LM3886 problem, HELP!

[Matthewong]

I'm building an amplifier using 2 LM3886 I.Cs. for stereo. Right now I'm facing humming problem. First the humming is very loud when I didn't connect the RCA cable from my pre-amplifier, after 3-4 seconds, my left channel will generate weird sound, something like shzzzzzzz, quite loud, only left channel, but it dissapear when I connect the RCA cable to the input. I have built power amps using TDA 2030 and 2050, both don't have such problem, only LM3886 got these problem. After connect the RCA cable from Pre-amp to the power amp input, I can hear humming sound near the speaker, not very loud, but audible when near the speaker. anyone out there can help me?

[sagarverma]

shzzzzzzz:::::its called 'pop'.

for hummmmm::use zobel at output.

[Matthewong]

just now I've do the zobel network, but It doesn't work. the humming still there. I use a 0.1 uF to connect from speaker output positive to speaker output negative.

[tja]

Originally posted by Matthewong
just now I've do the zobel network, but It doesn't work. the humming still there. I use a 0.1 uF to connect from speaker output positive to speaker output negative.

Shouldn't there be a resistor in the Zobel (in series with the cap) as well?

Take

[Matthewong]

I don't think it will work even I add a resistor to the 0.1 uF in series. right now I'm using 2 seperate boards for left and right channel. Will the problem coming from there? But I don't understand why humming become louder when I didn't connect the RCA from pre-amp? and the left channel generate weird sound after 3 to 4 seconds.

[tiltedhalo]

The wierd sound is input variance oscillations, caused by input ground loops causing the feedback network to slowly ring at frequencys above 1k. Make sure the input is referenced to ground with a resistor, if your input is 1k resistor use a 10k to 22k resistor to ground. Your zobel should be a 2.7ohm and .1uf cap also put a .74uh indutor inseries with a 10 ohm resistor to unload capactive coupling. Check your grounding really close!!

[peranders]

Matthewong, what kind of schematic have you used? If you short each input do you still get a hum?

[Matthewong]

Ok, I short the input positive and negative in the left channel and the humming is gone. If i disconnect the the RCA from pre-amp, not power amp, that's mean the power amp still connecting the RCA cable. The power amp is in good condition, no weird sound and no humming, but when I disconnect the the RCA cable on the left channel, the weird sound come and humming also very loud.

[Matthewong]

Hi, I'm back. I've done everything that you guys told to do, and now my power amp is nearly complete. But there is still have some problems. First is my power amp is still come out very low hum sound when I connect to preamp. Without connection to preamp, the humming gone. I've tried quite lot of preamp, all also same. Even when I just connect to preamp but preamp remain switch off, still come out hum. What is the problem there? Another problem is when I turn the volume up till almost 11 o'clock, one side of my poweramp output become distorted, seems like not enough power to drive the speaker. Before I modify, this problem never occur even I turn the volume up till 2-3 o'clock. Please help me!

[Matthewong]

By the way, now I'm using a transformer with 16.5 - 0 - 16.5 A.C output @ 8A. When I change to DC, it would be around 23v - 0v - 23v. I know this voltage isn't enough for LM3886 i.c and the worse is I use it for stereo. I would like to know which is best AC voltage for this I.C because I want to replace a new transformer for my poweramp. Can anyone help me?