LM1875 Running VERY HOT

[Sybex23]

I thought I would start simple using an LM1875 for a simple amp. I bought a little kit from quasar kits, and put it together. I have the 1875 board and power supply kits. They went together fine, and sound really great, but this chip gets so hot, it actually shut down after about 20 minutes. I am using an athlon CPU heatsink that has a thermal resistance of about .5C/W. The 1875 is the non-insulated variety, and it is bolted to the heatsink and uses arctic silver. Here are the voltage and current measurements:

V+/V- = 28 volts/ -28volts
Quiescent current - 52 mA
DC Offset Measurement - 10.3 mV

All of these values are easily within the chips limits, and the current and DC offset are actually decent values. In some of the posts I have read here, people say that they have chips that do not even get warm at full volume, but I am not having such luck. I want to build a couple of bigger amps based on 3875 or 3886, but if I can't keep this cool, then I am sure those will be worse. The thing is, this amp is dead quiet - no hum, no hiss, and sounds really good. The heatsink is isolated from the chassis. The power transformer (avel-lind. 18 -0- 18 toroid) and rectifier/filter board are well separated from the amp board, and the input and output leads are on opposite ends of the amp board and away from the power leads. Not sure where to go from here. Suggestions??

[carlosfm]

You shouldn't run this chip so close to maximum voltage (+/-30V).
Use it at +/-25V.

As for the heatsink, it's square, right?
For the Athlon they are square, I think.
It seams small to me.
Use those bigger ones from the PIII, or buy something else, bigger than what you have.

Another thing: you get +/- 28V from a 18-0-18 trafo?
Measure AC at the secondaries to see what you get.
If it's around 20V (as I suspect), I would claim my money back.

[Nuuk]

Another thing: you get +/- 28V from a 18-0-18 trafo?
Measure AC at the secondaries to see what you get.
If it's around 20V (as I suspect), I would claim my money back.

Won't that be dependant on mains voltage though Carlos?

[djQUAN]

are you using the heatsink with a fan?

[BrianGT]

Originally posted by Sybex23
[B]I am using an athlon CPU heatsink that has a thermal resistance of about .5C/W.

I would assume that the 0.5 C/W implies with a fan on. I would doubt that an average cpu cooler is enough for more than low listening levels, unless you have one of the monster overclocking ones with a fan.

--
Brian

[Sybex23]

Thanks for the responses.

1. Yes, the heatsink is square, and is supposed to be able to sink 85 watts when fan cooled (rated for athlon xp 3200+), and around 40 - 45% of that without the fan - if I read the data sheet on it correctly. Wouldn't that be enough for this small wattage?

2. The transformer lists the mains at either 110 (or 115 i can't remember) for its specs, and mine is a little higher at around 124 by my meter, so that would give me a little bump. If I need to reduce the voltage a little - say a volt or 2 is using a couple of diodes in series to drop 1 ~1.5V per rail good enough ?

3. I have not yet used the fan because I did't think I would need it for this little chip. I'll go ahead and try it with the fan. A question here though - is it better to add an additional 12 volt source to run the fan from, or can I just tap off of the positive secondary? In other electronics apps I just grab power when I need it from whatever is close, but in dealing with audio, I don't have a lot of experience. I guess I can try it and see if it colors the sound at all.

Thanks for the advice. I hope to get things resolved and move on to one of those 4780 kits from BrianGT. I bet I could make a whopper of a guitar amp with a bridge/parallel combo on those chips.

[carlosfm]

Originally posted by Nuuk
Won't that be dependant on mains voltage though Carlos?

Yes.
I still see trafos being sold around here with 220V primaries.
Mains voltage changed here from 220V to 230V around 10 years ago.
When I measure mains voltage sometimes I get almost 240V, so imagine a 220V primary trafo... It gets and vibrates.
And gives more voltage on the secondaries, of course.

[ofb]

1. Yes, the heatsink is square, and is supposed to be able to sink 85 watts when fan cooled (rated for athlon xp 3200+), and around 40 - 45% of that without the fan - if I read the data sheet on it correctly. Wouldn't that be enough for this small wattage?

well, going by the charts the 1875 puts out about 30w at 28v for 8 ohm speakers, so that sink should be pretty warm. try a p3 sink as carlos suggests. you should be able to find those for nothing now. the local used computer shop gave me a handful of old ones when asked. bigger would be better of course, but the rectangular p3 sinks might be enough larger than the athlon's.

[Nuuk]

This CPU heatsink is what I use on each channel of my VBIGC using LM3875 chips. They only ever get slightly warm!

[ofb]

how big are they, nuuk? they look pretty compact on your site, but i don't see anything i can scale them against well.

generic p3 sink is about 55x125mm, with fins in three sections. 23mm high in the middle and 32mm high for the ends. athlon sinks in cheap computers are a smaller square, and probably have optimistic ratings.

but yeah, i'm surprised that sybex23 is reporting "VERY HOT". a lot of people run fairly small heatsinks.


... sybex23 is using arctic silver paste and non insulated chips. wasn't there a problem that arctic silver is somewhat conductive? was it here or in an overclocker forum i noticed that? there was a little flurry about it around a year ago. i didn't pay much attention.