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Old 30th July 2012, 04:32 AM   #31
Arx is offline Arx  Canada
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Generally, as long as it's aluminum and your voltage is reasonable.
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Old 30th July 2012, 05:31 AM   #32
weiyan is offline weiyan  Hong Kong
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Quote:
Originally Posted by Arx View Post
Generally, as long as it's aluminum and your voltage is reasonable.
Thak you. They are using aluminum chassis. So, Sticking the chip to bottom of chassis no more heat sink required.

The a/c supply is 27V *2, is this voltage reasonable?

Last edited by weiyan; 30th July 2012 at 05:36 AM.
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Old 30th July 2012, 03:24 PM   #33
Arx is offline Arx  Canada
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if that's 27VAC x 2 then it's a bit high, and it'll run pretty warm (and you'll have to stick to 8 ohm speakers). You'll have to give it a test run to see. I ran some 3875s on 25VAC secondaries, and it did get quite warm, though not enough to be a problem on the chassis I was using. Currently I'm running mine on a pair of 14.4V transformers, and it doesn't even get warm enough to feel it.

It that's 27VDC per rail, it should be no problem.
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Old 30th July 2012, 03:45 PM   #34
macboy is offline macboy  Canada
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Quote:
Originally Posted by weiyan View Post
Thak you. They are using aluminum chassis. So, Sticking the chip to bottom of chassis no more heat sink required.

The a/c supply is 27V *2, is this voltage reasonable?
Don't forget that the chip's tab (on non-insulated versions) is at -V supply potential, not ground. You need to electrically isolate the chip from the chassis (or any heatsink) using appropriate measures, like silicone or mica insulating washers and plastic mounting screws or bushings.
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Old 10th February 2016, 02:00 PM   #35
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Default heat sink size!

found on datasheet a 'Power Dissipation vs Output Power' curve for 8ohm load and it references +/-40V power supply!
I will limit mine to 30W for speaker...
going to look at some heat sinks now and double check the thermal resistance numbers.
http://www.ti.com/lit/ds/symlink/lm3886.pdf
Figure 36, then Figure 34...
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Old 12th February 2016, 12:38 PM   #36
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Okay, I ran my transformer, rectifier and 10,000uF caps with three different loads!
The output voltage unloaded is about +/-40, but it runs from 30 to 34 loaded.
Should give me enough overhead for high peak to average signals with 15.5V rms average?!
Chart says thermal resistance of around 3.
Found a nice, small heat sink with thermal resistance of 2.1 and is only about 2.5" square. Wakefield p/n 517-95AB.
guess I'm running out of excuses...need to finish up ChipAmp pcb stuffing and run it a little. going to use 8ohm resistors at first?
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Old 13th February 2016, 11:53 AM   #37
AndrewT is offline AndrewT  Scotland
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Originally Posted by tanderson2009 View Post
.....................
The output voltage unloaded is about +/-40,..............
The maximum voltage for the 3886 is 84V (42Vdc).

What is the mains voltage range for your supply?
What was the mains voltage when you measured 40Vdc?

It would take an increase of just 5% on your mains voltage to exceed the 84V absolute maximum for your chipamp.
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Old 13th February 2016, 11:59 AM   #38
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by tanderson2009 View Post
...............
Chart says thermal resistance of around 3.
Found a nice, small heat sink with thermal resistance of 2.1 and is only about 2.5" square. Wakefield p/n 517-95AB.
guess I'm running out of excuses...need to finish up ChipAmp pcb stuffing and run it a little. going to use 8ohm resistors at first?
I recommend that you double the National guide value from their design chart Figure 34.

If @ 40V & 8r0 test load with Ta=25C, then National show 1.9C/W
I would suggest you use 0.95C/W per channel, for 8ohms @ 40V with Ta<=30C

From Fig34 I see 3C/W for 69V @ 25C & 8r0 test load. Is that what your read?
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Last edited by AndrewT; 13th February 2016 at 12:02 PM.
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Old 13th February 2016, 02:22 PM   #39
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Thanks, AndrewT! So good to have knowledgeable feedback!
First, AC Mains: just read VAC @ 124.1 with output +/- 39.4 VDC Unloaded, +/- 34.7 VDC at 0.54 ADC. so the output drops quickly with load.
I don't know how much higher the mains would get...this is already 10% high, i thought!
Also, the datasheet says the Max Supply 'No Signal' is 94 VDC... 'Signal' is 84. I feel like i'm splitting hairs with my poor amplifier's life!
I agonized at the transformer choice...this Triad line goes from 48CT down to 36CT. Maybe i need to go find an in between? there are some 22-0-22 VAC out there, but not many. what do you see for successful LM3886 projects? I do want maximum peak for my 30W speaker, clean signal, which is 15.5VACrms. i thought 2:1 is minimum. is LM3886 bad choice for this?
This is a 'plywood prototype' at this point...maybe i'll try it anyway...Would the $6 amplifier chip fail or would that cause the $50 speaker to fail? Sure wouldn't want the amplifier output stage to short and put that DC power supply across my speaker...my $50 speaker protected my 10cent fuse?
thx,
tom
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Old 13th February 2016, 02:33 PM   #40
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AndrewT thermal resistance!
I ran three different power supply load points and carefully looked at the charts. i will never have full power at +/-40...the transformer sags too much.
2x20ohm load yielded +/-30VDC; that would be audio power of 26Watts. Chip dissipation of 26Watts, thermal requirement at 40degC ambient 3.2degC/Watt. so my heatsink at 2.1 i thought should be okay...?
Of course, now i'm doubting my chip choice!!! HELP!!!
i don't have a huge investment in the LM3886, and will gladly keep the two ChipAmp PCBs for other projects...now i'm not even convinced about my 48VCT transformer...boy i love design!
But i am determined to play guitar through these two ten inch 30W Celestion speakers. Celestion said that higher peak voltages are not a problem as long as the voice coil heating stays below the 15.5Vrms value...
This still seems almost 'right-sized', please advise!
thx,
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