Causes of DC voltage leakage in general and in Harman kardon pm660 in particular

What are the causes of DC voltage leakage in general.
Big filter capacitors? Or output transistors.

Some amps have 5mv. Some 150mv

I have an issue with Harman kardon pm 660. Leaks 150mv dc at speaker terminals.

So say if it's a high miles unit and shows over 60mv dc leak would it be safe to say that it's capacitors related?
 

rayma

Member
2011-04-29 8:37 pm
Most current amplifiers lack an output capacitor. Instead they have +/- voltage supplies, and an approximate zero DC output. The feedback network may have a DC blocking capacitor, which forces unity DC gain, and then the output DC is equal to the input offset voltage. Otherwise a DC servo can be used, which filters the DC at the output and feeds back to the input to cancel it. Either way, typical DC output voltages can be tens of mV, but will vary with operating conditions.

Your amp instead has DC offset adjustments VR401 and VR402 for left and right. These can be adjusted for minimum DC output after warm up (see page 6), the spec is <10mV.

At the same time you can set the output stage bias current, if you have a reasonably accurate DVM that can properly measure 50mV DC. If not, leave it alone.

Harman Kardon PM660 - Manual - Hi-Current Capability Integrated Amplifier - HiFi Engine
 
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Thanks for suggestion. I have a fairly accurate multimeter. Will try to reduce dc bias. But visually what exactly is dc offset or bias.
I attached an image I found .
 

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rayma

Member
2011-04-29 8:37 pm
Thanks for suggestion. I have a fairly accurate multimeter.
Will try to reduce dc bias. But visually what exactly is dc offset or bias.

It would be hard to measure offset using a waveform, unless the sine amplitude is 100mV or less.
The whole waveform would look moved up or down, and not be symmetric vertically about the zero line.

Instead, just short the inputs and measure the DC voltage at the outputs.
After warmup, adjust the DC offset controls for <10mV DC at each output.
Then adjust the bias controls for 50mV DC at the test points shown in the manual.
Finally, repeat the DC offset adjustment for <10mV DC at each outout.
 
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