switching power supplies in amplifiers?

Some Class-D amplifier schemes need a regulated power supply because the open loop amplification is a function of the supply voltage. If the switching frequency is quite low, an unregulated power supply will make it difficult to control closed loop gain.

Decent Class D audio amplifiers use feedback these days, bypassing this issue.

Linear regulation is *far* too wasteful in this application.

Graham

It actually can be quite a challenge producing switch-mode power that will allow audio frequency circuitry to run with reasonable noise performance, while meeting EMC requirements.

As has been stated elsewhere, this often doesn't pay when the trouble is taken to do so.

It has been even more challenging, in the past, getting the audio-buying public to go for it.

Nowadays you can find a fair range of products that not only have switch-mode power supplies, but are in fact switch-mode power amplifiers as well. Automotive products were early to adopt the technology.

RL

One thing I've always mused over... what kind of damping factor do you get driving a speaker with a switcher?

...Jim Thompson

Musical instrument amps, like bass guitar for example, are available with SMPS. Some of the new amps are tiny - size of a large book - but produce 600W or better, amazing.

The thing I'd be worried about especially with a bass amp is whether the SMPS main filter caps store enough energy to carry a low-frequency signal over the 100/120 Hz minima in the rectified AC. I guess with a SM output stage you can efficiently use a much higher rail voltage than you'd use with a linear amp, hence tolerate much more rail droop. So maybe that's ok - or maybe the 50/60 hz comes through on big peaks?

It also seems strange to chop rectified line voltage to produce DC for lower rails, then chop that again to produce AC. Why not go direct from the rectified and filtered mains to the audio O/P? I guess you'd need an SMPS transformer running at 100's of KHz so your servo loop had enough resolution though... is that the problem? Or are amps like this available?

Damping factor will be determined by the amplifer not the power supply. DF is after all essentially a reciprocal meaure of output impedance.

Graham

Very few are like this actually.

Storing energy at ~ 320V is quite efficient actually when you consider that capacitor size is proportional to the CV product but energy is 1/2 CV^2. Supply rail ripple is actually probably *less* than with a conventional line freq transformer rectifier solution.

Try designing one ?

None that I know of.

Graham

I didn't say there were many - just that they're available.

That was the fact I was missing - I knew 1/2 CV^2 already - thanks.

Not this little black duck :-). But perhaps it's one of those things that are too hard because no-one's tried :-). That's why I put the question out there for a switching expert to try to answer, Terry Given perhaps? You out there Terry?

Clifford.

BTW, for an example, google for the MarkBass amps.

It was some time between 1974 and 1977. I don't recall the makers name.

For the zero negative feedback Class D amplifiers (e.g., most of the so-called 'pure digital' types such as TI Toccata), damping factor is much lower, since the output reconstruction filter adds its impedance to that of the output stage. A generous amount of negative feedback (e.g., Philips UCD such as Hypex products) brings output impedance down and damping factor up. Paul Mathews

"Paul Mathews"

These are very hi-fi, pure class D amps.

The " damping factor" is over 3000.

What a crock of shit you idiots crap on about.

........ Phil

I suppose the problem will be finding high voltage MOSFETs with a small enough gate capacitance.

NEVER post without quoting the context or the other poster's name.

Hit "options" - NOT "reply".

....... Phil

Sorry, but without context, I have no idea what this posting is about.

"Richard Henry"

........ Phil

Nope.

I havent studied class D audio amplifiers much. I did read a paper a few years ago IIRC in IEEE trans. power electronics, which tacked a class D stage onto a Self "blameless" amplifier, for an efficiency of 90% or so. I'd just do it that way, the modulator was cheap and the linear amplifier ends up acting in an error-cancelling role.

I've thought about delaying the audio a fixed amount, and use a (PFC) SMPS to follow a convex hull of the envelope, the hull geometry and delay length given by the smps bandwidth. This then powers a linear amplifier, which has extremely high efficiency because the power supply rails are always just where they need to be, regardless of the spectral content.

As the smps bandwidth increases, the hull "shrinks" around the actual signal. If a quick enough smps can be made, its a hop, skip, and a jump from there to making a dc-ac converter followed by a linear active filter.

At which point you might try making an actively filtered cycloconverter :)

Cheers Terry

In article , Terry Given wrote: [....]

You could make a second supply that runs at, lets say, 2 times the optimal voltage. If the swing suddenly goes out of bound for the main supply, the secondary one could be used to provide the energy while the main one gets up to speed.

You can also get fairly good efficiencies by just having 3 power supplies at fixed voltages. If you are working into a resistive load, the circuit is easy to do. If you need to drive reactive loads, the circuit uses a lot of parts.

Yes, that's been done quite a bit in high-power PAs. But it doesn't address the problem I was driving at, which is minimum weight for an instrumentalist (or band) to lug around, consistent with required power (500W at under 3Kg, amazing - but still two-stage switch-mode. The Zap Pulse amplifier modules are amazing too, but still need a PS.

Thanks all, it's been interesting. Genome's pointer to "ampliverters" was good too.

Clifford.