LM3875 into 4 Ohm

audioPT · 2005-07-06 3:47 am

I know this is a stupid question...

Can the chip hold a load of 4 Ohm?

With 24-0-24V?

dw1narso · 2005-07-06 4:05 am

I have mine (Brian GT's standard kit) drive my Dynaudio Audience 40 (4 ohm, min 3.9ohm) with ease... but I use 2x18V (25V rail)...

ocool_15 · 2005-07-06 4:12 am

I believe the datasheet says 40W into 4ohms but with those rails check the graphs in the datasheet to see the power youd have.

Mick_F · 2005-07-06 9:13 am

According to the datasheet the power of the LM3875 steeply drops below 8 Ohm. Use the LM3886 instead.

Mick

joensd · 2005-07-06 9:51 am

According to National´s calculator you´d still get around 50W out of it but the required heatsink theoretically would be too large.
(amp will be current limited as well)

Beyond 10W output power the power dissipation grows quite fast.
If you don´t need high power and install big enough heatsinks I don´t see a reason not to use it.

If you´d have to buy the chips first though get the LM3886 obviously or parallel two 3875.

Leolabs · 2005-07-06 2:50 pm

Please dont try this at home,unless u use lower supply voltage.Check the datasheet of LM3875.

joensd · 2005-07-06 4:11 pm

With 24-0-24V?

In my reply I was assuming that you mean 24-0-24V AC
so around +-35V DC.

If your rails are +-24VDC there´s no problem at all.

Please dont try this at home

I already did and guess what : nothing special happened; besides it gets hot very quick.

Of course I do assume that audioPT wants to build something with parts he´s got on hand.
If the transformer still has to be bought I wouldn´t choose 24VAV-secondaries. (rather 18VAC max.)

There are quite a few threads about this BTW. 😉

greets

audioPT · 2005-07-06 4:12 pm

Mick_F said:
According to the datasheet the power of the LM3875 steeply drops below 8 Ohm. Use the LM3886 instead.

Mick

I can't, the amp is ready! :bawling:

joensd said:
According to National´s calculator you´d still get around 50W out of it but the required heatsink theoretically would be too large.
(amp will be current limited as well)

Beyond 10W output power the power dissipation grows quite fast.
If you don´t need high power and install big enough heatsinks I don´t see a reason not to use it.

If you´d have to buy the chips first though get the LM3886 obviously or parallel two 3875.

Is this heatsink enough? 20x10x4 cm

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Leolabs said:
Please dont try this at home,unless u use lower supply voltage.Check the datasheet of LM3875.

It will be feeding this babes!!

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I've tested it in my bench with a 22-0-22V 150VA transformer with 10.000uF in each rail with 4 Ohm speaker and it didn't get hot...

An externally hosted image should be here but it was not working when we last tested it.

My doubt is if the chip live enough at mid power...

Peter Daniel · 2005-07-06 4:18 pm

There is no problem at all running LM3875 into 4 ohm loads with +/- 34V rails. But it will also depend how much you will be pushing the amps.

The heatsinks in a picture are fine.

audioPT · 2005-07-06 4:31 pm

Peter,

As I live in an apartment, the power can't go very loud... 😀

They will be running at half power, more or less...

Well, I'll try it in my room for a wile. If it gets :hot:

, I'll just turn it off...

BTW, the speakers I showed aren't finished yet 😎

carlosfm · 2005-07-06 8:59 pm

audioPT said:
BTW, the speakers I showed aren't finished yet 😎

Do you need help, this weekend?😉

:drink:

audioPT · 2005-07-07 12:54 am

I'll go to Famalicão saturday night...

But, we can have dinner friday or lunch saturday 😎

At this moment I don't have any amplifier working... But I'm working hard to let you hear my modest sound :devilr:

I'm changing my system and, as I said to you by phone, I'm painting some rooms in my place.

BUT,

You WILL see and ear my system.

And after it, feel free to :judge: