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Class D Switching Power Amplifiers and Power D/A conversion

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Old 20th February 2004, 10:54 AM   #11
subwo1 is offline subwo1  United States
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I have been working on the idea of using just a switching power supply also but am not happy that if active components are not used to switch the voltage on the secondary side, the positive and negative modulated outputs short together. However, I am considering using one switching power supply and half of a class D amp to produce two halves of an audio waveform. I think it is possible to make a simple decent quality class A style class D amp (Does that make sense? :-) ) if the power supply is set to produce constant current. Then only one fast N-channel mosfet is needed on the lower power supply rail.
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Old 20th February 2004, 03:39 PM   #12
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Given the nature of the signal from the typical source (CD-player), wouldn't it be a useful compromise to just let the volume control control the supply voltage (in addition to controlling the volume). Should work quite well for an integrated solution.

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Old 20th February 2004, 03:41 PM   #13
AGGEMAM is offline AGGEMAM  Denmark
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Quote:
Originally posted by runebivrin
Given the nature of the signal from the typical source (CD-player), wouldn't it be a useful compromise to just let the volume control control the supply voltage (in addition to controlling the volume). Should work quite well for an integrated solution.

Didn't you just describe the principle in every amplifier in existance?
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Old 20th February 2004, 03:44 PM   #14
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Uhm, not as far as I know. Imagine having a 3-gang volume pot, where two of the tracks control volume as usual, and the third controls the supply voltage - probably by means of a variable SMPS supply.

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Old 20th February 2004, 03:44 PM   #15
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Quote:
Originally posted by Ouroboros
But as a PSU it wouldn't need to track the individual waveform of the signal. It simply provides power to track the envelope of the signal.

it is the tracking of that envelope that's probamatic: at any given point, you cannot predict where the next "wave" would be. the "envelope" is more like averaging of historical "peaks". As such, it is delayed.

for example, if the last 5 seconds the highest peak is 20v. you add some margin and set your rail at 20+10=30v. what if the next peak turns out to be 40v? you get clipping.

If you increase the margin, then you sacrify efficiency. if you decrease the margin, you sacrifice sound quality.

Might as well go with a full switching mode amp.
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Old 20th February 2004, 08:21 PM   #16
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One has just to guarantee that the signal envelope doesn't change faster than the tracking supply is able to follow. This is achieved by a lowpass at the input of the linear amp and an envelope detector with faster rise- than fall- rate (which is of course controlled by the input signal before the lowpass).

But I by myself do still prefer a pure class-d solution.

The aforementioned peavey amp (plus corresponding patent) is in fact an SMPS that outputs audio.

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Charles
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Old 21st February 2004, 10:55 PM   #17
mwh-eng is offline mwh-eng  United States
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Sometime during the 70's, Sony produced a working prototype using Class D. It used a switching supply for the plus and minus DC rails. For the SMPS Class D audio section they switched at 300 KHz and had no feedback, if my memory is working today. I never remember them producing it for the public. The problem I see is bandwidth. The higher the switching frequency the higher the bandwidth. Using modern ferrite cores for the magnetics, core loss becomes an issue at the higher frequencies. Eddy currents and related proximity effects causes power losses in the copper. Using planer magnetics design reduces the copper power losses. A rule of thumb for usuable bandwidth in SMPS is 1/10 the switching frequency. This probably hold close to true for the Class D audio amp version of the SMPS. This makes me prefer to use the Class D for the base and lower midrange frequencies only. This is where most of the power is needed and where the Class D's efficiency is best. For the higher frequencies that require less power, a linear (non-switching) amp seems best. Of course, this means either bi-amping or tri-amping. Another potential problem with a Class D amplifier, especially at higher power levels is their switching noise. This can be conducted around the circuits, coupled electrostatically, or electromagnetically. It is not trivial to deal with unless you have a lot of experience in dealing with it.
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Old 23rd February 2004, 07:02 AM   #18
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Quote:
Originally posted by millwood




for example, if the last 5 seconds the highest peak is 20v. you add some margin and set your rail at 20+10=30v. what if the next peak turns out to be 40v? you get clipping.


That's why the delay in the audio path is required.
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Old 13th December 2010, 06:37 AM   #19
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I am new to this but I can clearly see that.... If the switching supply becomes too slow to keep up with voltage/frequency then it's probably a good idea to have a back up system where an almost instantaneous voltage (short duration only) is injected in the power supply output side to pull up the voltage fast enough till the supply caches up. Frequency dependant and only for a few milliseconds to hold the voltage at the level needed. I had this idea a long time ago. I believe Yamaha has a patent that sounds just like what I am describing however I have no idea how their system works (electronically)
can this be a logical solution?
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Old 17th December 2010, 01:05 PM   #20
AP2 is offline AP2  Italy
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Hi,
Interesting topic. I'm working on a PSU directly controlled by the audio signal (prototype already exists) measures are good, but is geared to sub for high ratings.
I would like to speak my mind, about the rapid response of a SMPS, after many tests, even with CPU usage, the best solution to speed response and sound very clean, sound identical to a toroidal with large capacitors (bass hot) is it is shown that the choice of the linear regulator voltage on transformer. This helped not to vary (as PWM) cycles on the transformer. This fact is important, normal pwm on transformer, the inductance is constantly changing, this causes many problems in the case of an amplifier with audiophile performance.
Under these conditions, you may work the best curve in the transformer, optimizing the losses on a fixed frequency. Certainly not as simple as they say, the linear regulator is very fast (150ns) and we had to think of a trigger to increase efficiency. Result is the same as a 500w transformer with 2x22000uF outputs (at + /-70V) and excellent voltage regulation line in just 500 grams weight.
PWM is good on continuos load PSU, it can offer very good line regulation/load also.

Regards

Roberto P.
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