Effect of feedback in Class-D amplifiers - diyAudio
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Old 21st January 2004, 11:00 AM   #1
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Arrow Effect of feedback in Class-D amplifiers

Recently , i came accross an TRANSAMP500AX amplifier which is using class-d topology and its listening tests performs very well and it surprised me a lot when i glanced the specs:
Inter Modulation Distorion=NIL
THD=0.001%
Switching frequency=500KHz
power out =500WRMS@4Ohms
This amp states that is doesnt use feedback , therefore there is no presence of IMD components in output.

I want to know that is this is possible that a Class-D amp without feedback will give such results.
CAN we also eliminate the feedback to get such results.
Then what type of effects does the feedback have on output of amp.
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Old 21st January 2004, 10:47 PM   #2
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lol, wot trannies does it use? and how are they biased?
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Old 21st January 2004, 10:50 PM   #3
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Biased?

He said it is class-d.
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Old 21st January 2004, 10:55 PM   #4
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good point

thats wot you get for not sleeping :S
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Old 22nd January 2004, 06:10 AM   #5
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Have you got any further info about the maunfacturer etc ? Please keep the following in mind:
1.)
Many manufacturers state that they would work without any NFB even though they definitely do.
2.)
In theory a precise PWM modulator, a stable PSU (!) and a perfect switching stage would give a perfect amp even without any type of feedback.

As we know, nothing is further from the truth in practice. But from the TI/TacT approach we know that very good switching amps are feasible without NFB.

It is generally advisible to build a switching amp in a way that it performs well, before any kind of feedback is applied. I know that it is feasible. Though I didn't measure the one I once developed without feedback, I assume that its open loop THD must have been below 3%. Now try to do this with an ordinary class AB topology !

Regards

Charles
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Old 17th August 2005, 06:23 AM   #6
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Hi Charles,

How about this setup...

Triangle generation with LM6172 than feeding it at inverting terminal of differential comparator LM360 & audio signal directly fed to non inverting terminal and differential outputs of LM360 are connected to IR2113 gate driver thus driving the Mosfets...

Could we use LM360 differential comparator in the place of LM319+CD41070 for both Hi and Lo pulses fed to gate driver circuit...[assuming single supply operation , no level shifting]
I think the outputs of LM360 are not open collector type, so no need of pull up resistor..i am i right
Could you please post your valuable comments about this...

with sincere regards,
K a n w a r
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Old 17th August 2005, 07:21 AM   #7
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This would be a simple, elegant and fast solution. But it would be more expensive as well, especially when the speed advantage is not needed or even detrimental.


Regards

Charles
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Old 17th August 2005, 07:24 AM   #8
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Quote:
Originally posted by phase_accurate
Have you got any further info about the maunfacturer etc ? Please keep the following in mind:
1.)
Many manufacturers state that they would work without any NFB even though they definitely do.
2.)
In theory a precise PWM modulator, a stable PSU (!) and a perfect switching stage would give a perfect amp even without any type of feedback.

As we know, nothing is further from the truth in practice. But from the TI/TacT approach we know that very good switching amps are feasible without NFB.

It is generally advisible to build a switching amp in a way that it performs well, before any kind of feedback is applied. I know that it is feasible. Though I didn't measure the one I once developed without feedback, I assume that its open loop THD must have been below 3%. Now try to do this with an ordinary class AB topology !

Regards

Charles

Hi,

Yeah tact and.... what is it.... spectron? Open loop class d.... I'm sure 90% of what their circuit consists of is correction circuitry though

W, what you've described is just not a high end open loop class d amp.

The required complication doesn't seem to warrant it?

Regards,
Chris
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Old 17th August 2005, 07:54 AM   #9
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I assume Kanwar doesn't use any open-loop topology anymore.

At least the older Spectrons use NFB as well (they even take it from the speaker posts).

Regards

Charles
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Old 17th August 2005, 08:06 AM   #10
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Hi Charles,

I understand some of them are using a digital form of "room correction"? That's either got to be with a microphone or a voice coil...... lvdt?

I'm assuming anything with a delay would have a local feedback/reclock loop.

No clock at all seems the only reasonable solution, and I dont' think you can have a feedbackless self oscillating design, though ucd is said to sound like it by design.

I guess you can do it with digital open loop but the added complexity just seems pointless to me, in part count alone.

Regards,
Chris
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