Using a 50v single supply (and trying to split it) with LM3886

[mudihan]

I have a quality linear regulated power supply that outputs 50v up to 3A. I am thinking about using resistors to split the single supply to make it +/-25v and use it to power a gainclone LM3886 amp that in turn will power a pair of electrostatic headphones (not much current needed there). I have a few questions:

1) Is the 3A rating on the power supply enough for the gainclone?

2) Is it possible to use resistors to split the supply, given the current?

3) If it's possible, what would be the power rating and impedance value for the resistors? And what types are preferred? (I have wirewounds, but just saw some Cement resistors)

Thanks!

[Rainwulf]

There is no way you could user resistors to do this. The amount of current they would have to sink to make yourself a virtual earth would probably tax the supply before you even had a chance to use it as an amp. The only way to ever make a half decent virtual earth with a single rail supply is to actually use another power op amp to dc amplify V/2 and use that as your virtual earth.


You are better of using the LM3886 in its proper single supply configuration, as shown on page six of this pdf.
http://www.national.com/ds/LM/LM3886.pdf

Its a fair amount more complicated to use in a single supply configuration, and a bit of waste really to drive ES headphones, which are hard enough to drive as it is.

Honestly, you are much better of using your regulated supply to make a class A amp. Much better at driving ES headphones, and its regulated, which pretty well solves the biggest problem most class A amps have when building them.

[mudihan]

Thank you for the reply. I will then look else where.

[pacificblue]

Originally posted by mudihan

1) Is the 3A rating on the power supply enough for the gainclone?

With ±25 V the LM3886 will deliver ~30 W into 8 Ohm and ~58 W into 4 Ohm. Your 150 W power supply will be sufficient for two channels.

Originally posted by mudihan
2) Is it possible to use resistors to split the supply, given the current?

You can use resistors to do that, because they will not carry a lot of current. The application in the datasheet adds a transistor to the resistors for better stability. The transistor is rated at 200 mA, so even if you don't understand the schematic, you can deduce that little current is needed.

I=U/R and P=U²/R

Originally posted by mudihan
3) If it's possible, what would be the power rating and impedance value for the resistors? And what types are preferred? (I have wirewounds, but just saw some Cement resistors)

Normal metal film types with 1/4 W rating will do. 100k is a commonly used value. If you don't like transistors, you can copy the implementation from the TDA2030's datasheet. There a capacitor is used for stability.

Don't forget that you need DC blocking caps at the in- and output of the amplifier in a single supply application.

[mudihan]

Since I don't like to have caps inbetween the amp and the headphones, I am considering the single supply version. Thanks for the reply.

[Rainwulf]

The single supply version does have caps between the amp and headphones.

The only way to avoid caps is to have a split power supply.

If you dont have one, this is the only decent way to make one from a single supply that is capable of sinking/sourcing a decent amount of current without wasting it as heat in large resistors.


http://www.techlib.com/craig/craigssplitsupply.htm

[mudihan]

My bad. I always omit NOT while typing. I meant "I am NOT considering the single supply version."

It's a terrible habit.

[Rainwulf]

Oh ok hehe.

Whats your plan? dual supply? class A? class A is capacitively coupled as well.

The only amp that is capable of driving direct will be dual supply.

[mudihan]

I also have +/-29v 100va transformer. After rectification, it will be around +/-40+ VDC, which is very close for LM3886/3875.

Well, I have to think about the whole thing. I am very familiar with National chips and have made a LM3886 myself. They are cheap and the PCBs are easily available. Doing class-A will need new research, which is almost impossible due to my time restraint. I am also trying to use existing materials, due to my budget limit.

For now, my newly acquired ES headphones have to rest.

[SpittinLLama]

Build two amps as single supply and run the speaker in BTL mode and you can eliminate any output coupling caps.

-TH