Class AB amplifiers with Switch Mode Power Supplies?

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#22
You can't beat a good old slightly over rated transformer and generous reservoir caps. I've built many amps over the last 50years and never use any sort of regulated supply, nor even a capacitance multiplier.


Builders seem to forget that the power supply is in series with the output transistors (of a discrete amp, but same applies to a chip amp) and must be considered as part of the amplifier.
Whereas its usually treated a a completely separate part which is assumed to have no impact on the sound.


Commercial manufacturers use SMPS for reasons of cost and compactness. Big transformers and smoothing caps cost more than the amplifier circuitry itself.
 
#23
DualTriode said:
Hello,

I just purchased a couple of these mono chip amp kits.

The idea is for testing THD+N and SINAD with linear vs Switch Mode Power Supplies. The amplifiers will be identical, The difference will be the power supply.

I am looking a recommendation for a known to be quiet SMPS.

Any recommendations?

Thanks DT

Mono HIFI Fever Audio Power Amplifier Board Kits LM3886TF 60W | eBay
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These are very quiet - they are single rail but you can put them in series for a dual rail. The mains protective ground is not connected to the output negative so these can be connected in serial. Then use an NTC to connect common 0v to chassis ground. I have used the 48v 800w version and there is no 60Hz/120Hz/180Hz leakage visible on FFT. The built in fan is noisy but if you are not running continuously at high power it can be disconnected, or best is replaced with a quiet Noctua fan. Or add your own DIY variable temp fan controller to keep fan off until needed.

DC48V 48W 60W 72W 100W 120W 150W 180W 200W 480W 500W 600W 720W 800W 1000W 1200W Led Lighting Transformer Power Supply 48V SMPS|Lighting Transformers| - AliExpress

Prices used to be much more reasonable at $43, now in the $70’s so a dual rail from Connexelectronic is cost competitive and they are much more compact.

One thing to watch out for with SMPS is that they don’t like to charge up large bulk caps at startup. You may need a solid state soft start between the SMPS and amp if that’s the case. But 4x 4700uF was not a problem for the 48v x 800w unit I tested.

You can also use something like SMPS800RE from Connexelectronic - a little noisier in terms of mains leakage through but quite suitable for most uses.
 
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#25
Line frequency residue from offline SMPS output typically comes from a modulation of the main bridge rectifier and filter cap high side DC ripple. Unlike linear supplies you can't just add a bunch of extra C at the output, and using oversized storage capacitors on the high side is impractical and dangerous. It can be quite the annoying issue in using SMPS for a linear amp PS. A couple solutions might be to use a buck/boost converter to provide a regulated high side DC supply. Or, a second non-isolated converter can be used after the off line circuit provides isolation; say an off line 12V@40A followed by one or more push-pull converters like car amp. I like this idea as it has more possible applications. Also, more than one circuit can be powered from the same "buss" without a separate isolated source, like a separate converter circuit for the +B_rail and one for the -B_rail. 494 chips are a dollar each and are everywhere.
Other parts such as ferrites, mosfets, Schottky rectifiers, and a small bit of magnet wire are pretty cheap. In addition, I find that generally an output inductor is very advantageous.😉
 
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#26
Dibya said:
For class AB avoid SMPS , it is going to fail with in 10 yrs for sure.#
Most medical equipment still uses transformer for some reasons.
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I work in medical devices, this is absolutely not true. I've come across almost zero mains powered devices designed in the past 15 years that have a linear supply. You do see plenty of legacy stuff in medical, only because no one wants to touch a perfectly good FDA approved device unless they have to.
 
#28
Hello,

I have seen claims that the quality of SMPS has made huge improvements over time. For me to pick one randomly off the shelf seems rather hit of miss to me.

Does anyone have any experience selecting SMPS for audio amplifiers? Is there an obvious selection to be made here?

Thanks DT
 
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#29
Alright, if this counts ....

I use a SMPS on my "What the heck, it's less then lunch" amp without problems. I am actually pleased with it. It is quiet and I don't notice any weird noise or odd sound.

But...I also use a LM723 linear PS for the TPA3116 Class-D board. It is also dead quiet and actually sounding very sweet as well.

I have no data to proof, Its just subjective.
 
#30
DualTriode said:
... SMPS for audio amplifiers? Is there an obvious selection to be made here?
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My obvious selection would be Hypex.
They make some of the best Class D amps in the world so they know switchers, they have a history as a reputable company, the power supplies are built to work with audio amps so have demonstrably low EMI, they have been objectively tested in this application by Audio Science Review and the results are fine.

Best wishes
David
 
#31
There is great difference using double single vs single double supply. The switching character needs to activate to supply power, in single mode, the power is half wave rectified, which requires to start and stop on each half alternate at bass notes, whereas in double symmetrical supply, the switching power is full wave rectified and runs more smoothly.
 
#33
"There is great difference using double single vs single double supply..."

Are you talking about the output? This depends on full bridge vs. half bridge vs. flyback etc. and not on single vs. dual supply.
Or is this about the voltage doubler at the input, to get 300V DC from 115V mains? In that case the primary ripple is much bigger and so must be the caps, or you get mains ripple at the output.

For PFC SMPS this is all irrelevant. At these power levels not a bad idea anyway, if not mandatory by now (did not check).
 
#34
"There is great difference using double single vs single double supply..."

I guess this statement is referring to driving a split rail supply (+V --> 0V--> -V) by using a single secondary and and controller to regulate the total output voltage (+V --> -V), and where half wave audio currents are referenced to the center 0V point, vs each output being separate and connected in series (+V --> 0V) + (0V --> -V), each one regulated by a separate control circuit.

It seems to me that for a non bridged push-pull power amp, if the center (GND reference) is "floating" between the regulated output (+V --> -V) then the large half wave audio currents that are referenced to that point would modulate it's potential relative to the +V and -V outputs. Linear supplies have large reservoir caps that help to decouple large low frequency audio currents but SMPS caps are small in capacity relative to AF.
 
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