Poweramp distortion

Hello everybody!
Does anyone know what is happening here:
[IMGDEAD]http://www.bilder-upload.eu/thumb/837e54-1454074722.jpg[/IMGDEAD]
[IMGDEAD]http://www.bilder-upload.eu/show.php?file=75b41a-1454074785.jpg[/IMGDEAD]
[IMGDEAD]http://www.bilder-upload.eu/thumb/75b41a-1454074785.jpg[/IMGDEAD]
[IMGDEAD]http://www.bilder-upload.eu/show.php?file=837e54-1454074722.jpg[/IMGDEAD]

It is a Wharfedale poweramp (descrete c3856 a1492) for a 15"-subwoofer. There is some crossover-distortion-like kink in the graph. Which is frequency dependant in the way, that its not always symmetrical (like real crossover distortion).
In the pictures you can see that at 153Hz its nearly symmetrical, while at 80Hz it shiftet.

This only happens under load. With open outputs its totally clean.
The amp doesnt lack any output power.
Has anybody a hint?

thanks!
ronny
 
Last edited:
Hello everybody!
Does anyone know what ist happening here:
[IMGDEAD]http://www.bilder-upload.eu/thumb/837e54-1454074722.jpg[/IMGDEAD]
[IMGDEAD]http://www.bilder-upload.eu/show.php?file=75b41a-1454074785.jpg[/IMGDEAD]
[IMGDEAD]http://www.bilder-upload.eu/thumb/75b41a-1454074785.jpg[/IMGDEAD]
[IMGDEAD]http://www.bilder-upload.eu/show.php?file=837e54-1454074722.jpg[/IMGDEAD]

It is a Wharfedale poweramp (descrete c3856 a1492) for a 15"-subwoofer. There is some crossover-distortion-like kink in the graph. Which is frequency dependant in the way, that its not always symmetrical (like real crossover distortion).
In the pictures you can see that at 153Hz its nearly symmetrical, while at 80Hz it shiftet.

This only happens under load. With open outputs its totally clean.
The amp doesnt lack any output power.
Has anybody a hint?

thanks!
ronny

Images don't open for me.

Have you checked the bias current?

Jan
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Here we go. These should open.

Looks to be a mix of crossover distortion and instability.

Capture.JPG

Capture2.JPG
 
what you are looking at is the output voltage. The output stage is clearly bias in class B and shows the signature crossover notch. The notch in the voltage waveform shows the cross from positive current flow to negative current flow and the discontinuity of Gm due to the class B bias. V=I*R or rather V=I*Z, as the phase angle of the speaker load is not zero degrees and shifts depending on frequency (should correlate to the speaker impedance curve) and this shifts where the crossover notch appears in the output voltage signal. Place a resistor as the load and the crossover notch will be in the middle at zero volts. :)

It looks like there may be a DC output? If this amp is not suppose to be class B then clearly something is wrong.