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Old 6th June 2005, 12:45 PM   #21
SSassen is offline SSassen  Netherlands
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Phase_accurate,

You're spot on, the current configuration with the transistors connected to gnd with the bases indeed greatly reduces the Miller effect and causes for a considerable speed gain.

Pierre, level shifting is indeed needed as the front-end, integrator and comparator, and the back-end, mosfet-driver and output stage, are running off of different power planes, hence interfacing them directly will cause for sparks to fly and parts to fry, not good.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
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Old 6th June 2005, 01:42 PM   #22
Pierre is offline Pierre  France
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Yes, I know, but what I was trying to suggest is that you can run the integrator and comparator with a single supply, then you can make their "gnd" be the negative rail. So you wouldn't need level shifting, the only matter is that you would need to AC couple the audio to avoid DC conflicts, and also the feedback.
I have done that in a synchronous design as a test, and it works, but I had some unstable DC offset at the output, but I am sure that can be solved easily.
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Old 6th June 2005, 02:24 PM   #23
SSassen is offline SSassen  Netherlands
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Alright, on second thought there's no need for a resistor across the C at the integrator, because the integrator simply cannot drift. The capacitor voltage is caught between the comparator and integrator. When the switch level is reached the comparator will switch and so will the output stage and hence the voltage across the capacitor will drop untill the low switch level is reached and then the process repeats itself. That's the beauty of the SODFA concept, courtesy of Tilman (Tillg) and Stephan (Beobachter) and a few others at hifi-forum.de

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Sander Sassen
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Old 6th June 2005, 02:37 PM   #24
SSassen is offline SSassen  Netherlands
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Pierre,

You're right about the solution you suggest, that would remove the need to level shift. However it will make the amplifier a lot worse in terms of performance, it might even not work properly at all anymore. Why?

If you add coupling Cs you're creating 1st order filters as well, that, despite their high Fres, will induce not linearities. Because you'll need a minimum of two Cs you're introducing a lot of non-linearities that will (greatly) reduce the performance of the amp in terms of harmonic distortion and overall linearity.

The voltage gain across the integrator will no longer be linear and that's at the basis of the concept, hence coupling Cs is not a good idea.

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Sander Sassen
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Old 6th June 2005, 03:00 PM   #25
Pierre is offline Pierre  France
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If you are talking about the coupling capacitor in the feedback loop, I can agree with you.
At the input, you will need a coupling capacitor anyway, or are you going to trust your preamplifier in terms of DC?

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Pierre
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Old 6th June 2005, 03:05 PM   #26
SSassen is offline SSassen  Netherlands
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Pierre,

Yes, the preamp will be DC free as that's the subwoofer filter I'll be using. That still needs to be calculated though, but my primary focus for now is the amplifier, as that's a challenge, a simple active subwoofer filter with variable phase, boost etc. is not that complex in comparison and is well documented throughout the net.

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Sander Sassen
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Old 7th June 2005, 12:41 AM   #27
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Hi,

Sure the integrator is switched and reverses direction, but now add an offset error to the input of the integrating op amp and what will happen?

As it stands it will work fine indeally, such as in a simulator, but with world world op amps you might find a different story. For the cost of a single resistor... I believe for the same reason you'll want to pay careful attention to part selection in this area as well, like find an op amp with low input offset error.

You've seen the electronics texts showing an "ideal" text book active integrator Vs the "practical" integrator? The difference is a single high value resistor.

"SODFA".... yeah I know it stems from the German audio forum but I question its originality, it looks (and sounds) much more like it is strongly based on Bruno's "SODA", only the only notable difference is the integrated IC driver and dual N channels.

I know Tiki brought that up on their forum before and the web translator didn't allow me to decipher their response to that very well at all.

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Chris
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Old 7th June 2005, 06:59 AM   #28
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The thread starter actually mentioned the original patent:

http://v3.espacenet.com/origdoc?DB=E...QPN=DE19838765

It is in German only unfortunately. It is quite well written IMO although I don't believe all the statements about its superiority above carrier-based amps (apart fom a 6dB increase in NFB). Its biggest advantage is that its output-frequency remains almost constant and signal-independant. Basically it is a variant of the classic rectangular generator using an OP-AMP.

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Charles
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Old 7th June 2005, 05:41 PM   #29
LBHajdu is offline LBHajdu  United States
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This amp looks very similar to one of international rectifiers demo boards for a referents level class-d amp.

Pat Allen originaly built one for his sub woofer and is now using it as has full range amp because it sounded so good. I would look into it, you may get some good ideas at least. You can find more information at : schematic of my future D amp for the pb12 .
If you could get rid of the inverter chip they use that would be nice, because it’s only available in a “very” small SMT package.
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Old 7th June 2005, 05:50 PM   #30
SSassen is offline SSassen  Netherlands
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LBHadju,

I'll have to disagree with you there but it is NOT similar to the IRF reference design, that design suffers from a few basic errors that I obviously avoided. Furthermore using SMD devices is not a bad thing, but rather an asset in high-frequency switching circuitry such as a class-D amp.

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Sander Sassen
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