LM1875 Heatsink Question

[nikon f]

I will be building a pair of the mono LM1875 kits from China, how long should the heatsink be for both amps? I will be running it at 24 volts.

 

[FauxFrench]

There is at least 10 different parameters that are important for how efficient a heatsink is in removing heat and for how much heat your LM1875 produces. It is a bit like asking: "I want to make a tent, how much clothes should I buy?".
If you show us the cooling profile with the most basic dimensions, we can give you an idea.

 

[adason]

You can get away with quite a small heatsink if you do not run it too hard. This is average size...

 

[johnr66]

Heat sink sizing is not trivial. You must consider the kelvin per watt rise of the heat sink, ambient temperature, air flow (inside a case will have worse air flow over the fins than external) and thermal conductivity from the chip to the HS. Having the HS not exceeding 60 Deg C. and using mica insulators is a good design. Avoid the rubber "sil pad" insulators as their thermal conductivity is worse.

To cut to the chase, a 11.5 x 5 high with 15 4 (cm) deep fins seem to work well keeping my amps cool for my listening use with ample margin. Remember no HS is too large. I always see people using too small HS sizes or orienting the fins in a non optimal way.

 

[rabbitz]

1.2°C/W or better for 2 mono amp modules (ref ESP).

I've used 1°C/W or better with good results and have used 2.5°C/W for a single channel which is a bit on the limit.

If running 4 ohm speakers, then you'll get more heat.

 

[chermann]

Yep


for 1 amp 2,8k/W heat sink is.small -lets say the minimum. it is OK if you don't have always party time. means max power at 24 rail.

 

[Bare]

A Heatsink block from a Computer CPU will typically do.

 

[tomchr]

A CPU cooler per LM1875 would be fine.

I suggest having a look here if you're interested in the theoretical background for heat sink sizing: Taming the LM3886 Chip Amplifier: Thermal Design – Neurochrome

Quick math suggests 2.1 K/W per IC for a 4 Ω load, assuming music signal played at clipping levels. 3.8 K/W per IC for 8 Ω load. So for a stereo amp 1 K/W would be good. A 50 mm tall (~2") chunk of the 8.0" serrated fin profile from HeatsinkUSA would be a good fit.

Tom