm »

PS: Speaking of damping factor Jack Sondermeyer (RIP) invented a circuit to allow amplifiers to deliver negative output impedance. Not only could he drive the amplifier's output impedance to zero ohms, be he could go further to compensate for 100% of the speaker wire losses, and even inside the box to compensate for crossover and voice coil impedance.

https://sound-au.com/project56.htm#s4
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Andre Vergison

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Re: Thermal limiter: why not based on RMS current?
« Reply #15 on: January 08, 2021, 01:21:45 pm »

This is a limiter type available to powersoft users...

Apparently to ElectroVoice users as well:
https://electrovoice.com/media/downloads/thermal_energy_management_and_protection_limiter_technology_v1-0.pdf

" Another  method  would  be  a  combined  approach  of  monitoring  the  voltage  and  current  delivered  by  the  amplifier.  Unfortunately,  this  would  make  the  parameters  depend  upon  how  many  speakers  the  user  had  paralleled  on  each  channel  and  would  be  limited  to  amplifiers  with  DSPs  on  board. "

That's what PowerSoft does. So EV doesn't like it. What they do is the following:

" Sample  by  sample,  the  TEMP  limiter  tracks  the  voltage  applied  to  the  loudspeaker  terminals  and  uses  this  to  calculate  the  instantaneous  temperature  of  the  voice  coil.  "

As discussed above, voice coil temperature computed from voltage won't be correct, due to the complex load.

Some striking quote though:

" The difference between a coil temperature of 250C and 200C is 1dB. "

Shiver.
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John Roberts {JR}

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Re: Thermal limiter: why not based on RMS current?
« Reply #16 on: January 08, 2021, 02:21:14 pm »

That sounds a lot like what powersoft is doing in their amps.

I believe an old Crown (Delta ohmega?) that would do the same sort of thing, or so I remember, but it has been a few decades.
Jack got a patent on his circuit. At the time Crown was "advertising" amplifiers with silly high damping factor, but as anyone with a grasp of math understands a modest length of speaker wire will dominate damping factor even in a combination using a zero ohm output impedance audio amplifier. 

JR

PS : I am not aware that this is some unresolved problem.

PPS: The Peavey variable amplifier source impedance technology was also used in solid state guitar amps, in the opposite direction, to mimic the loudspeaker/amp interactions common with higher output impedance tube guitar amps. Peavey even tweaked the guitar amp source impedance differently for high and low frequency for different sonic impact.
« Last Edit: January 08, 2021, 02:28:37 pm by John Roberts {JR} »
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Art Welter

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Re: Thermal limiter: why not based on RMS current?
« Reply #17 on: January 08, 2021, 03:40:58 pm »

Some striking quote though:

" The difference between a coil temperature of 250C and 200C is 1dB. "

Shiver.
At about 232C (451F) paper starts to burn, a lot bigger problem than 1dB loss ;^).

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John Roberts {JR}

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Re: Thermal limiter: why not based on RMS current?
« Reply #18 on: January 09, 2021, 10:25:51 am »

At about 232C (451F) paper starts to burn, a lot bigger problem than 1dB loss ;^).
One of the advances in loudspeaker technology over the years is development of higher temperature adhesives and high temperature wire insulation... resulting in power compression instead of outright failure, when things get really hot inside. When the VC gets hot and it's resistance increases and it makes less output from the same drive voltage

JR

PS: (232 x 9/5) +32 = 449.6  close enough to 451 for government work. The formers and components in immediate contact with the voice coils are probably not paper these days. 

 

 
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Riley Casey

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Re: Thermal limiter: why not based on RMS current?
« Reply #19 on: January 09, 2021, 03:54:59 pm »

Ray Bradbury would be aghast

...
JR

PS: (232 x 9/5) +32 = 449.6  close enough to 451 for government work. The formers and components in immediate contact with the voice coils are probably not paper these days.

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Re: Thermal limiter: why not based on RMS current?
« Reply #19 on: January 09, 2021, 03:54:59 pm »


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