Clipping only on negative side of wave.

[Johnblue]

I'm "racking" 8  console channel strips in a box.  I cut the pcb In Half, All the rerouting is done,  signal caps recapped, a few channels are finished and sounding good, pre-amp and EQs. 

I'm doing my fourth strip, testing the audio with a sine wave  and when I get to about 80% on the line amp,  I get a flat clip on the negative side of the waveform only,  the positive half looks good at full gain. All the other channels I finished test normal.

The sine wave looks normal, except the bottom squares off at about 75% of the negative side only. 

I'm wondering if there's a common explanation that can direct me what to look for. 

I had to wrap it up for the day but the next thing I'll try is to switch out the op Amps.

I'm wondering what the theory is as to why one side would clip and not the other.  I read about dc offset but couldn't really relate that too my problem.

 

[gyraf]

Impossible to tell without knowing the circuit of the section in question (where it flat-lines)

Maybe dead opamp, if such are used? Maybe dead transistor.

Jakob E.

 

[JohnRoberts]

Other possibilities are voltage sag in negative rail, or possible op amp current limiting from too heavy output load.

JR

 

[Audio1Man]

Since you replaced the caps, you may have installed  a PS bypass backwards on the Negative rail or have a DC offset.
Duke

 

[sr1200]

What kind of test signal are you putting through it and are you sure that your source is properly balanced?

 

[gswan]

You can bias the output stage of an op amp to prevent crossover distortion at very low signal levels. Are there op amps in the circuit which are biased in this arrangement?

 

[ricardo]

You have 8 of these and presumably some of these are working and don't have this problem.

Lop sided clipping is almost always cos the DC voltages have changed and become wonky.

The output of each amplifier has to be roughly in the middle of the most +ve & the most -ve supply volts.

For OPAs on the usual +/-15 or 24 bla bla supply.  This means the output of each OPA has to be near 0V.

Check the DC voltages on a good working unit and compare with your wonky one.  If you find discrepancies, the problem will be in the stage where this first appears.