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Chip Amps Amplifiers based on integrated circuits

LM3886*T* without mica foil - working!(?)
LM3886*T* without mica foil - working!(?)
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Old 18th May 2019, 02:15 PM   #1
SchiZoID is offline SchiZoID  Germany
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Default LM3886*T* without mica foil - working!(?)

Hey folks,

I just finished a long term project - building a Chip-Amp with the LM3886. Its a 6-Channel Amp, so 3x Stereo (for now only sub and fullrange driven - later then also tweeters)

The thing here is, I did't use isolators between the cooling blocks and the Chips - but knowingly that the -V power supply is connected to the body of the LM3886 I separated the blocks. So every chipamp has it's own aluminum cooling block with no connection to the ground.

I ordered 0.0035" thin mica pces - but still worrying to use them, because the heatsinks feel pretty hot - and with them the heat would be transferred even worse.
So question here: If the heatsink itself is isolatet but the chip to the sink is not, do I have to worry about any negative sideeffects? So far it works pretty well!

Thanks in advance!

Michael
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Old 18th May 2019, 02:29 PM   #2
leadbelly is offline leadbelly  Canada
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Nothing to worry about, unless the way you isolated cannot withstand a reasonable jolt: you want to be able to transport them without causing a serious short later on.
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Old 18th May 2019, 02:45 PM   #3
Panelhead is offline Panelhead
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What are your rail voltages? The chips should not generate a lot of heat. If the rails are up close to the limit the chips get hot quick.
Mica pads, shoulder washers, and thermal paste will be fine. Then you can ground all the heat sinks.
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Old 19th May 2019, 12:32 AM   #4
tomchr is offline tomchr  Canada
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+1 for thermal pads and goop.

If your heat sinks get hot, it's likely because they're under-sized. If you run the LM3886 at ±28 V, you can expect each LM3886 to dissipate 40 W when operated at clipping with a sine wave signal into a 4 Ω load. If you then run three LM3886es per heat sink, they'll certainly get hot.

I go through the thermal design around the LM3886 here: LM3886 chip power amplifier thermal design.
You may find it insightful.

Tom
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Old 19th May 2019, 04:52 PM   #5
SchiZoID is offline SchiZoID  Germany
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Thanks for the answers!

Here are two pics of the thing.
The heatsinks are about 3x6x8cm for each chip a single one.. and they're finely finned not massive, alu..
And - yeaah I underestimated the trafos my PSU (only snubberized - no chips) drive 34V +/- which is close to the edge.

And yeaah another "unprofessional" choice: I use a wooden housing (no problems with cellphone or radio signals so far - and it looks damn good )
so no problems with connection to the ground. The 34 Volts stay there safely.

Michael
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Old 19th May 2019, 05:46 PM   #6
tomchr is offline tomchr  Canada
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Yeah. Those heat sinks are designed for forced air flow. By boxing them in like that and by having the fins horizontal, you're not giving them a chance to work. They will get hot.

You don't need a regulated power supply for the LM3886. A regular transformer+rectifier+caps (snubber or no snubber) will work just fine.

Tom
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Old 19th May 2019, 06:54 PM   #7
00940 is offline 00940  Belgium
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A pair of little fans screwed at the back of your box would go a long way to keep the heatsinks cooler. Some fans are really very silent these days.

And yes, isolate your heatsinks. They're too exposed, you'll end up shorting them to gnd one way or another (Murphy's law will take care of that).
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Old 19th May 2019, 10:02 PM   #8
SchiZoID is offline SchiZoID  Germany
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Yes it's true that the fins are designed for airflow - but the principle of "surface area enlargement" should also do its part. - and horizontal or not they are exposed to the outside.

I think - if not just living with that state - lowering the voltage would be chance.
Is there an easy way - without exchanging the the whole toroids? (lowering R1 e.g.?)

Click the image to open in full size.
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Old 19th May 2019, 11:06 PM   #9
Tromperie is offline Tromperie  Australia
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Those pictures show a disaster waiting to happen.
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Old 20th May 2019, 12:49 AM   #10
Panelhead is offline Panelhead
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YOU HAVE TO INSULATE THE HEATSINKS. Thirty four volts dc may not kill you, but will not be pleasant.
A part of your heating may be airflow across the fins. I try to run mine vertical to increase the airflow. Plus slot the top and bottom panels.
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