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mix current drive with voltage drive at LF?
mix current drive with voltage drive at LF?
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Old 17th July 2017, 01:04 PM   #51
keantoken is offline keantoken  United States
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Actually the modulation is entirely due to the LF content of the input. So a good HP filter improves it dramatically. Rumble filter by default?
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Old 17th July 2017, 09:35 PM   #52
Zero D is offline Zero D  United Kingdom
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Crown amplifier circuit diagrams/manuals etc etc, are available Directly from Crown Audio - Professional Power Amplifiers In particular, download & look @ Macro-Tech MA5000VZ schematics
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Old 20th July 2017, 08:05 AM   #53
IanHegglun is offline IanHegglun  Australia
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Default Oops in Post #46 :o

Quote:
Originally Posted by IanHegglun View Post
To show the response time of the Birt method of measuring the voice coil resistance using a 1mA DC bias I have first made a voice coil model with electrothermal feedback.
Oops. The voicecoil resistance temp. co. was not correct in post #46. Source "B1" needs to include "Re". It was out by a factor of 6.

The attached files are the corrected ones for post #46.

Fortunately it doesn't change the time to reach 150 degC vc limit.

But it does change the power level to the speaker where thermal runaway occurs using current drive; it should have been 100W (not 600W).

This mistake also affects post #47 for the Birt sims. I will update them as a following post.

My apology to all.
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Old 20th July 2017, 08:30 AM   #54
IanHegglun is offline IanHegglun  Australia
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Default Oops in post #47

Quote:
Originally Posted by IanHegglun View Post
Oops in post #46...This mistake also affects post #47 for the Birt sims.
The mistake in post #46 fortunately does not affect the time constants so we get similar outcomes with the mistake fixed.

But the power levels of the sim need to be reduced to stay below thermal runaway; in this case below 100W.

The corrected sims are attached.

There is still the large initial transient when the 20Hz starts and about a 2 second delay to settle to a reasonable temperature reading.

I'm still thinking of a way to suppress this transient. Using a Bessel filter didn't seem to help. Any ideas?

Keantoken's posts #50,51 scales Vin to the power amp (eg a two quadrant multipiler/volume) with V(Res4) to increase the gain in proportion to the real-time vc resistance reading, which works after it has settled -- but the first part of the 20Hz is modulated too much to be usable; illustrating clearly the need to suppress this initial transient error some how.
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Old 8th August 2017, 09:51 AM   #55
IanHegglun is offline IanHegglun  Australia
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Default Update

Quote:
Originally Posted by IanHegglun View Post
...There is still the large initial transient when the 20Hz starts and about a 2 second delay to settle to a reasonable temperature reading.

I'm still thinking of a way to suppress this transient. Using a Bessel filter didn't seem to help. Any ideas?
Hi All,

Recap: This thread covers using current drive of loudspeakers at MF+HF and with voltage drive at LF -- current drive at at MF+HF reduces speaker distortions which mainly occur in the MF region while undamped resonance boominess that is the problem with pure current drive. It does not need an equaliser to remove the resonance boominess of pure current drive and is not sensitive to T/S parameter drift with temperature and aging. But there's a problem -- voltage drive at LF brings back volume compression and reduced damping at high power -- two effects which which pure current drive eliminates. So what to do? To overcome volume compression and damping loss at high power we can build compensation for voicecoil resistance change into the power amp feedback path -- this can be done by generating a temperature variable negative resistance to 'eat up' the extra resistance of the voicecoil as it heats up. But to do this compensation we want an accurate estimate of the voicecoil temperature in real time otherwise you get unwanted gain modulation a.k.a. distortion. The Birt method was suggested as a simple way to obtain the voicecoil temperature in real time without any calibration, by forcing 10mA DC through the speaker and use a 1Hz LP filter is used to remove the AC music signal. But with a 20Hz tone-burst we get a massive DC transient and it takes a few second to settle. This is unusable for compensating the voicecoil resistance change in real time. BTW David Birt's compensation worked well on a mid-range speaker (>300Hz), but for a woofer down to 20Hz we get this massive transient a few seconds settling. After trying several ideas none were able to suppress the transient to less than 10 degC, and the shortest settling time was 0.5 second which is still not fast enough for real-time compensation and fast speaker protection.

So what to do? I have returned to my thermal model (Post #30-32) but now with a RC thermal model instead of the self-heating 0.1 ohm resistor.

But a thermal model needs to be set to suit the speaker. Some bench tests are needed to see if we can get away with using typical thermal model values, possibly based on diameter, power rating, etc. Anyone interested? Unfortunately, I am still several months away from any bench tests.

Until tests are done I'll include the Birt method for use with with a steady test tone for calibration.

The extra parts for the Birt measurement and tone generator are minimal; they could be part of a speaker protection PCB. To calibrate a speaker you flip a switch to engage the test tone, wait 10 seconds and read a DMM and adjust a thermal model trimpot to get the same voltage as the Birt temperature. Then return the calibrate switch back to Run and it's done.

The attached simulation shows the basic circuit in operation with a woofer in a cabinet. The top plots compare the Birt temperature reading with the thermal model reading (these plots are after after calibration). Notice the Birt plot transient (brown) and then several seconds to settle to within 10 degC. This is with a 100Hz sinewave starting at V=0 at t=50ms. Notice also the thermal model reading (pink) has no transient and no settling time delay. Nice.

The lower two plots are the same information as the top plots but scaled in degC (1V/degC). The input voltage to the power amp was chosen as 15V to give a final temperature of 70 degC, which is safe for calibration. The green line at 150 degC the limit for most speakers. BTW T(vc) red plot is under T(vc2) plot, showing the excellent accuracy from the I^2*Rvc estimator after calibration. Nice.

As for the basic circuit (attached):
  • The 1st thermal model is for generating the voicecoil dc resistance change with power -- it is not required for a practical circuit when a real speaker is used.
  • The 2nd thermal model is for estimating the voicecoil resistance using I^2*Re*(1+0.004*Tvc2) for a compensation circuit (not included here). The 2nd thermal model can use a MOSFET for squaring and another MOSFET for the temp. co. multiply to keep it simple.
  • Voltage source V4 is for thermal model calibration -- B4 changes B2's resistance. In a practical circuit B2 is a trimpot for calibration of the thermal models thermal resistance. B2 could be an E2Pot so a micro could do the calibration automatically.
The calibration frequency should be above resonance to get enough power to heat up the voicecoil without hitting the limits of voicecoil travel. On resonance the impedance may be too large to get enough heating. For a woofer 100Hz to 300Hz region can be used for calibration. Above 500Hz a woofers voicecoil inductance starts to limit the voicecoil power.

This temperature estimation method can also be applied to a pure voltage drive power amplifiers -- to remove driver compression, damping loss and crossover misalignment issues. Once this temperature estimation method has been validated by bench tests, a separate thread could be started for those who want to apply it to voltage drive power amplifiers and speaker protection.
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Old 13th October 2017, 04:56 PM   #56
Jerry R is offline Jerry R  Netherlands
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Default Why compensating for temperature?

I see a lot of discussion about compensating the rise of temperature in the voice coil. I donít see the problem.

Since May (2017) I have an active current-drive configuration in use. Consisting of:
* 2-way loudspeakers with Scan-Speak drivers and acoustic suspension and without passive filters
* A four channel current-drive amplifier, based on the TDA 7293
* A miniDSP DDRC-24 as pre-amplifier, with Dirac Live and a four channel DAC

It sounds amazing. So I got rid of my Denon amplifier and Acoustic Research speakers.

I use Linkwitz-transformers in the DSP for resonance damping and crossover by shaping the f and Q of the drivers both at the low end and the high end of the driver response. I use acoustic suspension in stead of bass reflex, so the woofers are stable below resonance. The woofers have a fs of 30 Hz, in the enclosure it gives a fb of 60 Hz. The Linkwitz-transformers can change that to a resonance with a f of 30 and a Q of 0.85 (or even lower, the f and Q are adjustable!). The woofer has sufficient displacement (Vd), and music has low power in the lower region, so no problems and distortions here.

Again, I donít see the problem of raising temperatureÖ

I think the only influence of a rising temperature (as with every rise in impedance) is that the voltage needed for creating the current rises. It has no influence on the sound! If the needed voltage is too high the amplifier starts clipping.

A nice feature of the TDA 9293 that is has a clipping indicator. In the above mentioned configuration I have never seen it clipping. And it can play loud! I have seen it clipping with the Acoustic Research speaker during testing. That was with female voices. I think because of a huge impedance rise at crossover.

Another thing is that voltage-drive has the greatest power demand where the impedance is the lowest and the power in the music the highest (with the 50-50% border around 350 Hz). The greatest power demand with current-drive is where the impedance rises, at the ends of the bandwidth, where the musical power is the lowest. And, around the resonance, where the impedance rises, the demand is even lower than in the middle of the audio range because of the Qms. The higher the Qms the lower the power demand. Also, I selected 4Ω drivers to further lower the power demand.

Then the T/S parameter drift. The result of a parameter drift is a change in resonance frequency and/or Q of the resonance. Both have the effect of a bump or dip in the frequency response. Some comments on this:
* I havenít seen this. I think the effect is minimal.
* A recompute of the Dirac Live DRC compensates the disturbances, if there is any.
* A more fundamental solution is to recalibrate the Linkwitz-transformers in the DSP by measuring the driver response with close range miking.

So, I would suggest to use current-drive only, also for the low frequencies.
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Old 15th October 2017, 05:35 PM   #57
soundbloke is offline soundbloke  United Kingdom
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The audibility of thermal modulation (or its absence) frequently appears as a topic for discussion in this forum often without proper qualification, as does the associated subject of voltage vs. current drive.

For the record, unless anyone has evidence to the contrary, we can assume that the thermal time constant in a typical driver is so long as to render thermal modulation of its output inaudible.

The exception to this conclusion arises where we employ a multi-way system and where one band in that system compresses more than another. Here then thermal modulation produces a change in the frequency balance of the complete loudspeaker - normally manifesting itself as a 'softening' of bass notes.

Since there is little low frequency difference in stereo channels, applying a similar argument to stereo loudspeakers, for example, appears unnecessary. In passive loudspeakers, however, we might also postulate an additional change in the crossover alignment.

Also for the record (as I have mentioned in other posts on this forum), I have been using a modified version of Birt's incredibly elegant solution for some time as a means of more than just implementing current drive - albeit always in an active system. In high power applications (i.e. studio monitors and the like), the lack of softening in the bass registers is audible and IMO implementing current drive is worth the effort. However, in all situations including more normal domestic applications (admittedly still active loudspeakers), I can report that the absence of MF colouration due to magnetic non-linearities using current drive has IMO always proved by far the most significant improvement.

Last edited by soundbloke; 15th October 2017 at 05:39 PM.
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