LM3886 Hum - Help

[john65b]

I bought the following LM3886 Board from APEXjr ...has 7 connectors...from tracing the circuit, I have determined the following board pinouts (no documentation exists)

Pic here

http://apexjr.com/images/LM3886Board.jpg

ANYONE FAMILIAR WITH THIS BOARD?

From top of pic

Wire 1: + Speaker Output
Wire 2: Vin- (line in+)
Wire 3: GRND
Wire 4: Vsupply -
Wire 5: Mute
Wire 6: Vin+ (Line in -)
Wire 7: Vsupply+

With mute disconnected, no sound at all (muted - duh). With Mute connected to Vsupply- with 34Kohm resitor, nothing but hum...

I have +/- 19.5 Volts and I am quite sure the Power Supply is good to go. +/- volt rails are correctly connected to the proper leads on board

The entire setup is on a wooden board and have the negative speaker output, transformer ground, connected to the GND (wire 3)...no chassis or chassis grounded.

Chip is insulated type, so no issues with heatsink grounding.

Driving me crazy!!

ANY HELP OUT THERE??

[john65b]

Looks like all I needed to do was connect wires 2 and 3 and violla..duh.

Don't know how I missed that...

Thanks

[john65b]

Actually, put in the Mute resistor between Mute wire to pin 8 and V-Supply rail, and mute capacitor to signal star (observe corrrect polarity - + on ground)

And then separate Power ground star from Signal ground star, the reattach the two ground stars with a wire to complete the grounding circuit.


Funny how everything always works when you folow the schematic.

[kec]

John,

How do they sound??? I just bought 8 of these but haven't hooked them up yet. Steve is already sold out of these - they went very fast. At $2.50 a pop it was a no brainer.

I think the next step would be to swap out the 2 ceramic caps for better ones.


Thanks,
Ken

[john65b]

Sounds great. I need to fianlize the wiring, as they are hanging all over the place, and need to separate the two ground stars, and then reconnect with single wire (from what I've read, helps sound).

I also have not put the correct Mute resistor in (22k vs 30k) per the LM3886 spreadsheet, nor have I put the mute cap to ground in either. This should be the reason I still get a pop when I turn on the power.

First thing I would mod is the Feedback resistor. 15k ohm seems low, and according to spreadsheet, reduces gain considerably. Spec says 20 - 22k ohm.

Damn, I bought 10 and wanted another 10. Wanted to experiment with 3 in paralell per channel to get 200+ wpc. Still looking for a monster toroid to do all six (100VA per LM3886, so 600VA), but a 25V 600VA toroid would weigh in at 15 pounds... yikes...

[kec]

First thing I would mod is the Feedback resistor. 15k ohm seems low, and according to spreadsheet, reduces gain considerably. Spec says 20 - 22k ohm.

Good point.

Damn, I bought 10 and wanted another 10. Wanted to experiment with 3 in paralell per channel to get 200+ wpc.

Yeah, I emailed Steve a couple of days ago and he had 20 left - should have bought 10 more.

Still looking for a monster toroid to do all six (100VA per LM3886, so 600VA), but a 25V 600VA toroid would weigh in at 15 pounds... yikes...

I've got 2 Avel Lindberg 30-0-30 160VA that will be perfect for 2 monoblocks. They've been sitting around waiting for a project like this

Keep us posted on your progress - I'll do the same.

-Ken