Help with first Power Supply for LM1875

[simonlarusso]

Thanks for your information, it is most appreciated. As this is my first build I have a steep learning curve and everyones input is helping a lot

Originally I was using the two aluminium sides of the casing for heatsinks (80 mm high, 230 mm deep and about 6 mm thick). Do you feel these are adequate? One side for each IC of course.

Also the IC isolator which came with the kit is a rubber pad about 2-3 mm thick. I have applied heat compound between the IC and pad then pad and heatsink. However when I feel around the area where the IC is mounted it does not seem to be getting that warm. This seems to me that the heat is not tranferring through the pad. Would you recommend a better pad? Maybe a mica one?

[pacificblue]

Originally posted by simonlarusso
Originally I was using the two aluminium sides of the casing for heatsinks (80 mm high, 230 mm deep and about 6 mm thick). Do you feel these are adequate? One side for each IC of course.

That is not easy to calculate, and depends also on whether it is black anodized or not and on the mounting direction. My guesstimate is they could work most of the time, but maybe not for party or live SPL.

Originally posted by simonlarusso
Also the IC isolator which came with the kit is a rubber pad about 2-3 mm thick.

According to the instruction sheet there should have been a silicon washer. It should be a tenth of the thickness you mention.
Could the rubber pad be supposed to go between screw and IC as damage prevention?

Originally posted by simonlarusso
However when I feel around the area where the IC is mounted it does not seem to be getting that warm. This seems to me that the heat is not tranferring through the pad.

Looks as if you found the culprit.

Originally posted by simonlarusso
Would you recommend a better pad? Maybe a mica one?

Yes, if the original silicon washer has disappeared.

[simonlarusso]

The only washer I got with the kits were metal. This is the pad that I got in the kits:

http://www.maplin.co.uk/Module.aspx?ModuleNo=11889

As you can see it must go on the back of the IC with the raised hole going through the hole on the IC. But it is very thick.

Does anyone know where I can source a mica pad or tape in the UK?

[pacificblue]

Originally posted by simonlarusso
The only washer I got with the kits were metal. This is the pad that I got in the kits:

It does not look 2-3 mm thick in the photo. And it should also work. The FAQ section says it should be used without thermal grease.

[simonlarusso]

The ones I have are quite thick, but the one in that photo does look thinner. I will have to do some testing with the ones I have and no thermal grease. I might also buy these ones and see if they are thinner.

Thanks for your help, I am sure I will keep you posted

[simonlarusso]

I reseated the ICs on the heatsinks without an thermal compound and made sure they were good and secure. They chips are definitely cooler and I can comfortably keep my finder on them with no load.

So I think that solves my problem but I still need to get some 8 ohm speakers to test the amp fully.

[pacificblue]

Good news.

[simonlarusso]

Right I have managed to blow both the fuses on one of my amp boards! I think this was cause by plugging the input into my laptop. Any ideas why the laptop would have caused the fuses to blow? (I have been running my amp from an iPod or portable CD players so far)

[AndrewT]

it is an AC coupled amplifier, so It can't be DC offset problems.
No RF attenuation, could that be a problem.
The amp oscillated before and may be unstable just waiting for a high frequency whistle at it's input.
Did you hot plug it? That could have blown the fuses if it's unstable.

[simonlarusso]

I think I might have hot plugged it. I am getting some new fuses tonight so I can check nothing else was damaged. Also getting the 8 ohm speakers I have been waiting to get to test the amp properly.