# Class D audio amp power rating

Discussion in 'Electronic Design' started by N Cook, Jun 22, 2007.

1. ### N CookGuest

For a bi-polar D class amp ( 2 n-channel mosfets) being powered from +38V
and -38V what would be the ball-park maximum instantaneous and continuous
RMS wattage driving into 8 ohms and not demanding THD requirements and only
40Hz to 18KHz handling.?

2. ### EeyoreGuest

38 volts peak > 180W into 8 ohms ( 90 W rms).

In reality it'll be less due to device on resistance.

Graham

3. ### MooseFETGuest

At the 40Hz end, you can have a huge THD and get nearly a 38V peak
square wave. Ignoring the drop in the output devices:

P = 38^2 / 8 = 180W RMS

For a not very distorted signal it will be

180W / sqrt(2) = 127W

4. ### N CookGuest

Thanks for that.
What would it be for say 10 percent THD

5. ### BobGuest

That is correct if the amp does not have an output transformer.

If an output transformer is used the power output will be limited by
the amount of
current that the MOSFET's can handle.
I'm not going to do parametric searchs on MOSFET's right now but I'd
guess 500watt would be achieveable. In practise paralleling FETs or
using
an H bridge might make more sense than looking for the highest
capacity
since MOSFET that can switch fast enough.

Bob

6. ### N CookGuest

I was trying to establish whether the amps I was repairing could be
correctly rated at 100W rms output , variously stated 150 , 180W "music
power" into 8 ohm with a THD at maximum power of 10 percent.
What would the likely wattage of thermal loss be for such a 100W rms rated
amp at say 20 and 50 watt rms output? As part of dummy load testing I was
monitoring the output vented air temperature in use at different power
levels, for the next time I come across such amps.
I appreciate they are thermally more efficient but I could not find what
temperatures to expect in a given ducted and vented cooling airflow

7. ### EeyoreGuest

How about you show me a Class D amp WITH an output transformer ?

Graham

8. ### EeyoreGuest

Do you just make this stuff up and blow it out of your ass ?

Maybe you'd like to show your calculations for these output figures and how you got
there without any data whatever ?

Graham

Graham

10. ### EeyoreGuest

Depends hugely on device on resistance.

Graham

11. ### BobGGuest

Can you see 3% distortion on a scope? Someone told me you couldnt, but
I never had a scope and a thd meter hooked up at the same time. Maybe
10% thd would be a 'slightly clipped' sinewave?

12. ### Tim ShoppaGuest

Class D can be done quite efficiently - 80 percent in a lot of off-the-
shelf designs. So I would not expect the power dissipation to be much
more than 10 watts for 50 watts output. (Efficiency isn't flat over
the entire power range but around half to full power you'll be in the
sweet spot for efficiency).

In some circumstances class-D efficiency can reach 90-95-98 percent in
certain applications but most audio amps seem to max out around 80
percent efficiency in real-life commercial products.

Vented air temperature by itself cannot be directly mapped into power
dissipation without a metric buttload of other parameters and thermal
and hydrodynamic/convection modeling. But if you want, just put a
power resistor in the same box and see how much power it's dissipating
when the vent temperature matches. It's not a perfect comparison but
there are just so many variables in play.

Tim.

13. ### John PopelishGuest

If the 3% is caused by clipping or zero crossing distortion,
almost certainly. If it is a combination of only second and
third harmonics, not at all certain.
I think 10% would be an obviously clipped sine wave.

14. ### mpmGuest

Someone wrote (& I couldn't decide who..)

My response: Blah, blah, blah...

If you don't define what a term means, (i.e., "music power") it can
mean anything.
And that's usually whatever the Marketing department thinks it should
mean.

Maybe it's the SPL developed by the system while it is accelerating
down the center of a black hole!

Oh wait, there wouldn't be any sound in space?
(Let's just keep that tidbit to ourselves....)

-mpm

15. ### BobGuest

There are plenty of class D amps with output transformers, they are
mostly for
100volt line public address system use rarther than 8 ohm though thats
just
a matter of the transformer ratio, there are 100volt line class D amps
modules
with output transformers that can be paralleled to drive 8ohm loads.

Googling for "100volt line class d" finds a few examples.

The BV440M model on the page below will do 440watts with a single
22volt supply.
You can see the transformer in the picture.
http://www.baldwinboxall.co.uk/vigil2amps.htm

Here's a class D amp brochure that mentions the output transformer.
Tony Smith from this company has ocasionally posted to s.e.d on the
subject of
class D amps.
http://www.current-thinking.com/pdfs/WEB_PERMA_USER_Inst.pdf
No need to be obnoxious. Perhaps you should look up the meaning of
"guess"
in a dictionary.
The original poster specified the psu voltage and a single pair of
fet's. Based on
what I'v seen of switching FET's in equipment I estimated that a
single FET can
switch 15amps at a few hundred kilohertz. I said I though 500watts
would be
achieveable, I didn't say I think that is the maximum possible.

Bob

16. ### EeyoreGuest

I rather doubt there are plenty actually. With modern devices it's more sensible to use
higher voltage supplies and direct couple.

Yes, so it's irrelevant to the OP's question isn't it ?

Graham

17. ### BobGuest

Systems that need battery backup generally use 24V or 48V single
supplies and have
output transformers.
Output transformers can mitigate various practical problems in public
A high power amp without a transformer will generally need one side of
the output
connecting to earth due to EMC and safety standards. In office
buildings where hundreds
of meters of PA cabling and lots of junction box's are thrown in by
the contractor that
bid lowest and the occupier moves the partiton walls around
relatively often so
Mr Important can have a larger office it is a common occourance to
find one wire in a
loudspeaker line is short to earth somewhere. With an output
transformer sound still
comes out of the loudspeakers, without a transformer it often dosn't.
It was relevant to the original hypothetical question about what is
possible, before
the OP told us he is repairing an amp and it is relevant to your
demand to be
shown class D amps with output transformers.

Your demand implied that you believe calss D amps never have output
transformers.
People will think more of you if you can accept when you are wrong
getting stroppy.

Bob

18. ### EeyoreGuest

That wasn't the OP's question !

You decided to answer a completely different question you made up for yourself.

Graham

19. ### EeyoreGuest

Bob wrote:

You're talking complete unabated drivel now.

Point to a single IEC standard that would require doing that.

Graham

20. ### Phil AllisonGuest

"John Popelish"

** With any respectable * analogue * scope - 3% waveform distortion
with a sine wave is easily visible.

If the sine shape has become asymmetrical ( ie 2nd harmonic ) - it shows up
as one maxima being larger than the other.

One can easily see slight peak amplitude compression or a slight inflexion
near the zero line that when checked on a THD meter is about 2 or 3 %.

Very small amounts of peak clipping can be seen - little as 0.5%

Forget doing this with a digital scope where all sine waves look distorted
and noisy anyhow.

........ Phil