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Old 24th February 2007, 10:50 AM   #1
Dag is offline Dag  Sweden
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Default SMPS vs. Class-D.

Hello!

I hear people complaining about noise and spurs from SMPS and I also hear people rave about Class-D amps and this is confusing me.
I know from experience that SMPS can be really noisy (bad designs!) but it must be much easier to design a good low noise SMPS than a low noise, low spur, low distortion, wideband etc. class-D audio amp???

To me SMPS+analog amp sounds lika a better idea than trafo+class-D.
(SMPS+Class-D with syncronized switching frequencies is probably the future, but we are not there yet)

I have just compared a LM3886 amp with the Zap Pulse 2.2se (with regulated PS at +/-40V) and I do prefere the LM3886. Am I doing something wrong?? What are your experiences with different combination of analog/digital PS and amps???
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Old 25th February 2007, 08:30 AM   #2
ssanmor is offline ssanmor  Spain
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It is difficult to get a good and quiet design using both Class-D amplifiers and SMPS. However, it is possible. We (coldamp) have been using and selling this combination with no problem, obtaining better sound quality than with linear PSUs, if everything is laid out carefully and cabling is properly done.
The future is, no doubt, SMPS + Class-D. Nobody said it was the easier way, though.
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Old 25th February 2007, 10:12 AM   #3
Pano is offline Pano  United States
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I've done a lot of testing and listening to class-d with battery, linear and switching power supplies. As ssanmor indicates above, much depends on the implementation.

Class-D amp by their very nature kick a lot of noise back onto the power rails. Usually much more noise than any well designed power supply. How well the power supply deals with the amp noise goes a long way toward quality.

There are fans of battery supplies, because batteries are the ultimate quiet supplies, right? Wrong. Batteries are very quiet until you turn on the amp, then the noise floor rises considerable. It seems that the battery can't cope well with the RF noise generated by Class-D amps. A good linear or switch mode supply can.

As many of you know, SMPS will have a lot of HF noise, much more than a good linear supply or battery, but it will tend to have less 50/60 Hz than a linear supply. But even that noise is far below the noise floor of the Class-D amp. Once the amp is running the noise spectrums are quite different from supply to supply.

Leaving the noise issue behind, how well does the power supply regulate and how does that regulation sound? That is were a SMPS often takes the upper hand. I don't know why (yet) but they do seem to sound better than other supplies, at least with Class-D. A good linear supply can also sound very nice. And of course even a battery supply could be regulated and filtered, if you wanted to.

Where does that leave us? In limbo, sort of. I would not use a SMPS for a sensitive or high gain circuit such as a preamp, but in a Class-D amp most SM noise will be drowned by the noise of the amp. And the SMPS noise can be filtered out to a large degree. They are not as bad as you might think, and in fact are quite good when used properly.
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Old 25th February 2007, 10:38 AM   #4
ssanmor is offline ssanmor  Spain
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Only a couple of additions to Panomaniac comments, that has done a very good job in explaining why SMPS can sound better with Class-D:

Usually, switching frequencies of Class-D amplifiers are much higher than the switching frequency of SMPS's. The only problem that may arise are "beat" frequencies between these frequencies that may fall within the audio band, producing subtle high pitched (and even variable frequency) noises. However, practice has demonstrated that it is quite easy to filter out the rails noise so this effect disappears, so it is no longer an issue with good SMPS implementations.

About regulation: linear power supplies are almost never regulated, and this is specially true as power increases, due to efficiency and heat dissipaton issues. On the other hand, it is easy to make regulated SMPS, that will hold rails voltage up to full power with no problem. This translates in a deeper and more powerfull bass, mainly, although it has sonic impacts in the rest of the audio band. Further, if you add a respectable output capacitance (most SMPS manufacturers fail to use it due to increased costs and add a few hundred microF per rail arguing that the refresh rate is much higher than in a linear PSU), you get the best from both worlds: efficiency, excellent load regulation and high energy reservoir.

As a curiosity, we have been manufacturing both linear and switching power supplies, and almost all customers that have used both of them with our Class-D amplifiers (and others), agree in that they get a better overall sound when using SMPS...
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Old 25th February 2007, 09:48 PM   #5
Pano is offline Pano  United States
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And again - I agree with all above.

I didn't mention in my post that I was running tests on the low power T-Amps for which it's easy to build a linear regulated supply. Larger, more powerful amps are rarely regulated, as ssanmor mentions.

I'm also a fan of a large capacitance reserve, no matter what the supply. There are a lot of sonic advantages. The disadvantages seem to be cost and weight.

Power supply design and implementation is very important in an amplifier. Because the amp is a power supply, basically.
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Old 26th February 2007, 11:10 AM   #6
Dag is offline Dag  Sweden
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Thanks for your answers and explanations.

Well, it seems like it is already possible to make a good SMPS+Class-D amp but I still think SMPS+analog amp is also a very good solution, especially if you use chipamps (LM3886/TDA7293) for an active speaker. I do not think that I would like to use a SMPS with 6 Class-D amps in the same box but SMPS+6 chipamps should be no problem (cheap and small size : ) Has anyone tried???

The advantages I see with SMPS is good regulation at high currents and high efficiency. The smaller caps needed is probably good because smaller caps will have a larger bandwidth (resonance at a higher frequency) and better quallity (at least for the same price).
The high frequency regulation will also be good at taking care of the back generated current (Back EMF???) from the speaker.
Maybe a SMPS has a better isolation to 220V/GND because of the MUCH smaller transformer...and that is always good.

I also think that in a good SMPS the LF noise (<20k) can be very low and that the hf noise (switching frequency etc) might not be a big problem because it will not end up in the audio band (this is not really true..there will always be some second order effects that will transform the hf noise to lf noise....)

Too bad that it is almost impossibe to find data/measurements on the noise and output impedance vs frequency and output current.




ps smps can also be very good in cd players and preamps. One exampel is the Linn CD12.
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Old 26th February 2007, 11:43 AM   #7
ssanmor is offline ssanmor  Spain
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Quote:
I do not think that I would like to use a SMPS with 6 Class-D amps in the same box but SMPS+6 chipamps should be no problem (cheap and small size : ) Has anyone tried???
Have a look at:
http://www.coldamp.com/opencms/openc.../aplicaciones/

This one was used for a classic music concert hall. You will understand that this is one of the most sound-quality demanding applications you may find.
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Old 28th February 2007, 02:31 PM   #8
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Hi SMPS-lovers.

It sounds to me, as if the option to put a filter, as part of a battery-supply, in front of the amp is not existing to you folks!

I'd like to see a SMPS beating a high-quality battery-supply.

Cheers
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Old 28th February 2007, 02:34 PM   #9
Pierre is offline Pierre  France
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Quote:
'd like to see a SMPS beating a high-quality battery-supply.
I'd like to see a high-quality battery supply feeding a 400W amplifier (directly)
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Old 28th February 2007, 04:49 PM   #10
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Hi Daq,
You have to synchronize the SMPS and class D amp to a master clock.
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