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Class D Switching Power Amplifiers and Power D/A conversion

CLASS AB Amplifier Vs CLASS D Amplifier
CLASS AB Amplifier Vs CLASS D Amplifier
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Old 16th June 2012, 04:14 PM   #111
5th element is online now 5th element  United Kingdom
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That depends, I don't know if performance like that of the ncore is achievable if the filter is outside of the loop. I am talking of distortion here rather then anything else. The ncore is rather special in how low it's measured distortion is and no other class D amp with the filter outside the loop even comes remotely close to matching it.
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Old 16th June 2012, 09:49 PM   #112
ViennaTom is offline ViennaTom
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Quote:
Originally Posted by ChocoHolic View Post
Class I can be temptating, but is basically just multiphase - which of course deserves a new letter....
Take care, aside multiple obviously temptating properties it has less obvious down sides:
- The two paralleled stages must have excellent DC-properties, otherwise you will get unpleasant DC balancing currents.
- The switching residuals that are being injected back into the modulator through the feedback have double frequency of the modulator carrier and the resulting distortion mechanism appeared difficult to cure, while I found a simple but extremely effective method for traditional designs.

Both topics are not necessarily show stoppers. It is more a question of your target applications.
For very high power the extra efforts for the DC-balancing won't bother in costs and complexity. Also nobody will panic, because of slightly higher THD - and of course there might already exist appropriate cures of these specific double-fs-feedback/modulator distortions.
The two stages need not have excellent DC-properties, since DC bias has to be actively controlled anyway to avoid runaway, which is inevitable w/o active control. This topology basically needs 2 control loops: 1st for output voltage, 2nd for bias current.
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Old 17th June 2012, 10:18 AM   #113
ChocoHolic is offline ChocoHolic  Germany
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Originally Posted by ViennaTom View Post
This topology basically needs 2 control loops: 1st for output voltage, 2nd for bias current.
Yup, the 2nd control loop is what I adressed by the 'extra efforts for the DC-balancing'. I also agree that the inherent DC-accuracy , or better inaccuracy, without bias/balancing loop, will almost guarantee trouble except we use lossy resistors for paralleling - or air coils with high Rdc in the filter.
==> which is leading us again to the discussion, which solution might sound better. The one and only question.

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Originally Posted by opc View Post
In what ways does a class D amplifier offer better sonic performance than class A and class AB?
Please stick to technical justifications, and not "I own X brand of class D amplifier and it sounds the best"
Regards,
Owen
In fact it is easy to reach clarity about technical advantages and disadvantages of different solutions, but the impact on sound is not always easy to foresee.
Many sonic myths might have their origin from certain set ups, where the sonic effect was real, but the given explanation was incomplete or sometimes just rubbish.

Back to class I / dual phase:
What is the benefit of biasing the two stages with a certain DC instead of biasing them to zero differential bias (==>balance)?
Basically I would just expect that the two typical areas of non linearity (in the range were load current and peak filter ripple equal) would be shifted.
In any case I would expect that the output nonlinearity of the dual phase is less than in single phase, because of the time shift between both dead times at the critical load current ranges.
I did not dig into full depth of this circuitry. So I am curious about Tom's findings (in case you intend to dig deeper).
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Old 17th June 2012, 10:46 AM   #114
ViennaTom is offline ViennaTom
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"So I am curious about Tom's findings (in case you intend to dig deeper).[/QUOTE]"

As usual, my simulations focus 1st on getting the topology to work stable with as high loop gain as possible. Since I am simulating in the time domain with simetrix, I can only see a sine wave, square plus Overshoot, etc. at the output with the switching ripple superimposed. I can also try to find effective output impedance vs. frequency by using different load impedances and comparing the voltage. But: For me, there is no way to determine if the distortion is 0.02 or 0.01 % THD, as long as the distortion is not visible from the output waveform.
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Old 17th June 2012, 01:33 PM   #115
ChocoHolic is offline ChocoHolic  Germany
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Originally Posted by ViennaTom View Post
As usual, my simulations focus 1st on getting the topology to work stable with as high loop gain as possible. Since I am simulating in the time domain with simetrix, I can only see a sine wave, square plus Overshoot, etc. at the output with the switching ripple superimposed. I can also try to find effective output impedance vs. frequency by using different load impedances and comparing the voltage. But: For me, there is no way to determine if the distortion is 0.02 or 0.01 % THD, as long as the distortion is not visible from the output waveform.
Well, 0.01% is already pretty nice. In fact in my simulations the distortions caused by switching residuals that are feed into the modulator by the feedback were more in the range of >0.1% - especially at higher modulation levels, say above 50% of max output voltage.
I am using LT spice and let it calculate the FFT from the output wave form.
But of course you are 100% right, that simulation of low distortions is sort of gambling. ...have to build a proto, in order to countercheck theory and simulation with reality.

Last edited by ChocoHolic; 17th June 2012 at 01:36 PM. Reason: ...repair of quoting..
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Old 18th June 2012, 07:49 AM   #116
CENTRAL is offline CENTRAL  Greece
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What would you think of these measurements?

Frequency response 0.3 Hz – 33 kHz -3 dB points, 8 Ohms load.
Frequency response -0dB/+0.2dB 20 Hz - 20 kHz, 8 Ohms load
Frequency response -0.2dB/+0dB 20 Hz – 20 kHz, 4 Ohms load
THD+N, 1 W /8 Ohms 0.004% A-wgt.
THD+N, 1 W/4 Ohms 0.006% A-wgt.
THD+N, 100 W/8 Ohms 0.01% A-wgt.
THD+N, 180 W/8 Ohms 0.07% A-wgt.
THD+N, 275 W/4 Ohms 0.07% A-wgt.
S/N ratio 117 dB A-wgt. ref 200 W/8 Ohms.
Channel separation 84 dB 1 kHz, 200 W/8 Ohms.



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Old 18th June 2012, 11:45 AM   #117
ViennaTom is offline ViennaTom
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@Choco: Munich is not too far from Vienna - If you happen to come over to my place for some other reason maybe, I could show you my Simetrix simulations in exchange to your LTspice sims and we could work together, maybe for some hours...
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Old 18th June 2012, 12:52 PM   #118
Melon Head is offline Melon Head  Australia
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I am Class A man myself but I have just purchased my first Class T pcb.
When I have put it together I will let you know what I think.

Edit: On paper at least what I like about the Class T amps, is that the distortion does not increase with reducing impedance (according to the graphs I have seen), as opposed to Class AB amps which distort more as speaker impedance falls.

Last edited by Melon Head; 18th June 2012 at 12:57 PM.
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Old 18th June 2012, 08:00 PM   #119
darkfenriz is offline darkfenriz  Poland
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Quote:
Originally Posted by ChocoHolic View Post
- The switching residuals that are being injected back into the modulator through the feedback have double frequency of the modulator carrier and the resulting distortion mechanism appeared difficult to cure, while I found a simple but extremely effective method for traditional designs.
Congrats, Markus, you've therefore achieved what not many came close to.
For the record, all my intergate&reset, sample&hold and similar efforts have been a plain failure :P
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Old 18th June 2012, 08:11 PM   #120
ViennaTom is offline ViennaTom
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"The switching residuals that are being injected back into the modulator through the feedback have double frequency of the modulator carrier and the resulting distortion mechanism appeared difficult to cure, while I found a simple but extremely effective method for traditional designs."
Since in class I, no adverse body diode / dead time issues exist, I would suggest to use a high performance schottky diode and ramp up the frequency as high as possible. Why not go 2 x 500kHz or greater? Then the residuals will get way smaller.
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