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Old 9th April 2007, 04:43 PM   #11
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I think Andrew may be right depending on the heat sink size and the supply voltages. If you can't see any oscillation on a scope then there is probably not enough to worry about.

-SL
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Old 9th April 2007, 08:22 PM   #12
SBob is offline SBob  Sweden
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OK, I am feeling re-assured that there is no oscillation going on. It is only using 50mA (which seems about right from the data sheet) and I cannot see anything on the scope. I will try the AM radio thing though, but since other peoples 3886s also gets warm I think mine is normal.

I just found it odd that it gets warm both with and without any load, but the heat sink on my test rig was a lot smaller than jaycees. (In fact, it was a flat piece of aluminium 25X12 cm for both channels. I guess the fact that my 3875 never even gets warm made me over confident...)

I was looking at the data sheet and +-27V into 4 ohms will produce about 40W, which will need a heat sink with 1.9 C/W. So I will have to get a big heat sink, which will hopefully keep it cool.

Thanks everyone for your help!

/Robert
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Old 9th April 2007, 10:19 PM   #13
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by SBob
It is only using 50mA (which seems about right from the data sheet) ....I just found it odd that it gets warm both with and without any load, but the heat sink on my test rig was a lot smaller than jaycees. (In fact, it was a flat piece of aluminium 25X12 cm for both channels. I guess the fact that my 3875 never even gets warm made me over confident...)

I was looking at the data sheet and +-27V into 4 ohms will produce about 40W, which will need a heat sink with 1.9 C/W. So I will have to get a big heat sink, which will hopefully keep it cool.
sounds like you built it before comprehending the datasheet design procedure!
Might as well jump in with both feet. The worst is I'll only sink.
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Old 9th April 2007, 10:49 PM   #14
jaycee is offline jaycee  United Kingdom
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you might want to check for good thermal coupling with the heatsink. In doubt, sand the heatsink with some fine sandpaper to make sure it's flat. If you are using isolated heatsinks (a good idea!) Mica will do but make sure it's quite thin, and you have to use paste thinly.

Personally, I used the grey "sil pad" things with mine (the TO3P ones fit just fine) and i dont have a heat transfer issue. Saves the mess of paste, too.
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Old 10th April 2007, 04:25 PM   #15
EAH is offline EAH  Panama
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Please:

Can you tell me what is :
grey "sil pad" things

EDUARDO AH.
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Old 10th April 2007, 06:07 PM   #16
jaycee is offline jaycee  United Kingdom
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http://www.bergquistcompany.com/tm_sil_pad_list.cfm
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Old 10th April 2007, 08:22 PM   #17
EAH is offline EAH  Panama
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TY
I will try

EDUARDO AH.
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Old 10th April 2007, 08:28 PM   #18
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Quote:
Originally posted by SBob
OK, I am feeling re-assured that there is no oscillation going on. It is only using 50mA (which seems about right from the data sheet) and I cannot see anything on the scope. I will try the AM radio thing though, but since other peoples 3886s also gets warm I think mine is normal.

In fact, it was a flat piece of aluminium 25X12 cm for both channels.

I was looking at the data sheet and +-27V into 4 ohms will produce about 40W, which will need a heat sink with 1.9 C/W. So I will have to get a big heat sink, which will hopefully keep it cool.
Your cooling isn't much. You've better try a serious heatsink. Check ELFA how 1.9 W/C heatsink look like.
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Old 6th May 2007, 07:47 PM   #19
SBob is offline SBob  Sweden
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I finally got around to finishing the amp. As you can see in the image I built two heatsinks from scrap pieces of aluminium. They seem to be enough since they only get luke warm when playing at fairly high volume.

I used the approximation formula 50/sqrt(Area) = C/W to get an idea of how big they shold be.

It seems that the 3886 requires a lot more heatsinking than the 3875 in my previous GC.

How it sounds? Much better, I think, than the NAD 7240 it replaces. (I use it to drive the woofers in an active system. Another GC is driving the tweeters, which might explain why I prefer to match it with another GC)
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Old 27th June 2013, 10:00 AM   #20
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Default LM3886 warmer than LM3875 with no signal

Quote:
Originally Posted by SBob View Post
Hi, I have just finished building a 3886-based Gainclone. However, I suspect that it is oscillating since it is getting warm, but not hot, even when there is no music playing. My old Gainclone, with a 3875, has always been very cool when it is not playing.

So, my question is: is it normal that a 3886 GC is getting a bit warm? Or should I start searching for oscillation? Would be nice to know to avoid going on a wild goose chase...

Some data: Voltage is +/- 27 volts. No zobel or LR (yet). Point2point. Star grounding with separate signal and power stars (the same set-up as my previous GC)

/Robert
The LM3886 has a higher idle current than the LM3875 so it will will be warmer than LM3875 even with no signal as it has to dissipate more power.
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