Help with circuit

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
The problem is high frequency oscillation and instability. Nothing else would cause this.

You need to examine your construction method, layout and grounding... as well as checking the component values agree with the design.
 

Domce106

Member
2015-05-26 9:21 pm
The strange thing is that I have two other of these circuits made and they work fine. I had this one and it worked, then I burned the chip by accident (short circuit). Changed the burned components. Didn't work. Rebuild it from scratch and it still doesn't work. The only thing that is in this version from last time is the inductor. Could the inductor be faulty and cause the burning. I have checked many times for assembly error, but I could not find any. Thank you for your replies.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
The inductor shouldn't be a problem at all and tbh its a recommended fitment in most amps. Its purpose is to isolate the amplifier from capacitive loads that can and do cause instability problems with many amps.

If the amp is properly designed and properly laid out and constructed then it should be 100% stable with no load (such as a speaker) attached, and in that condition the inductor doesn't even come into play.

Do you get the problem if you short circuit the input to ground ?
 

Domce106

Member
2015-05-26 9:21 pm
I do not know what is the problem. I had it connected to my preamp (which i fixed not long ago), but apparentally the preamp does not work as well (I did not expect that, because I fixed it and it worked fine). So now i tested it without the preamp and I does not burn any resistors anymore. But still, no music coming out from the amp. I connect the leads from my phone jack straight to the amp connectors and the speaker gives of this crunchy sound, it is pretty loud and does not sound like any music with distortion, more like random noise. And no, connecting to ground does not cause the burning, something in my preamp must be broken (again).
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I don't really know what to suggest to you although I suppose its possible the chip has been damaged in all this.

Is the DC offset correct at near zero volts ? That can be a good clue to trouble... and I'm assuming you haven't an oscilloscope to troubleshoot this ?

Also check the speaker is OK and that it has not been damaged by the amp.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Yes, its possibly the chip.

All you can do is go around the chip and measure the voltages to see if anything stands out (beside the obvious offset). Check the positive supply is correct. Check the feedback network is OK and connected OK. Check that the input pin (10) really does connect to ground via the 47k (R4) and that the 10 ohm (R9) is OK and correctly connected to ground.