Denon POA 4400 DC Offset

[gheadhifi]

Hi People,

I've got a Denon 4400a that has a DC offset of 45mv on both A and B channel outputs.

When trying to adjust the bias as per the service manual I cant reduce this figure down and lower no matter what adjustments to the variable resistors for the bias.

I'm wondering if 45mv is too high as sometimes when the amplifier has been running for a while it will produce a sharp crack sound with the transients of say a kick drum, the amp is nowhere near maxed out maybe at a 3rd of the level so its not a headroom issue.


Would a 45mv offset produce this issue? The amplifier has been fully recapped with new capacitors on the main amp and power supply boards and some faulty transistors were replaced. Is it likely a mismatch in the transistor values causes a higher DC offset than recommended?

I'm pretty good with a soldering iron and have a multimeter, oscilloscope and signal generator for testing.

Any help would be greatly appreciated to expand my knowledge of the offset issue further.

Thanks

D

 

[satpaul]

Hi,
Here is the service manual on page 6 there is a setting.Hi Pavel

Denon POA4400A power amplifier - Service Manual Free Download

Download Denon POA4400A power amplifier ✅ Audio equipment,digital audio, home audio, professional audio systems service manuals and repair information

www.eserviceinfo.com

 

[Powertech]

Looking at the service manual there is no DC offset adjustment, only bias. The book states that the DC offset should be below 100mV. Seems a bit high, but that's what it says.
The bias is adjusted to read 8mV across TP1 & TP2 using VR1 with no input or load connected. The manual has a part load setup described using VR2 as well to set the optical bias circuit.

 

[satpaul]

DC offset is given by a complementary pair of output transistors. If you changed them, were they paired? But first, set up the optical bias circuit according to the manual. Then measure the offset Pavel