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Old 9th October 2006, 07:46 PM   #1
Razbi is offline Razbi  Finland
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Default LM3886 Stereo Amplifier problem

Hi!

I'm finishing my project of stereo amplifier with snubberized power supply based on Brian's Kit (http://www.chipamp.com). Yesterday I powered the amp and measured DC offset between the output terminals. It started with 110mV and then went down towards zero. Is this normal?

Second problem is the power LED. It doesn't go on. I tested the circuit with another led (certainly working one) with same results. Then I tested the resistor and diode on circuit, they're fine, but I'm not sure about the capacitor. How can I test that? I didn't get any voltages or currents measured from resistor's ends when amp was powered.

I also tried the amp with test speaker, but didn't get sound out of it.

I attached a couple of pictures, if they're helping any.

I would be very pleased if you can help me out of this problem.

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Old 9th October 2006, 08:14 PM   #2
AndrewT is online now AndrewT  Scotland
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Hi,
can you keep the safety earth attached to the chassis and disconnect the audio ground from the chassis?
Keep the audio ground floating above the chassis and add a disconnecting network from audio ground to safety earth. Do not remove the nut securing the safety earth, instead add the disconnecting network above the first nut and then use a second nut to secure.

The disconnetcing network can consist of a ceramic cap // power bridge diode// power resistor// power thermistor, or any combination of these.
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Old 9th October 2006, 08:15 PM   #3
AndrewT is online now AndrewT  Scotland
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Hi,
your input cables from RCA to PCB are run separately. Try twisting them tightly together.
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Old 9th October 2006, 11:59 PM   #4
john65b is offline john65b  United States
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Your power supply board is fully populated with all eight rectifier Diodes, so it is set up for a transformer with Dual secondaries (4 wires). If you have a center tapped transformer (3 wires) you need to modify the power supply board and get rid of 1/2 the diodes and put in a few jumpers.

If you are running a centertapped transformer, see Brian's LM3875 Instructions on how to jumper the right diodes. The LM3886 instructions do not identify how to wire a centertap transformer.
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Old 10th October 2006, 06:36 AM   #5
Razbi is offline Razbi  Finland
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Quote:
Originally posted by john65b
Your power supply board is fully populated with all eight rectifier Diodes, so it is set up for a transformer with Dual secondaries (4 wires). If you have a center tapped transformer (3 wires) you need to modify the power supply board and get rid of 1/2 the diodes and put in a few jumpers.

If you are running a centertapped transformer, see Brian's LM3875 Instructions on how to jumper the right diodes. The LM3886 instructions do not identify how to wire a centertap transformer.
Transformer has dual secondaries. But thanks still.
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Old 10th October 2006, 10:03 AM   #6
Stuey is offline Stuey  Australia
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Try reversing the LED - I think the instructions have the LED wired incorrectly.

Stuey
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Old 10th October 2006, 11:24 AM   #7
AndrewT is online now AndrewT  Scotland
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Hi,
can you measure the voltage across the LED and across the feed resistor?
The LED should have about 1.5V to 2V across it.
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Old 10th October 2006, 12:37 PM   #8
Stuey is offline Stuey  Australia
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I was pretty sure I'd wired my LED up as instructed in the kit and the LED didn't work, hence my post. I reversed mine and it worked...

Cheers

Stuey
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Old 10th October 2006, 01:01 PM   #9
Nordic is offline Nordic  South Africa
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Heck, you have space left for 2 more channels....

Nice strong build there
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