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Class D Switching Power Amplifiers and Power D/A conversion

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Old 15th May 2006, 08:10 PM   #511
IVX is offline IVX  Russian Federation
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..Unbelievable, the new ferrite better in highs, but worse for middle in four times! I never seen such effect of the output coil. In my experience, properly gaped core give almost the same THD figures for many (if not to say "any") ferrites. I can't understand what is the reason for such THD "equalization"..
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Old 15th May 2006, 08:20 PM   #512
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IVX: Yes i was also baffled by this effect. But it is probably because the lower permeability requires a lower biasing of the core, and thus gives the higher THD at low freq but lower THD at higher freq. Thats the only explanation i can find at this point.

This new ferrite material also features lower THD at power than the ferrites i have used before. See below:

(40 Watts RMS 8 Ohms )
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Old 15th May 2006, 09:17 PM   #513
kims is offline kims  Denmark
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Hi Lars

The THD as function of Rpsu is given by the follwing eq for BD-mode:

THD = Rpsu*M^2/(4*Rload)

for AD-mode this eq does not hold - but will at least be lower than that.

Rpsu = 6mR, M=0.94 (max modulation), Rload=8R

6mR*0.94*0.94/(4*8R)=0.016% - not far from the measured results.

With these NON-feedback designs the demand for feedback is moved from the amplifier to the power supply - some where the 50/60Hz signals from the mains need to be filtered and regulated - so for a 6ch design using a common PSU, we have one feedback system e.i. the SMPS, handling the mains frequencies, 100Hz ripple and harmonics thereoff.

Making the SMPS having low output impedance is hardly any issue today with the knolege on feedback systems used in manny class-d amps (just a SMPS with a varaible reference voltage).

Take any class-d amp with feedback, feed it with a dc-voltage, and you have a very good SMPS - and it can even sink current (that many SMPS can not).

The THD graphs you show looks very good - what material is used for the inductors (we use Micrometals -2)

rgds,

Kim
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Old 19th May 2006, 12:07 PM   #514
mikeks is offline mikeks  United Kingdom
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Default Re: 0.01% THD

Quote:
Originally posted by kims
Low THD is not nessesarry a measure for good sound -
Wherever did you get this idea..
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Old 19th May 2006, 01:26 PM   #515
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Default Good sound vs THD

Hi Mikeks

I realise I'm probably opening a can of hysterical and angry worms here, but this is always good fun, so:

I would firstly ask you, have you ever heard what you would classify as 'good sound'. Anywhere, anytime, anyplace?
Maybe you have? Surely yes?

And was that sound reproduced through loudspeakers?
Yes?

Well then the 'good sound' you heard was subject to THD far in excess of the of 0,01% you would set as a criterion for amplifiers. A typical loudspeaker has THD+N of the order of 0,5% to 1% at best, perhaps more if driven hard. Including harmonic and an-harmonic distortion.

Therefore it would certainly seem that to get 'good sound' one could safely forget about all THD unless it was above about 0.1%.


Think holistic.

Have a good weekend and Skol
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Old 19th May 2006, 02:07 PM   #516
mikeks is offline mikeks  United Kingdom
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Default Re: Good sound vs THD

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Originally posted by John Hope
Therefore it would certainly seem that to get 'good sound' one could safely forget about all THD unless it was above about 0.1%.


I agree...Cheers!
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Old 21st May 2006, 01:04 PM   #517
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quote:
Quote:
Originally posted by John Hope
Therefore it would certainly seem that to get 'good sound' one could safely forget about all THD unless it was above about 0.1%.
That would certainly seem like a good excuse for not solving the weak spots of your amplifier, get your office sleeping pillow out, and forget all about making 'good sound' into great sound.

Cheers

Lars
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Old 21st May 2006, 03:09 PM   #518
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Lars

I was making a point to Mikeks.

So why do I - or any other amplifier designers - pursue these goals of performance then? Probably because we can. And we will never be satisfied. But it sure beats the hell out of watching the TV, playing Sudoku, or building ships in bottles.


Cheers
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Old 23rd May 2006, 10:50 PM   #519
matjans is offline matjans  Netherlands
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nothing beats watching reruns of 3d rock from the sun

on a more serious note, has anybody tried building a power-opamp based psu for use with a ti tas5152 or similar? i was thinking about an opa549 or something similar.

no flames for not using an smps please
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Old 17th December 2007, 01:53 AM   #520
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Quote:
Originally posted by Eva
Power Supply Rejection Ratio is 0dB in these systems!!
Why PSRR is more important in class-D amps then in say SET amps?
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