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question about paralleling amps

I'm considering building an audio amp using some LM3886's from National. One thing that slightly worries me is that on one of the plots of power vs load resistance, it really takes a dump below 4 ohms. I currently have speakers that dip to three ohms, so I am considering paralleling two to reduce the load on each. It doesn't mention paralleling amps in this datasheet, but in another it just sez to match the gains and put a .1 Ohm resistor between each of the amps.

One question is that the datasheet sez that each amp sees twice the load resistance, but doesn't each amp actually see a .2 ohm resistance to the other amp? Will that cause stability problems?

The next question is if there is some kind of a standard trick, using feedback or other gizmitry, to better load balance the amps and also reduce the output impedance to nearly zero (or even slightly negative to undo the effects of speaker cables).

Reply to
alan
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I haven't tried this, but (assuming it requires external feedback networks) it might work better if you parallel the chips, and use a common feedback network for the two. That will get resistor tolerances out of the picture.

Tam

Reply to
Tam/WB2TT

I read in sci.electronics.design that alan wrote (in ) about 'question about paralleling amps', on Mon, 27 Dec 2004:

Unless the 3 ohm region is very extended in frequency and is at the low frequency end of the spectrum, you almost certainly don't need to worry about it. Paralleling amplifiers could run you into much more grief.

That works for some amps and not others.

It could. Also, if the drive to one amplifier fails, the other sees very nearly a short-circuit.

There are ways to do that, but really you almost certainly have no need. Stability issues occur with these techniques.

You really don't need to do that. Remember that the voice-coil resistance is by far the largest resistance in the circuit (and it varies with temperature, which effectively means programme content), so that reducing the cable resistance cannot have a significant effect.

If you want to be very precise, find out which cables the loudspeaker designer used when developing the product and use those cables. The sound will then be **as the designer intended**.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

This will completely defeat the purpose of putting the current sharing

100 mOhm resistors in there in the first place.

--Mac

Reply to
Mac

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