Chinese LM3886 Board having problems

[mattthegamer463]

Originally Posted by Andrew Eckhardt

The trouble with rail fuses is they are "designed" to protect the amplifier, not the load. Depending on the behavior of the IC one rail might make it to the load if that's all it's getting. So without knowing anything about the IC ('specially if it's shorted), if you use rail fuses, use an inline fuse too, or watch out.

Well, now I have a fuse on the live wire from the wall, and I have 1A fuses on the positive of each speaker terminal, hopefully that will protect the speakers from a number of unpleasant situations.

[ted4412wilt]

Your torroid voltage is too high. The ebay ad calls for 28 volts DC which needs 20 volts AC although 22 volt AC would probably work. How big are your heat sinks and did you use Artic Silver or something similar? As others have pointed out you must have low DC offset. If you use your own bridge rectifier and a regulator to drop the voltage your torroid will be fine.
Good luck,
Ted

[mattthegamer463]

Originally Posted by ted4412wilt

Your torroid voltage is too high. The ebay ad calls for 28 volts DC which needs 20 volts AC although 22 volt AC would probably work. How big are your heat sinks and did you use Artic Silver or something similar? As others have pointed out you must have low DC offset. If you use your own bridge rectifier and a regulator to drop the voltage your torroid will be fine.
Good luck,
Ted

How Now Brown Cow?

Voltage: AC 28V-0-28V 5.5A

My transformer outputs AC 25-0-25 and 420VA = 8.4A, tons of spare current on the amplifier side.

[sivan_and]

I could see only 2 To220 power diodes..and a small bridge rectifier (3 amp) probably for the preamp IC....

[ted4412wilt]

My mistake. It does call for 28 v AC so I guess the builder does not know what he is talking about. Maybe he is using half wave rectifier which will give lower voltage and poor performance. If you use a bridge rectifier and voltage reglation this might work.

[poynton]

Originally Posted by sivan_and

I could see only 2 To220 power diodes..and a small bridge rectifier (3 amp) probably for the preamp IC....

Try a bridge rectifier and 2 regulators for the +- op-amp supply

Andy

[danielwritesbac]

Temporarily, and until the amp is sorted out, put a 2200uF cap in-series with the speaker + wire.

That way, when you blow one of the DC rails fuses and not the other, the consequences of 40 volts DC offset will be blocked by that new cap.

Output cap--its better than a speaker fire.

[sivan_and]

Originally Posted by danielwritesbac

Temporarily, and until the amp is sorted out, put a 2200uF cap in-series with the speaker + wire.

That way, when you blow one of the DC rails fuses and not the other, the consequences of 40 volts DC offset will be blocked by that new cap.

Output cap--its better than a speaker fire.

BUT THAT WILL GIVE U A CATASTROPHIC CRACKING SOUND !!! if there is DC at the output and the 2200uf charging thru the voice coils !!!

[danielwritesbac]

Here's the LM3886 datasheet from National Semiconductor.
http://www.national.com/ds/LM/LM3886.pdf
You will find the output cap on page 6.

[mattthegamer463]

Originally Posted by danielwritesbac

Here's the LM3886 datasheet from National Semiconductor.
http://www.national.com/ds/LM/LM3886.pdf
You will find the output cap on page 6.

I have looked at the circuit, and following the trace from pin 3 (output) pin of the op-amp goes directly to the screw terminal of the speakers; there is no output capacitor. Could this be a source of trouble?