Lm3875 Help

[Tristanc1]

roughly how big were the old heatsinks compared to your new ones?

The new ones are 7cm long by 6cm deep by 7 cm tall. The old ones were roughly the same size, a bit longer, but not nearly as deep, only 3 cm or so, with less fins(about the same size I think as the ones in your amp, maybe a bit smaller). I don't think they were aluminium either if that makes a difference, quite a bit heavier the old ones were. I took off some of the thermal compound aswell, I put a bit too much on the first go round. Hope that helps!

Anyways, it made a huge difference, they've been on for around 3 hours straight, at a pretty decent volume and the heatsink is barely warm, chip's cool to touch aswell. I'll take some pics at some point.

Thanks for the info on the transformer aswell, they still kind of mystify me...but i've been pouring over Rod Elliots articles on them and that's helping...a lot to take in all at once though...

Tristan

 

[markiemrboo]

The new ones are 7cm long by 6cm deep by 7 cm tall. The old ones were roughly the same size, a bit longer, but not nearly as deep, only 3 cm or so, with less fins(about the same size I think as the ones in your amp, maybe a bit smaller). I don't think they were aluminium either if that makes a difference, quite a bit heavier the old ones were. I took off some of the thermal compound aswell, I put a bit too much on the first go round. Hope that helps!

Anyways, it made a huge difference, they've been on for around 3 hours straight, at a pretty decent volume and the heatsink is barely warm, chip's cool to touch aswell. I'll take some pics at some point.

Mine are actually really quite large then I think. They're about 11cm long and probably about 6/7cm tall. Not sure about the depth though. The really hot one is probably only 2cm deep. The other one might be about 4cm deep.

I guess maybe I just need bigger / better heatsinks Still hunting about for some good proper passive heatsinks like you might find in commercial amps. I can't seem to find anything in the UK at a reasonable price.

Thanks for the info on the transformer aswell, they still kind of mystify me...but i've been pouring over Rod Elliots articles on them and that's helping...a lot to take in all at once though...

Tristan

I am still learning too. There's just so much to learn Rod Elliots site helped me quite a lot!

Decibel Dungeon is also good if you don't know about that http://myweb.tiscali.co.uk/nuukspot/decdun/ It is the website of a member here, 'nuuk'

 

[!]

Out of curiousity, roughly how big were the old heatsinks compared to your new ones?

I actually still have trouble understanding how everyone says their chipamps barely get warm Mine get pretty hot with what I would consider quite large heatsinks!

I don't know the specs for those sinks but to approximate and put things in perspective, years ago I used equivalent size, similar design slot 1 CPU heatsinks to passively cool CPUs known to operate at about 15W /full load. They got too hot to comfortably hold your finger on for more than a single-digit # of seconds when the airstream from an adjacent PSU fan was removed during bench testing. The LM3875 datasheet http://www.national.com/ds/LM/LM3875.pdf , page 10, shows LM3875 dissipation could easily reach 15W.

Thus, I have to think your situation is normal, the heat is merely a product of input voltage and load during operation, and of course ambient temperature makes a difference too, that different people are comparing different conditions. The circuit could be oscillating, you could measure with a scope or measure per channel current consumption and compare with the other channel or the datasheet figures.