LM3875/LM3876 load question.

It just so happens that i am currently designing a very small and extremely cheap integrated amp based around the LM3875, LM1894 and PGA2310.

The LM3875 should run into a 2ohm load but output power will be down to around 25W and to avoid the protection circuit activating you will have to decrease this further leaving you with a usable power output of 15W rms with rather large heatsinks ..... you could run multiple LM3875's in parallel with small value (0.01ohms) resistors at the output of each chip.

Just keep the supply rails low say +/- 20V and the voltage gain of the amp very low and make sure the input signal is fairly small (it may need a set amount of attenuation on top of any variable attenuation used as a volume control).
 

Jamie F

Member
2001-12-15 12:33 am
UK
LM3886 has greater power handling, but will still activate its protection at very low voltage swings with this sort of load, unless you come up with an ingenious method of getting rid of the heat.

My woofers present a 2.5R load (2 parallel 5R units on each side), but I decided to bi-amp, so each driver gets its own LM3886.

If this isn't an option, parallel operation helps, but doesn't double the benefit. Though the current is shared, unfortunately the relationship between current and temperature rise isn't linear.

Also this design won't do 2R, the approach may help...

http://www.
national.com/appinfo/audio/files/BPA-200_Application_Note.pdf
 
If the current sharing is equal or close enough to it and you use a heat sink with half the thermal resistance or give each chip its own heatsink with a low enough thermal resistance then you will get ~90% increase in current handling.

Also rest assured that the LM3875, LM3876 and LM3886 will work into a 2 ohm load just with very much reduced power output ... but as stated above they really are useless into a 1ohm load unless you parallel 4 or more of them. Therefore, using the bridged / bridged-parallel designs are not appropriate for a 2 ohm load as due to the nature of bridging each chip will see 1 or 2 ohms respectivly.

Jamie F,
As for the ingenious way of getting the heat out of the chip ..... they're called peltier (or thermoelectric) coolers.
 
I just saw the LM3886 datasheet, hadn't noticed it before...

I wanted to know if it could handle 2ohm load, because I wanted to bridge two of them to drive a 4ohm load for more power output, but if it's power output at 2ohms is rather low (since it would have to be) I guess this idea just plain sucks. Unless there's another way to use multiple LM3876's or LM3886's for greater power output for driving a 4ohm driver... (I'm currently reading the link partyjups gave me, maybe those will shed some light on the subject).
 
Well. The LM3886 has a power output of 68w into 4ohms, I was wondering, do I really need more then that?

I mean, some people build 20w amps, and then I see 500w amps and higher (why such a big difference?). I'm going to use it for home listening, and while I like loud, I don't want something that makes me have to wear earplugs...

And after reading partyjups link on bridging and using LM3886's in parallel I think I better not try that on a first attempt, just more chance of me doing something wrong... :(
 
downhere said:
Peltiers are just good at lowering chip temperatures, e.g removing heat, but they do not do much in reducing overall heat production. I think a previous thread had something about it... overclocking circles also have alot of info on the subject

Yes i know so you then stick a massive heatsink (maybe with a fan) on the other side of the peltier or better yet a water cooling rig ..... oh the price we pay for this hobby of ours.
 
But with peltiers they also produce heat of their own. And you need one rated higher then the wattage of heat you intend to move. They're rather inefficient.

You'd only use a peltier to cool something below ambient. Because whatever you'd use to cool the peltier would cool whatever you want to cool, down to pretty near ambient anyways. Even in CPU overclocking peltiers don't have many uses. Try dissipating 300w+ of heat from from a computer...
 

downhere

Member
2001-10-03 10:24 am
Yeah peltiers are just used for overclocking, it may improve the sound I guess, by producing a steady temperature, but peltiers introduce their own problems. E.g condensation, you have to insulate practically everything around the peltier, high heat on one side of the peltier, and watercooling the peltier, which is a huge problem by itself.
 
A peltier wouldn't exactly produce a steady temperature. A rise in heat output of the device being cooled would result in a rise in heat being transfered from the cold side to the hot side of the peltier, therefore also increasing the amount of total heat having to be dissapated. While I may seem to be putting peltiers in a bad light, they do have their uses and are excessively usefull in some applications. They do for one work rather well if you ever intend to build a DIY mini-fridge :).
 
JoeBob said:
Well, I've found a solution to my problem, I'll just bi-amp and use two LM3886's per channel, still powering 4ohms but gives me more power. Everyone's been telling me to bi-amp anyways...
Excellent solution. If you're not already aware of it, this site has much info on biamping and projects for LM3886 amps and active crossovers:
www.sound.au.com
If you already have 4-ohm speakers, fine, but if you haven't got them yet check the 3886 specs carefully. Depending on your supply voltage and heat sinks, you might actually get more power into an 8-ohm load.