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Old 24th May 2010, 12:04 AM   #1
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Question LM3876 parallel into 6ohms. Possible?

So I was recently gifted several 50w 3876 kitsets. Ex stock from a storeowner friend.

I have four of 'em, and I'm wondering if it's possible (and what changes would be necessary) to run two of these kits per channel in parallel into my 6ohm speakers

Also I'd love to hear recommendations for heatsink values and power supply rail voltages (if they need changing for this).

I've got at my disposal two 25Vx2 toroids, one 160VA and one 300VA and four 80V 8000uF capacitors (big cans. I plan to use them all on the power supply as I have no use for them elsewhere). Is regulation necessary for a good sound, or is 10u/1u/100n snubbing enough? (Again, I have lots of these snubbers lying around. All are metallised polyproplene)
Was planning on making a cheaper heatsink by bolting several computer CPU heatsinks (fans removed) to thick piece of aluminium (Have tonnes lying around and retailers charge through the nose for simple metal boxes + heatsinks..)

OR

Would I be better making these into a stupidly powerful bridged parallel monoblock amp? This is really my last resort if I cant use these as two parallel pairs. If I was to do that, I'd need to eventually build a second amp - or use this one for a sub to annoy the neighbours

Scanned in the circuit diagram. Any other info needed, ask away.

Cheers in advance.

Click the image to open in full size.


Also volume control - I have some double ganged 50k pots, or some single gang 10k pots. double ganged makes life easier, but is 50k too high?
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Old 24th May 2010, 02:50 AM   #2
infinia is offline infinia  United States
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Hi
I think building up a normal gain clone with the parts you have should be good for 6 ohm speakers. After your intial success you could plan for future upgrades if any.
I wouldn't recommend parallel unless you really like a challenge, Or a potential for hair pulling experience seems appealing. Esp. on a PCB NOT designed for parallel operation. If you are really determined, you could experience it vicariously by reading up on an similar example here.

Yes 50K pots should be good enough here, but you will need to add a good DC blocking cap between the pots wiper and the amps input. A 1uF to 4.7uF should be fine for this, polyp. film cap gets extra credit.
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Old 24th May 2010, 05:50 AM   #3
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Just build them,If you are planning on giving them a sever test in to the 3 ohm range then post your results. jer
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Old 24th May 2010, 05:56 AM   #4
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Sorry iI didn't mean to be so sever ,Just build them, and post your resuslts.
Opamps are opamps.
If you follow the proper guidlines you should turn out with some thing decent. jer
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Old 24th May 2010, 07:34 AM   #5
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With 35 V rails and 6 Ohm load you should definitely use a parallel setup for two reasons. One is heatsinking and the other is the 5 A current limit per IC. A resistive 6 Ohm load would already test that limit. A real speaker that dips below its nominal impedance asks for more current. For the same reasons bridged-parallel is not a too promising choice, because it has the same current limit and makes the heatsinking issue worse, especially for a neighbor-annoying subwoofer.

The least changes for parallel operation are
- skip L1 and replace the 10r 1W resistor with 0r1..0r47 and a higher rating of 2 or 3 W.
- match the feedback resistors to 0,1 % or better on the parallel channels.
- don't waste the 10 f film caps on supply bypassing. If you have surplus of them, use them in parallel to replace the feedback cap.
- the 100 nF should be as close as possible to the chip, not as in the schematic on the high side of the fuse.

Consider the option to skip the feedback cap and employ DC servos for each IC.

It is always difficult to say whether CPU heatsinks will do. For one thing that depends on the CPU they are designed for and for another it depends on your way to use the amplifier. Does it become hot where you live and do you crank the music up every now and then? Then don't dismount the fans, but use a temperature-controlled speed regulator on them. Without the fan between 1 and 4 of those heatsinks per IC could be necessary depending on the conditions. With fan 1 will be enough most of the time and 2 should do the trick, whatever the conditions.
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