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Old 6th July 2005, 02:47 AM   #1
audioPT is offline audioPT  Portugal
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Default LM3875 into 4 Ohm

I know this is a stupid question...

Can the chip hold a load of 4 Ohm?

With 24-0-24V?
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Old 6th July 2005, 03:05 AM   #2
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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)...
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Old 6th July 2005, 03:12 AM   #3
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I believe the datasheet says 40W into 4ohms but with those rails check the graphs in the datasheet to see the power youd have.
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Old 6th July 2005, 08:13 AM   #4
Mick_F is offline Mick_F  Germany
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According to the datasheet the power of the LM3875 steeply drops below 8 Ohm. Use the LM3886 instead.

Mick
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Old 6th July 2005, 08:51 AM   #5
joensd is offline joensd  Germany
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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.
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Old 6th July 2005, 01:50 PM   #6
Leolabs is offline Leolabs  Malaysia
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Please dont try this at home,unless u use lower supply voltage.Check the datasheet of LM3875.
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Old 6th July 2005, 03:11 PM   #7
joensd is offline joensd  Germany
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Quote:
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.

Quote:
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
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Old 6th July 2005, 03:12 PM   #8
audioPT is offline audioPT  Portugal
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Quote:
Originally posted by Mick_F
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!

Quote:
Originally posted by joensd
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

Click the image to open in full size.

Quote:
Originally posted by Leolabs
Please dont try this at home,unless u use lower supply voltage.Check the datasheet of LM3875.
It will be feeding this babes!!

Click the image to open in full size.


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...
Click the image to open in full size.

My doubt is if the chip live enough at mid power...
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Old 6th July 2005, 03:18 PM   #9
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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.
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Old 6th July 2005, 03:31 PM   #10
audioPT is offline audioPT  Portugal
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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 , I'll just turn it off...

BTW, the speakers I showed aren't finished yet
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