Bootstrapping LM3886

[Stocker]

Some will go much higher than that, but none that "we" usually use... It may be spendy to acquire but they are out there.

[janneman]

Originally posted by xplod1236
[snip]I have 20 of them (free samples--I was surprised that National sent me 4 packs of 5 of them in a row) This will be used as part of a bridgeable sub amp, so performance doesn't matter that much. And it will be easier to do with the LM3886, because I can keep the rails at +/- 35v, instead of +/- 15 as with a regular opamp. (A regular opamp wouldn't swing to 75v, would it?)

You got me confused now. I thought you were going to bootstrap the LM's on +/- 60 (90?) volt supplies. That can be done with opamps also. If you want to use +/- 35V, does that mean you decided to junk the bootstrap scheme?

Jan Didden

[xplod1236]

I thought you were going to bootstrap the LM's on +/- 60 (90?) volt supplies.

I am.

If you want to use +/- 35V, does that mean you decided to junk the bootstrap scheme?

No. The 35v supplies are modulated with the output. At 0v output, the chip supplies will be +/- 35v. With the chip running, the positive supply will swing to +75v, and the negative will swing to 0v, and the other way around. The main supplies will be +/- 75v.

Sorry for the confusion.

[janneman]

OK, got you. So, the basic issue is this, if I am correct:

What is the best way to built an audio power amp, starting with a lot of LM3886'es and a +/- 75 V supply, right?

Jan Didden

[xplod1236]

Yes and no.

Yes:

I have 20 LM3886s and a 75v supply, and I want to build an amp. Now I would ask your question:

What is the best way to built an audio power amp, starting with a lot of LM3886'es and a +/- 75 V supply?

No:

I already have a design (attached), and I'm looking for comments about it. I am using output transistors, because I want the amp to be bridgeable. Putting 150v (+75v and -75v) across the LM3886 is not a good idea.

Will this amp work? I did simulations of it, and it turns out ok. Does anyone see any problems with it? Which transistors should I use for the supply and output?

The writing on the schematic is small, so for those of you who can't read it, here it is (from left to right):
First is the input (signal generator), then an 80 hz low pass filter, then a phase splitter with a gain of 5x, and finally the amps (25x gain). The overall gain of the amp is 125x for single-ended, and 250x for bridged.

[Upupa Epops]

By this connection is shifted also DC input voltage on LM's and input of this circuit will be destroy .

[Pjotr]

Originally posted by xplod1236

bootstrap2.jpg

Be careful adding extra power transistors at the output. Because of the extra phase lag you are risking stability problems.

BTW, I am missing a proper bias set-up to get the output drivers in proper class AB. Now it will unnecessarily suffer from cross over distortion.

Cheers

[xplod1236]

Would I get enough phase lag from this setup to cause problems?

How would I go about biasing this output stage?

[xplod1236]

I just realized that I don't need to bias the output transistors. The feedback is connected after the output transistors, and the opamp will compensate for the transistor bias. My simulation showed this as well.

This is from a document I found online. If anyone wants it, email me at xplod1236@wowway.com.

The loss across the bias network and across the transistor have been compensated for by the op amp.

The crossover distortion is gone. During the crossover time, when both transistors try to turn off, the output of the op amp makes a jump of several volts to bias the opposite transistor on. So the flat spot in the output has been removed by the step in the op amp's output voltage.

They even show graphs of the "jump", which looks exacly like my simulation (attached). The blue line is the opamp output, and the green line is the transistor output.

[xplod1236]

Does anyone see any other problems with it?