Normal temperature of LM3886 - diyAudio
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Old 5th April 2007, 08:41 PM   #1
SBob is offline SBob  Sweden
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Default Normal temperature of LM3886

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
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Old 6th April 2007, 10:27 PM   #2
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It should be similar in temperature to the LM3875 under the same conditions of voltage, heat sink, and input signal. If you suspect there is some oscillation then add the snubber, R+C, and that should cure it.

-SL
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Old 7th April 2007, 07:45 AM   #3
SBob is offline SBob  Sweden
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The conditions are almost identical, the only thing differing is the chip, the 10k mute resistor and the PS has two 2200uF caps/chip instead of one 10000uF/rail. But the wiring is of course a bit different, which could explain that it picks up something to make it oscillate.

I tried looking at the output with the oscilloscope, but could not see anything apart from a little noise, ~1 mV. I also tried building a simple RF detector with a diode, condenser and a resistor, but the voltage over the resistor was zero when applied to the output.

I will try adding the R+C network (there are already 100nF caps over the PS caps) and see if it helps, but it would have been fun being able to actually see the oscillations.

/Robert
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Old 7th April 2007, 08:05 AM   #4
jaycee is offline jaycee  United Kingdom
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Add decoupling close to the IC as well, never hurts...

My own LM3886 amp (with a PCB) is stone cold when idle, and it only has the output inductor+resistor, no zobel... but I have 100nF on each rail close to the IC, and 330uF on the board (with 2x4700uF per rail off board).
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Old 7th April 2007, 08:26 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
3875 datasheet shows 30 to 37mA quiescent current.
The 3886 datasheet specifies Iq<85mA
That difference alone can account for the temperature difference.
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Old 7th April 2007, 07:23 PM   #6
SBob is offline SBob  Sweden
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I have now tried adding an R+C network (4R+100nF), but the chip is still getting warm. I measured the quiescent current to 52 mA, which apparently is within normal range.

I'm wondering, if it were oscillating, would it not draw more current? Is there any way of actually "seeing" the oscillations?
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Old 7th April 2007, 09:37 PM   #7
Nordic is offline Nordic  South Africa
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Yep, I wouldn't say my lm3886s run cool, and they are on 10 x 10 x 4 cm sinks each.... Are both equaly hot...? If not, look at the bad channel's speaker with an ohmmeter...

Likely your scope isn't good enough to see HF either... so you cant rule out oscillation....

I think if it doesn't go into thermal protection during normal use, you are OK.
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Old 9th April 2007, 12:12 AM   #8
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Default ditto

Each of my 3886s are on an AMD heatsink. Warm if under half volume, hotter than the bad place if above. I re-attached the original PC fans as I use this amp in my garage and CRANK-IT-UP! I run it at near top volume as all the way will hurts your ears in just a few seconds - but there is no distortion!
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Old 9th April 2007, 12:27 AM   #9
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A handy way to find HF oscillations is to use an ordinary transistor radio tuned between AM stations. Bring the antenna near the DUT and see if you hear anything. Don't short anything out! Maybe cover the antenna with some shrink tubing or similar. This can work to find RF coming from power supplies as well. It's not definitive, but another tool to add to the arsenal. IMO, you probably don't have oscillations, or you would have seen it on the scope. It's probably just the quiescent current of the device heating things up.
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Old 9th April 2007, 01:31 AM   #10
jaycee is offline jaycee  United Kingdom
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Ah, the smaller heatsink might explain why mine is stone cold. Mine is 160mm wide, 25mm deep and 75mm high that's for both channels though.
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