(1)4780 vs. (2)3886 for 4 ohm load

[WWWJD]

Hello all,

I just went through building my first 3886 kit, dual mono. I must say I love this little amp. Picked it as a winter project and have really enjoyed it. All that's left is a case, and now that it's warm, this project is just about put to bed. Plan to do a red oak shell that wraps around the perimeter, just need some table saw time.


I built this amp, just because, and because I eventually want to start designing my own speakers and what not and have a small powerful desktop amplifier. I think I hit my goals successfully; finished dimension will be 10.5W x 4H x 9D. It runs a pair of 4 ohm Usher Towers beautifully and really enjoy listening through this amp (imac piped through inboard bantam DAC).

I've enjoyed this one so much, I want to build another one... more powerful, more portable. No other really good reasons other than just because. So, I've been reading through National's data sheet somewhat, and it doesn't really come out and say that the 4780 will run a 4 ohm load in parallel. What do you guys think? I've read elsewhere that I may be better off trying to strap dual 3886 per channel vs. the single 4780 for reasons of heat dissipation and pin spacing difficulties. I'm sure down the road, the amp will see 8 ohm loads as well.

[ChrisA]

Originally Posted by WWWJD

...
I've enjoyed this one so much, I want to build another one... more powerful, more portable. No other really good reasons other than just because. ...

If you want more power, the very best way is to bi-amp the speakers. Build a four channel amp that is like your two channel amp.

The next step up the food chain is to parallel the two bass channels.

If getting power to the speaker drivers is the goal it makes much more sense to place the crossover BEFORE the power amp so that it gobbles up a few less watts. Also if the x-over only has to handle line level it can be very in-expensive or for constant budget much more sophisticated.

Back in the old days amps were expensive. But now these LM3886 chips sell for under $10. There is not reason not to lets ever speaker driver have it's own amp. in an "amp per driver" design.

[AndrewT]

Originally Posted by ChrisA

Back in the old days amps were expensive. But now ......chipamps sell for under $10. There is no reason not to let every speaker driver have it's own amp. in an "amp per driver" design.

Yes, what can be wrong with an amp per driver philosophy? It's the PSU and chassis that are expensive.
A multichannel amp located right beside the speaker terminals with very short cables must be better than centrally located amps and long speaker cables.

[olblueyez]

Did you strip the HK pot out of an AVR or were you able to order it somewhere. If you can order it then I may change my mind about not adding a volume control to my next project.

[WWWJD]

No, I made that one with some acrylic and a blue velvet pot. It's 2-1/4" diameter.

[olblueyez]

Nice work

[pacificblue]

To address the initial question. Two parallel channels can drive a 4 Ohm load, because that is the same as each channel driving an 8 Ohm load.
Heat dissipation is of course an issue. The datasheet specs the same 125 W Pd max for the LM3886 as for the LM4780. That makes it obviously difficult for the latter to dissipate double as much heat without any restrictions.
Choose the LM4780, if you want the power only as headroom. Choose two LM3886, if you want to use the power continuousy.

[fastbike1]

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