Hello everybody!
Does anyone know what is happening here:
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
Does anyone know what is happening here:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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:
Thanks Mooly, for the Pic-links!
Jan, I Havent checked anything yet. Just I/O-behaviour.
I will disassemble it on monday, maybe there are some bias-trimpots
Jan, I Havent checked anything yet. Just I/O-behaviour.
I will disassemble it on monday, maybe there are some bias-trimpots
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.
It looks like there may be a DC output? If this amp is not suppose to be class B then clearly something is wrong.
You're right CBS240. A loadresistor causes the centered notch.
what do you mean by "DC output"? i can't measure a dc component in parallel to the load.
i think the the amp is supposed to work class AB.
what do you mean by "DC output"? i can't measure a dc component in parallel to the load.
i think the the amp is supposed to work class AB.
So i biased up the output transistors. Now everything is smooth as baby-sinewave.
The trimpot + fixed resistor in the bias circuit weren't able to go low enough, so i paralleld the fixed resistor with another one.
thank you!
The trimpot + fixed resistor in the bias circuit weren't able to go low enough, so i paralleld the fixed resistor with another one.
thank you!
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