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Class D Switching Power Amplifiers and Power D/A conversion

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Old 28th December 2003, 09:40 AM   #111
MWP is offline MWP  Australia
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It seems that the output filters are a source for a lot of problems with PWM amps.

My understanding is that they exist to stop the speaker cables from becoming HF antennas.

So.... if the output stage of the amp was close enough to the speaker (like inside the box), could the O/P filters be removed?
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Old 28th December 2003, 10:59 AM   #112
Pan is offline Pan  Sweden
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It would be interesting to se distortion measurements of a pwm amp with pre-filter feedback.... measured both before and after the filter..?

Is the filter really adding this "much" distortion? In such case a normal passive loudspeaker filter would be suspect to add "lots"
of distortion as well... does this explain why people report such great benefits from going active?

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Old 28th December 2003, 11:11 AM   #113
subwo1 is offline subwo1  United States
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That is why I would avoid passive crossovers, even ones which are air core. I hope to try air core filters on a PWM amp even though the feedback is taken after the filter.

MWP, some manufacturers seem to do that already for cell phones, etc., at least according to data sheets which describe "filterless" class D circuits.
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Old 28th December 2003, 12:52 PM   #114
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It seems that the output filters are a source for a lot of problems with PWM amps.

My understanding is that they exist to stop the speaker cables from becoming HF antennas.
Maybe in theory.........

In reality, most still become antennas. Especially long ones.

Some of the PWM moduls discussed here probably work fine in powered speaker systems, but would be an EMI disaster with 6 m speaker cables.

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Old 28th December 2003, 02:29 PM   #115
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Texas Instruments have output filterless PWM chips such as the TPA2000:

However increasing power levels would make it difficult to keep EMI within regulations limits for this approach. But for at powered PC speaker, laptop, TV set or transistor radio, a chip like TPA2000 would be the perfect choice. On the other hand you can almost build a cheap analog amplifier of 2W without heatsinking as well, so the bucks you spend on the PWM would only give you extended battery life, and added 'cool effect'.

Speaker Wires as HF noise emitting antennas: in any PWM amplifier there are two different EMI problems: The switching frequency 300 - 500 kHz typical could give you problems in AM radio reception, on long wires, and the other is VHF - UHF remains from the switching slopes of the power stage. Both can be dealt with.

6 m. speaker cables have a good antenna effect at 12 - 18 MHz (quarter wavelength) , so to give any substantial transmission you need a lot more than 1 V rms. But if you want it can be removed simply by means of an output filter and subsequent parallel resonant circuit in series with the output. At the 500 kHz (Frequency has to be known), the resonant circuit has a resistance of 50 - 100 Ohms, while a 100nF has 3 Ohms. So the remaining carrier noise is attenuated 15 - 30 times. If you need more attenuation, simply add a choke to the grounding filter cap, to form a seral resonant circuit at the same freq. Typical output of this type is 0.5 - 1 V rms without filtering. After resonant circuits the remaining voltage will be 20 - 66 mV RMS. The serial impedance of the resonant circuit is low, as you only need around 1 uH and 100 nF (not exact values!!).

On the other hand if your speaker cable is 180 Meters long, then you might have a real problem with a 500 kHz carrier frequency.

VHF / UHF residuals mainly transmit from the power supply wires, and the output circuit itself. NOT the speaker wires (if just simple measures are taken to prevent it).ī

The most effective way to get these emissions down is using ultra short (HF emitting) signal path, good fast power decoupling and a 4 layer PCB can help a lot too, as most of the HF energy gets caught in the ground layers.

AIR cores for output filtering, we started out with that in the beginning, thinking the low THD would benefit sound quality. But as it turned out the Ferrite cored ones sound much better, even if the THD is higher. Why the preoccupation with THD?
Further the air cores significantly adds to the HF emitting part of the signal path. You have something like 3 Meters of coil wire with 50 - 200 MHz slope energy on it.
Now that's what i call an antenna!
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Old 28th December 2003, 04:10 PM   #116
Pan is offline Pan  Sweden
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"Why the preoccupation with THD?"

With the new high rez digital performance of very low distortion and very high dynamics, and also new generation speaker drivers with 0.1-0.01% THD at 90-96dB (mid and high fr.), it seems like it would be possible and beneficial with amps at least 1/10 the distortion of speakers, if not as low as the distortion of the digital sources.


I remember reading some time ago, about you using 1MHz switching in the Zapulse... why not now, would it not be beneficial for the high frequency performance of the amp?

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Old 28th December 2003, 05:04 PM   #117
km is offline km
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nelson i think it would be very interesting to hear what work you did on pwm amps in the past and what suggestions you have for further improvements , any chance you could share your thoughts with us here?

about thd i agree with you completely , one can focus so much on the thd meter one completely forgets to listen (no names mentioned but i think some of you have got the picture)

about emi i think we have a couple of good examples of "emi-disasters" , wasnt that a suitable describtion of your zappulse 1.0 lars?

lets see what we can do about listening tests and comparisons , we are looking at 2 projects with finished pwm amplifiers for the hifi market at this point , i think you should be able to hear one of them within reasonable time (and others of course too)

what drivers are you referring to with 0.01% thd at 96 db pan?

best regards - karsten madsen -

ps. i do not agree that active crossovers in hifi speakers is better than passive ones , long time experience says that the opposite is very often true....
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Old 28th December 2003, 05:45 PM   #118
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There is no real dispute that these amps are not as evolved as the more traditional design and I cannot see buying one for much over $1K it is surprising that the consumer can get adequate performance from the Panasonic recievers that are going for under $300.

I can't say they are as good as my reference gear,but they do very nicely for value orientated consumers that go to BB or CC. They outdo the run of the mill mass market AB class junk recievers for a clean presention and great Bass which is what I think they do good.No glare or strident sound. Not bad for 300US bucks.

I agree with the comments posted by NP
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Old 28th December 2003, 06:31 PM   #119
Pan is offline Pan  Sweden
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Seas Excel W22 about 0.1% at 96dB in the mids. Accutons diamond tweeter 0.01% at 90dB in the high range. Think there are some ribbons with very low THD as well at 90dB, but donīt remember any specific brand/driver. Maybe I was not so clear in my post.

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Old 28th December 2003, 06:39 PM   #120
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Pan: All versions of ZAPpulse can run 1 MHz, however i am not convinced there is much difference in high frequency performance, when running or 500 kHz or 1 MHz.
But i can see why it could obviously be expected so.

km: 1.0 you are right, it had way too high EMI.
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