switching power supplies in amplifiers?

[Pooh Bear]

I've both seen and heard class C audio. It was a PA system used for
voice. It sounded very bad but you could understand what was said.

How long ago was this ? Do you recall who the manufacturer was ?

Graham

 

[Nico Coesel]

Actually IEC61000-3-2 let them off the hook on account of the low
contribution from such devices.


Power amplifiers don't use regulated supplies.

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.

 

[Pooh Bear]

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

 

[legg]

Hi all,

Are home audio amplifiers typically powered nowadays with switching
power supplies, or do they still have a massive 60hz power transformer
inside?

I went to Best Buy yesterday, and was surprised at how heavy many of
the home audio amplifiers were...

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

 

[Jim Thompson]

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

 

[Clifford Heath]

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?

 

[Pooh Bear]

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

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

Graham

 

[Pooh Bear]

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.

Very few are like this actually.

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?

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.

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?

Try designing one ?

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?

None that I know of.

Graham

 

[Clifford Heath]

Very few are like this actually.

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

...capacitor size is proportional to the CV product

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

Try designing one ?

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.

 

[Clifford Heath]

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

 

[Ken Smith]

How long ago was this ? Do you recall who the manufacturer was ?

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

 

[Paul Mathews]

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

 

[Phil Allison]

"Paul Mathews"

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.

** Crown ( yes, the famous one) produced their K series a decade ago.

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

http://www.crownaudio.com/pdf/amps/136713.pdf


The " damping factor" is over 3000.

What a crock of shit you idiots crap on about.





......... Phil

 

[Nico Coesel]

Very few are like this actually.


Try designing one ?


None that I know of.

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

 

[Genome]

"Paul Mathews"


** Crown ( yes, the famous one) produced their K series a decade ago.

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

http://www.crownaudio.com/pdf/amps/136713.pdf


The " damping factor" is over 3000.

What a crock of shit you idiots crap on about.





........ Phil

So, what part of a damping factor of 3000 @ 10-400Hz and the application of
negative feedback whistled over your head?

DNA

 

[Phil Allison]

** Learn to post properly - Google fuckhead.

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

Hit "options" - NOT "reply".





........ Phil

 

[Richard Henry]

** Learn to post properly - Google fuckhead.

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

Hit "options" - NOT "reply".

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

 

[Phil Allison]

"Richard Henry"


** Go drop dead - fuckhead.



......... Phil

 

[Terry Given]

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.

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

 

[Ken Smith]

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,[/QUOTE]

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.