My very first Class D pwm (switching) amplifier. - Page 11 - diyAudio
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Class D Switching Power Amplifiers and Power D/A conversion

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Old 20th June 2003, 09:21 AM   #101
ssanmor is offline ssanmor  Spain
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I know. It was me!
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Old 20th June 2003, 10:38 AM   #102
koldby is offline koldby  Denmark
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LOL

Could be very interesting to take the feedback from HT side of outputfilter and use that as integrator filter instead of the LF356/C8 integrator

Koldby
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Old 20th June 2003, 11:00 AM   #103
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This Danish thing is indeed one of the self oscillating versions of a pwm amp.
The ICE power modules by B&O are self oscillating and also some amplifiers by Philips.

The oscillating frequency of the above circuit is determined by the integrators timeconstant and the following schmitt-trigger's hysteresys voltage (it is in fact the very same principle that you use when building multivibrators with OP-AMP, one capacitor and three resistors).
Some more modern self oscillating amps make use of phase shift alone to determine the oscillating frequency (i.e. they use a comparator rather than a Scmitt-trigger). Some proactively include the output filter in order to achieve the necessary phase-shift to achieve reliable oscillation (specially the Philips one , which some even claim to be one of the best amplifier principles available today).

Regards

Charles
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Old 20th June 2003, 11:53 AM   #104
ssanmor is offline ssanmor  Spain
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Charles, do you think that making it self-oscillating really makes a difference with respect to the traditional PWM modulator in sound quality?
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Old 20th June 2003, 12:09 PM   #105
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I personally don't like the approach, though it has two major advantages:

1.) simplicity
2.) higher NFB factor due to the fact that the unity gain point is a the switching frequency rather than half of it as it is the case for an ordinary PWM amp.

The disadvantage is that you can't synchronise multiple amps to the same frequency (or even to an integer multiple or divider of an SMPS switching frequency), which could reduce IM problems on the supply rails (which in turn can feed through to the output).

As you can see, I used a quartz controlled square-wave oscillator whose output was made available to off-board devices.

Regards

Charles
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Old 20th June 2003, 12:13 PM   #106
ssanmor is offline ssanmor  Spain
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Sorry, but I don't understand what you mean with NFB factor and its comparation with conventional PWM.

Sergio
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Old 20th June 2003, 12:29 PM   #107
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NFB factor is the gain ratio between open-loop gain and closed-loop gain. It determines by how much THD, IMD and output impedance is reduced by NFB.

The unity-gain point of a class-d amp (being a device that does discrete-time processing) is defined by it's switching frequency.

In simple terms: If you can move the unity-gain point up by one octave (which is automatically the case for the self-oscillating principle) you will achieve an additional 6dB of feedback for a single first-order feedback loop.

Regards

Charles
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Old 20th June 2003, 12:34 PM   #108
koldby is offline koldby  Denmark
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Couldn´t there be another advantage in the self osc. circuit: Freedom from:
Noise leakthrough from the triangel and inaccuracy (jitter) in the clock??

Koldby
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Old 20th June 2003, 01:25 PM   #109
JohnG is offline JohnG  United States
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"The unity-gain point of a class-d amp (being a device that does discrete-time processing) is defined by it's switching frequency.

In simple terms: If you can move the unity-gain point up by one octave (which is automatically the case for the self-oscillating principle) you will achieve an additional 6dB of feedback for a single first-order feedback loop."


I find this whole thread to be very interesting. I had a couple of questions.

Regarding the "unity gain point", what exactly is this defined in reference to? For the conventional PWM PS or amp, I assume the duty-cycle "d" is the input. I believe the gist of what you are saying is that the response of the power stage to changes in "d" decreases with increasing frequency of "d". Is not the response more limited by the output filter, or are you talking about a response of the averaged power waveform before an output filter?

Also, is the frequency of the self-oscillating system defined, or does it vary with input signal? The input is no longer "d", but instead a reference voltage to an integrator, if I understand correctly.

Thanks for an interesting discussion,
John
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Old 20th June 2003, 02:28 PM   #110
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With unity gain point I mean the frequency, where the gain of the feedback LOOP drops to 1. The phase-shift at this frequency determines the so called phase margin (i.e. the amount missing to 180 degrees) and hence by a large part the step response of the system.
The desired phase margin is 90 degrees usually though this is hard to achieve if you take feedback from the filter.
If you take it before the filter, this target is quite hard to MISS (something designers of linear amplifiers only dream of !!).


The frequency of most self-scillating designs is quite independant of input signal.
The switching frequency of delta-sigma amplifiershowever is heavily signal dependant.

Regards

Charles
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