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Old 15th June 2004, 02:52 PM   #1
angel is offline angel  Norway
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Lightbulb Compression cancellation with dual voice coils

I don't know if anyone here has tried this, but I'd like some feedback on this idea..

As I understand it, power compression with dynamic drivers results primarily from voice coil heating and the positive temperature coefficient of the wire resistance. The same effect also alters other T/S parameters, like Qes and, hence, Qts.

Using a dual coil driver in a similar fashion to the output transformer of a push-pull valve amplifier, except with MOSFET transistors and a highish DC current, I'm thinking this should cancel.

Essentially, the dissipation in the coil remains constant, meaning that heat-derived power compression is eliminated. Also, since the coils are in counterphase to each other, the standing current may not displace the cone, depending on the winding technique. The AC is in-phase, of course.

Combine this with a biquad transform and a sealed enclosure, so as to match room gain, and I'd guess one would be getting very decent and natural bass
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Old 15th June 2004, 02:58 PM   #2
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Since this would be resemble a "class A device" efficiency would be quite low. And that counts for both - driver and amplifier !

In a "normal case" the peak to average ratio would make dissipation in the voice-coild significantly lower than your proposal.

For midrange and tweeters current-drive would be one solution to compensate for the thermal resistance-increase.

Meyersound has a solution that seems to work at the lower end as well, but I don't know how they do it.
Maybe they use the poweramp offset-voltage to measure the DC resistance of the coil and apply the result to some control circuitry.

Regards

Charles
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Old 15th June 2004, 03:13 PM   #3
angel is offline angel  Norway
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Quote:
Originally posted by phase_accurate
Since this would be resemble a "class A device" efficiency would be quite low. And that counts for both - driver and amplifier !
Yes, unless you employ feedback, the amplifier has to be operated with a balanced single-ended output stage, hence low efficiency. For those of us who enjoy the SoZ, for example, it should be quite bearable, though.

However, I fail to see how the efficiency (by which I assume you mean efficiency and maximum SPL) of the driver is reduced. Essentially, the DC cancels out, and produces no force, while the AC, being in phase, acts as if the voice coils were paralelled?

Quote:
In a "normal case" the peak to average ratio would make dissipation in the voice-coild significantly lower than your proposal.
Yes. And this variation between peak and average is the main source of dynamic compression in the speaker.

Heavy-duty speakers should have no problem with this dissipation; for example, the JBL2226 can dissipate 600Wrms continous. (Although that driver is not, to the best of my knowledge, available in a dual-coil version.)

For less rugged speakers, just keep below the rated continous dissipation.

Quote:
For midrange and tweeters current-drive would be one solution to compensate for the thermal resistance-increase.
Current drive is indeed an option, regardless of which type of driver you are using, but few, if any, drivers are designed for current drive, meaning that you will need to have quite a lot of compensation circuitry.

Quote:
Meyersound has a solution that seems to work at the lower end as well, but I don't know how they do it.
Maybe they use the poweramp offset-voltage to measure the DC resistance of the coil and apply the result to some control circuitry.
Sounds doable, but hardly high fidelity? They would, I guess, be using an automatic gain control circuit that aims to keep the DC current constant. Such a circuit can be difficult to implement without loss of fidelity, and it would only respond to changes in the DC characteristic of the load, whereas my approach, admittedly at a very high efficiency penalty, would cancel changes in the AC characteristic without the use of an AGC circuit or similar complexities.
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Old 15th June 2004, 04:30 PM   #4
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I wouldn't think that the speaker efficiency would be reduced, but rather that the power handling available for music would be reduced. although the forces produced by the opposing coils would cancel, the resistive heating would add.

For normal home use, it seems to me that this is a non problem. Since an average of a watt or two isn't going to heat the voice coils much anyway. Short peaks of a few cycles aren't likely to produce much of a temperature rise due to the mass of the voice coil assembly.

I'd be curious to hear the results of your testing, when you get to it. Adire has high power dual voice coil drivers that may be suitable.
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Old 15th June 2004, 08:54 PM   #5
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Quote:
the resistive heating would add.

........ leading to unnecessarily wasted energy and therefore reduced overall system efficiency. That's what I meant above.

Regards

Charles
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Old 15th June 2004, 09:37 PM   #6
angel is offline angel  Norway
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Quote:
Originally posted by BobEllis
I wouldn't think that the speaker efficiency would be reduced, but rather that the power handling available for music would be reduced. although the forces produced by the opposing coils would cancel, the resistive heating would add.
Yes, you will pretty much limit yourself to the long term max power. I don't see this as much of a problem, since short term power handling is rarely more than some 6dB or so above long term, and most woofers are excursion limited before they hit dissipation limits.

Since you are essentially using each coil as the load for one leg of a balanced single ended output stage, you do not reduce the power handling available for music: the total power remains constant, regardless of signal level. You are just alternating the same net current between two coils.

Quote:
For normal home use, it seems to me that this is a non problem. Since an average of a watt or two isn't going to heat the voice coils much anyway. Short peaks of a few cycles aren't likely to produce much of a temperature rise due to the mass of the voice coil assembly.
First off, it depends on your definition of home use. At realistic listening levels and full bass extension, you are going to peak more than a couple of watts into anything but the most efficient of woofers.

Secondly, discussing averages is not really interesting. We are talking here about dynamic compression, not static conditions.

Thirdly, due to the time it takes for the heat to distribute itself into the voice coil assembly (thermal time constant?), the voice coil wire is going to be heated quite a lot during a peak, causing dynamic compression. After the peak has passed, this heat has time to dissipate.

Quote:
Adire has high power dual voice coil drivers that may be suitable.
Thanks for the tip.

Quote:
Originally posted by phase_accurate
........ leading to unnecessarily wasted energy and therefore reduced overall system efficiency. That's what I meant above.
Ahh. Then we agree. The arrangement *does* waste a lot of energy to minimize nonlinearities by keeping conditions constant.
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Old 15th June 2004, 09:54 PM   #7
Svante is offline Svante  Sweden
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Hmm... A few thoughts here.
First, isn't the motion of the coil an important component of the cooling of a modern speaker? If that is case, the coil would end up warmer at standstill, and there would be a reverse compression effect.
Second, if the speaker is continously loaded with max power, the resistance would be higher, and this would lead to a higher Qts, which would have to be taken into account during the design, and this would also mean that the speaker would need some warmup time until it would sound as intended.
Third, I think that the lifetime of the driver would be seriously shortened if it was run at full power continously. But of course, I don't know.

I think it is a thought-provoking idea, but I think there is a reason why it is not done. Compression is a too small problem to go through all of these efforts, IMO.
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Old 16th June 2004, 08:55 AM   #8
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Quote:
Second, if the speaker is continously loaded with max power, the resistance would be higher, and this would lead to a higher Qts, which would have to be taken into account during the design, and this would also mean that the speaker would need some warmup time until it would sound as intended.
Good thoughts.
The warmup time of the coil can be controlled however, since it is possible to measure DC resistance of the driver and control the heating current accordingly.

But I still don't like the idea.

And there's another drawback: Not only the DC resistance influences Qes, also the magnet's temperature, which has a very large time-constant compared to the voice-coil. And this effect would be more difficult to control than the coil's Re.

The two best things against thermal power compression is the use of generously dimensioned voice-coils (both drivers of my two-way system use 3" coils !) or the use of efficient drivers with light coils.


BTW: The intentional heating of voice-coils is indeed used for some outdoor installations (using low power ultrasonic noise) in order to keep humidity out of the air-gaps.

Regards

Charles
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Old 16th June 2004, 10:06 AM   #9
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I have a design at home from, I think, Electronics World, for a subwoofer that uses a dual coil driver, where the second coil is used in the feedback path of the amp. If I remember correctly, the claim was of less than 1% distortion at 20Hz.
Would this work up to mid-range frequencies?

Arnie
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Old 16th June 2004, 11:37 AM   #10
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Default Compression is mostly mechanical

Before you worry too much about the electrical aspects consider that power compression is mostly a mechanical problem. When the cone/voice coil travel approaches Xmax the 6dB/octave excursion equation means that at lower frequencies the suspension puts finite limits on output capability while at higher frequencies the cone is able to travel freely with much higher input levels. If the driver is mounted in an enclosure that adds acoustic resistance, be it a bandpass or a horn enclosure, that acoustic resistance is also non-linear, being another source of limiting cone movement at lower frequencies but not high frequencies and thus being another source of non-linear response.
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