6 channel LME49830 power amp

[MJL21193]

HI Andrew,
What would the line AC have to be for a 40-0-40 to break 75VDC?
I have measured mine several times (admittedly, our electrical system seems to be very stable here) and I've never gotten over 57VDC after rectification and smoothing.

[AndrewT]

rather than a simple "yes/no", I gave him "how to".
Hopefully he now knows "how to" in future.
Your simple "yes/no" means he and others keep coming back with the same question because it doesn't help them learn.

230:40Vac 1000VA transformer produce 58Vdc at the amplifier

even for a low regulation transformer my "normal" supply voltage gives a PSU voltage with quiescent current set to 210mA above your maximum. A "how to" gets around this conflicting information.

[MJL21193]

The how to was fine.
It was the bickering and finger-pointing about who's responsible for your flaky power grid over there that I had enough of...

[pacificblue]

Originally posted by AndrewT
Harmonisation of the 220/240Vac is the only reason the UK had to lower the previous 226Vac limit to 216Vac.
Our systems are properly designed, properly built and well run, even after privatisation.

Lowering the allowed limits does not oblige the energy suppliers in the UK to lower their standards. And if you say your systems are still fine, what harm has been done?

The reason for harmonisation rather lies in international industrial interests than in European force. You still pay in £ instead of €, and still drive on the wrong side of the street, both of which does not give me the impression that Europeans can force a lot upon you.

And the reason, why the tolerances are so high can rather be found in the US of A, with their very different infrastructure than in Europe.

[cuibono]

Ha, you guys.

Thanks for the how to, it seems there is always something more to learn. I'll be ordering parts tomorrow, and hopefully have some modules up working within a couple of weeks.

So far my line voltage has been 123V +/-.5V

[cuibono]

Lets do a little off the cuff power dissipation estimation...

from the an-1850 pdf:
PD(AMP)MAX = VCC^2/(2pi^2RL)
where Vcc is the sum of the + and - rails (guessing 108V for my case) and RL is the resistance of the load, lets assume 8ohms. The modules have four devices total, and the above equation gives 74W total power dissipation, each device sees about 18.5W.

My woofer will be about 4 ohms, which gives total power dissipation of 148W and 37W per device. Not bad if you derate the devices to 90W max for SOA.

There are a few things I don't understand. Is this calculation for a signal swinging to the rails? Isn't power dissipation higher with the signal at about half the rail voltage?

Lets calculate total quiescent power dissipation. The woofer will be biased at about 100mA - would this give 10.8W loss with no signal? The tweeter I plan on biasing up to 1A, so I guess the power dissipation will be 108W total. How's that sound?

So for the tweeter module working into 8 ohms, does the 108W of quiescent power loss add to the 74W of max power loss, meaning each device will possibly be seeing 45.5W power loss at maximum?

The other thing I don't understand is how changing the number of devices changes the power dissipation - it seems like fewer devices would lead to less power available and therefore less power dissipation, but the above equation doesn't factor it in. Seems like the power dissipation per device would actually be higher. I was hoping that using fewer devices on the tweeter module would help compensate for biasing it higher, but it looks like it will just make them blow faster..

[AndrewT]

Originally posted by AndrewT
check with your supplier.

Originally posted by cuibono
So far my line voltage has been 123V +/-.5V

? advice ?

[cuibono]

sorry, I was being glib. I understand your point.

[MJL21193]

Originally posted by cuibono
Lets do a little off the cuff power dissipation estimation...

from the an-1850 pdf:

Hi,
I looked at the amp you want to use. You will use National's board layout? It's huge and I can't see three of those fitting on a 5.5" x 12" heatsink.
I think you can find a better, more economical amp design to work with.

[cuibono]

Hmmm, you might have a point. The National pcb is about 5.75" wide, and the heatsink is about 5.38". I had missed that. I could redo the pcb without too much trouble, but I'd really rather not.


EDIT: the same seller has 8.5" wide extrusions, which should work just as well.