LM3886/3876

[jrc4558]

Hi all! I looked over the datasheets, but got lost in a multitude of subscripts... I was looking for the current consumption of these chips. Can you advice me on how much current do they draw and how big a transformer will I need to power that beast? Thanks!

[Nasse]

I have 2x24 volts 180 VA toroidal transformer, I think it might work for stereo hifi use, 2x22 or 18 volts might be safer

[R.G.]

For any solid state power amplifier, the power for the amp circuit itself is usually inconsequential compared to the power delivered to the load.

The right question is "For an XXXX amplifier delivering YYWatts to an 8 ohm speaker, how big a transformer/power supply do I need?"

Here's a copy of my posting on the same topic from the ssguitar forum:
   

Re: Is this PT ok for LM3886 amp
« Reply #3 on: October 27, 2006, 05:16:27 PM »
   
Let's play transformer sizing.

The LM3886 datasheet says, as joecool85 notes, that 96V is the absolute maximum. That is, if you apply 96.01V, you must assume that the LM3886 will immediately transform itself into a Darkness Emitting Diode, or DED.

So let's see what that voltage is. DC and RMS AC are related by the square root of two, so for 96V, you have 96/1.414 = 67.89Vct on the transformer.

But wait - that's a maximum, which presumably happens at the very peak of your AC line power, which also varies. If you get 120Vac +10%, that's 132Vac. So we need to know what a 132Vac to 67.89Vac transformer is when it's at home at 120Vac. That's 61.72Vac.

But wait - that ignores loading. Transformers are rated with "regulation" which is really the no-load to full-load sag from losses. In transformers of the 100VA class, 5% "regulation" is pretty normal, so a transformer rated at 58.78Vac with 5% regulation is actually going to be 61.72Vac at no load.

We're finally down to the absolute maximum transformer voltage. I would not use anything over 58Vct on one of these and expect them to survive line surges.

What is the minimum?

I'd go with what National says if they said it clearly. But they don't. It's hidden. This stuff has to be gleaned from about three or four of their performance charts.

They say that the chip will do 80+ watts into 8 ohms with a +/-40V supply, and into 4 ohms with a +/-30V supply. Can you **guarantee** that you will never connect a 4 ohm load to it? If so, you could use +/-40V. If not, you might use +/-30V. There is more calculation behind this, but those are fair maximums because National suggests a 1 C/W heatsink to do those powers and voltage.  1 C/W heatsinks are HARD and expensive.

For longevity, it make a lot of sense to not dissipate all of the heat in one package. These things are CHEAP compared to heatsinks. So use a lower voltage, or use a cheaper heatsink and use two or three LM3886's in parallel.

So if you have to have the most, most power you can get, use a 56Vct transformer (that being a relatively standard value)  and two LM3886's in parallel for 8 ohms. If  you can stand the difference between 68W and 45-50W, back down to a 48Vct transformer which is easy to find. If you're going to use this for 4 ohms loads, back down to 44Vct transformers.

How much power? Music power varies. If you use a transformer rated at 1.8 times the amplifier rated RMS power, it can run a sine wave at that power forever. If you use a transformer rated at 1.0 times the amplifier rated RMS power, it can run music with a 50% crest to average power factor forever. Most music has between a 0.1 to 0.25 crest factor, so you're pretty safe with a transformer rating in VA equal to the amplifier rated power.

When I do LM3886's, I look for 44Vct or 48Vct transformers rated at 2A. If I run them into 4 ohms, I will use two or three LM3886's in parallel. Get over 56Vct and a power surge will kill your chips one day.