I just finished an lm3886 amp kit and the sound output is barely audible. There is a wavering or oscillating sound. Voltages to and out of the power supply are fine. There was 0 on the speaker output before I plugged in speakers. What am I doing wrong? I have everything grounded.
Thanks Folks.
Thanks Folks.
Check you've got the components in the right location. If the feedback resistors (e.g. 22K/1K) are reversed this can happen as you'll end up with around 0.38dB gain.
Go over the board again and again to spot any errors.
Go over the board again and again to spot any errors.
Check that the power supply is in good working order, correct connection and compliance with the manual.
Thanks all
Thank you for the replies folks. I have checked:
when checking power supply I noted: Both +and - voltages are roughly the same. When power is removed, the + voltage decays much slower than the - voltage. ??
Will try to post some pics
Thank you for the replies folks. I have checked:
- Sound source...ok No Mute
- Boards for proper locations of components...think its good
- Check power supply
when checking power supply I noted: Both +and - voltages are roughly the same. When power is removed, the + voltage decays much slower than the - voltage. ??
Will try to post some pics
Attachments
From what I see you have a stereo amp ?
One 3886 is at one end of heat sink and other 3886 is at the other.
Did you isolate them from the heat sink ?
V- connects to 3886 tab.
On my first LM3886 amp I made the mistake of pushing down LM3886 right down on to the pcb.
The back of the front pins were shorting on the back line of pcb pads !
I put the next one in a little higher up and it was fine.
One 3886 is at one end of heat sink and other 3886 is at the other.
Did you isolate them from the heat sink ?
V- connects to 3886 tab.
On my first LM3886 amp I made the mistake of pushing down LM3886 right down on to the pcb.
The back of the front pins were shorting on the back line of pcb pads !
I put the next one in a little higher up and it was fine.
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did not isolate, thought the lm3886tf versions only needed thermal paste to help with heat transfer.
Yes Nigel, Stereo amp. I checked all the pins on both chips. looks ok. It has to be a power supply issue? Both amp boards exhibit same issue. 24 volt ac in from toroid getting 36 volt dc out of power supply. Again the negative side of the power decays very slowly, when power off, after 5 minutes still reading 30 volts. Positive side drops pretty quickly about 1 volt after 5 minutes. Am I missing something here?
A resistor bleeding voltage from V+ to ground?
You need to check the actual resistance to marked values.
Possibly wrong locations?
Bad chips?...That rarely happens, but if they heated up for some reason, or were nearly shorted. Should not happen in genuine chips, no idea if fakes are in circulation.
But I would start with checking the inputs, and follow the signal path.
You need to check the actual resistance to marked values.
Possibly wrong locations?
Bad chips?...That rarely happens, but if they heated up for some reason, or were nearly shorted. Should not happen in genuine chips, no idea if fakes are in circulation.
But I would start with checking the inputs, and follow the signal path.
The power supply is common, try checking the supply with some sort of dummy load, that means not connected to the amp boards.
Maybe that is faulty
Maybe that is faulty
How is pin 8 of the LM3886 connected ? It should be with a resistor of around 20K to -V to get the chip out of mute (so at least 0.5mA is drawn).
The only other thing I can think of is a defective chip.
edit: Thought of something else. It looks like there is also a capacitor on pin 8. Check this capacitor isnt bad. You can try removing it, its not necessary for the amp to work - it simply gives some startup delay
The only other thing I can think of is a defective chip.
edit: Thought of something else. It looks like there is also a capacitor on pin 8. Check this capacitor isnt bad. You can try removing it, its not necessary for the amp to work - it simply gives some startup delay
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Don't forget that there can be up to 85 mA bias current flowing from the positive to the negative rail, fed by the +ve power supply cap(s) when the power is removed. However, there is no balancing current flow in the opposite direction that woulld discharge the -ve power supply caps. Is that some kind of problem? Nope..unless you poke your finger at it and try to discharge the power supply caps through your body (little risk of harm at 30V but not everyone has a sense of proportion when it comes to any risk, however small).
Think of the LM3886 single supply rail circuit. There, the same current flows from rail to ground and no one is the wiser if ground remains charged to 0V😉