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Class D Switching Power Amplifiers and Power D/A conversion

The simple tweak for NFB
The simple tweak for NFB
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Old 3rd June 2005, 02:21 PM   #1
phase_accurate is offline phase_accurate
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Default The simple tweak for NFB

There was some discussion regarding pre/post filter NFB with all their advantages and disadvantages.

I once divised a scheme that allows for an intermediate soultion.

I haven't tried it out personally so far but from the math point-of-view and from simulations it should work. I do however not know if it is sonically interesting at all.

It doesn't show the load-independancy of frequency response that post-filter topologies do, but it is a little better than pre-filter NFB in this respect. The simulations show a behaviour a little like the ps-audio HCA-2 amp, maybe they are doing something similar.

The whole idea is based on a transient-perfect crossover that is used to take RF-content from before the filter and AF content from the filter output.
Application is easiest on class-d topologies that use both - a 2nd- order output filter and an inverting/summing integrator.

So which transient-perfect crossover is the most simple one ? The first-order crossover of course !

Stay tuned to see how it is done !

Regards

Charles
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Old 3rd June 2005, 02:57 PM   #2
phase_accurate is offline phase_accurate
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O.K. here we are again.

First the simple part:
We take the NFB from the filter input by a first-order high-pass filter. Because it is feeding the virtual ground represented by the summing node of the integrator we can use an RC series circuit.

The little more complicated one: We want to make a first-order lowpass out of our 2nd-order one. We will therefore have to multiply its output with a term Gain * (1 + sT). With our summing integrator this is again simple: We use a parallel RC circuit.

Below you will see the diagram for this. The original feedback resistor RF is substituted by the four parts RS, RP, CS and CP.
Both caps are of the same value, so are the two resistors.
The two resistors have the same value as the original RF. The caps are caltulated as

C = 1 / (FilterCutoffFrequency * 2 * Pi * RF)

The circuit works best if the output filter has a Q of 0.5 but it should be possible to tweak it for other Q values (which is dependant on load of course). In some cases a Zobel across the output might not be out of place.

There is also a possibility to have an intermediate solution between classic pre-filter NFB and this one. For this purpose RF is left in place but having a higher value. A simple example: Increase all resistors to twice the original RF value, redimension the caps to half and then you will have a 50%/50% classic-pre/mixed NFB solution. The ratio between the solutions can be changed at will by chosing the respective resistor ratios.

As already mentioned this is not a double loop topology but a mixture of pre- and post- filter NFB.

Regards

Charles
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Old 3rd June 2005, 03:24 PM   #3
Lars Clausen is offline Lars Clausen  Denmark
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Charles: Thanks for this suggestion. I wired up a ZAPpulse module to see what happened. Your idea works fine, but it has a flaw, when you start it up without a load, the switching frequency will drop uncontrolled, and a lot of heat is generated in the output choke. Any suggestions?

When you start it up with a normal load, however it works just fine. Didn't listen to it though. THD seems to be unchanged compared to pure post-NFB.
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Old 3rd June 2005, 03:26 PM   #4
phase_accurate is offline phase_accurate
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Quote:
Any suggestions?
Maybe a Zobel ?

Maybe another idea will arise.

Regards

Charles
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Old 3rd June 2005, 03:29 PM   #5
Lars Clausen is offline Lars Clausen  Denmark
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It's rather embassaring, I made a mistake, and placed the feedback wire in the wrong place. When i did it right, like in your drawing it works perfectly!

And the Zobel: it IS in good place
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Old 3rd June 2005, 03:37 PM   #6
phase_accurate is offline phase_accurate
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So we are all ears about how it sounds.

Regards

Charles
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Old 3rd June 2005, 04:05 PM   #7
Lars Clausen is offline Lars Clausen  Denmark
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Doing comparation listening tests right now, two exactly the same amplifiers, except one is pre-filter NFB, and one is 'Phase Accurate NFB' .... will post results a little later ..
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Old 3rd June 2005, 05:33 PM   #8
Lars Clausen is offline Lars Clausen  Denmark
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Here is my first impressions: I first used that track 6 of Niels Lan Doky, Dreams album. I tried to listen different passages, and compare the sounds, but it appeared i was hearing two different tracks. Even if it was in fact the same track. But there was so much difference in the sound, that it was hard to believe that i was listening to the same.

The original ZAPpulse made a nice airy top, that really comes out in the room. But the new 'Phase Accurate' modified ZAPpulse, cleared up the room, and drawed a crystal clear picture of the high pitch instruments. The 3D effect is holographic! Some sounds where the floorbass turns slightly, 1:46 minutes into the track, this sound you almost miss with the original ZAPpulse, but with the modified ZAPpulse, it stands out, so you can't miss it! The Bass is right in front of you, while the rest of the band appears to be a few meters further back. Fantastic!
Bass dynamics seem to be untouched by this system, compared to pre NFB. Meaning: Good!

Now that was the good news.

The bad news is that female vocalists get what i call 'the Corrs effect'. This sounds like they are singing into a plastic bowl. Hollow in a way. It has nothing to do with the corrs, only that it is very obvious when you listen to the Corrs. (If you have this effect in your amplifier).

This was only my first impressions, i will continue the work with this and hopefully come up with something that is an improvement in every aspect.

If i measure THD it also seems to be more or less the same like before. (When the amplifier was pre filter NFB'ed).
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Old 3rd June 2005, 07:10 PM   #9
JohnW is offline JohnW  Czech Republic
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Lars,

I once saw an AES paper or patent the showed FB taken from both paths – or it might have been differing positions, not both combined at once – so I got thinking along the lines of Charles…

I played with a similar arrangement – DON’T use ceramic caps for C positions even COG - I made the early mistake of using COG ceramics - although they measured very well, they resulted in a VERY strong sonic signature - a hard edge to piano notes etc - even values like 47pF!!! I wonder if you’re experiencing a similar effect?

Charles suggests trying a 1:1 ratio in the first place, however with a good quality inductor, I found increasing the ratio in favor of Pre-LPF FB improved the audio quality, however insuring enough FB is taken after the inductor to reduce load dependency.

I would be very interested in your sonic evaluations with differing FB ratios splits.

Watch those Caps….

John
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Old 4th June 2005, 12:59 PM   #10
Hurtig is offline Hurtig  Denmark
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I belive Bang&Olufsen Powerhouse has patended the "double loop" design, used in their ICEPower amplifiers. But I may be wrong...
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