LM3875 into 4 Ohm

audioPT · 6th July 2005, 02:47 AM

I know this is a stupid question...

Can the chip hold a load of 4 Ohm?

With 24-0-24V?

dw1narso · 6th July 2005, 03:05 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 · 6th July 2005, 03:12 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 · 6th July 2005, 08:13 AM

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

Mick

joensd · 6th July 2005, 08:51 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 · 6th July 2005, 01:50 PM

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

joensd · 6th July 2005, 03:11 PM

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

audioPT · 6th July 2005, 03:12 PM

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

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!!

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...

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

Peter Daniel · 6th July 2005, 03:18 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 · 6th July 2005, 03:31 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

, I'll just turn it off...

BTW, the speakers I showed aren't finished yet

6th July 2005, 02:47 AM	#1
audioPT diyAudio Member Join Date: Aug 2002 Location: Right here	LM3875 into 4 Ohm I know this is a stupid question... Can the chip hold a load of 4 Ohm? With 24-0-24V?

6th July 2005, 08:51 AM	#5
joensd diyAudio Member Join Date: Dec 2001 Location: Germany	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. __________________ jens

6th July 2005, 03:18 PM	#9
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	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. __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Lm3875 Rev C	sumacSK	Chip Amps	41	2nd January 2006 01:51 PM
yet one more lm3875	karma	Chip Amps	8	27th July 2005 08:48 PM
WTB: LM3875 pcb	vdi_nenna	Swap Meet	7	27th July 2005 03:19 PM
LM3875 60W Amp	juiceman	Chip Amps	3	18th February 2005 06:28 PM
What LM3875 to buy?	AJ Bertelson	Chip Amps	12	18th January 2004 02:40 PM


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

6th July 2005, 03:05 AM	#2
dw1narso diyAudio Member Join Date: Mar 2004 Location: Jakarta	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)...

6th July 2005, 03:12 AM	#3
ocool_15 diyAudio Member Join Date: Nov 2004 Location: sk	I believe the datasheet says 40W into 4ohms but with those rails check the graphs in the datasheet to see the power youd have.

6th July 2005, 08:13 AM	#4
Mick_F diyAudio Member Join Date: Jun 2005 Location: Cologne	According to the datasheet the power of the LM3875 steeply drops below 8 Ohm. Use the LM3886 instead. Mick

6th July 2005, 01:50 PM	#6
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Please dont try this at home,unless u use lower supply voltage.Check the datasheet of LM3875.

6th July 2005, 03:31 PM	#10
audioPT diyAudio Member Join Date: Aug 2002 Location: Right here	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

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read