Very strange behavior/malfunction on My Denon X6400H

[PHD]

Hi

While I was listening to music from HEOS via headphones connected to my X6400H, the audio on the right headphone speaker started to sound weak, but when I touch the main pre-out RCA cables which connect the X6400H to an external amp, the audio on the right headphone either stops completely or returns to normal.

The music from the speakers connected to the external amps works fine and is unaffected by touching the cables (the cables are of excellent quality).

This very odd behavior makes me suspect that the internal headphone amp in the X6400H is fed directly from the main speakers' pre-outs and when I press on the pre-out RCAs, due to some loose connection inside, the audio supply to the headphones is interrupted.

Is this a common design in such a high-end AVR? It looks like they have cables or PCB tracks that bridge directly between the main pre-outs near the RCA connectors, and the internal headphone stage.

I'll open the case and see if I can locate the problematic connection inside. I did try to replace the pre-out to external amp cables, but it made no difference. This is very disappointing from Denon...

 

[Chrispy]

I'd suspect the cables more than the jacks on the avr, but hard to know. What do you mean the cables are of excellent quality? Do they fit properly?

You might try different pre-outs and see if the problem follows.

FWIW I've never had an issue with any jack on any avr except for a front hdmi when I tripped on the cable, that ended the use of that jack.

 

[PHD]

I just noticed that any analog cable I touch on the back, such as the subwoofer output, will cause interference or dropout on the right headphone speaker. This looks like an analog PCB issue. Perhaps some soldering needs a reflow. And yes, just downloaded the service manual, and the headphones are fed directly from the high-level front speaker's output, via a relay + attenuation resistors... There is no dedicated headphone amp. Even if I press hard on the top cover (metal case) it may cause the right headphones to mute...

 

[MaxwellsEq]

I'll open the case and see if I can locate the problematic connection inside. I did try to replace the pre-out to external amp cables, but it made no difference. This is very disappointing from Denon...

If you have swapped left and right cables all the way though the chain and it stays on the same channel, then it could be the amp. Have you tried different headphones - since that would eliminate them being responsible.

If it is a fault, it won't necessarily reflect poor quality at Denon. All products shipped at volume can have faults - it's just a statistical inevitability. The fact it was OK to begin with means it passed their factory tests OK, but perhaps some small contaminant has meant a solder joint is not mechanically sound.

 

[PHD]

OK. It does not stay on the same channel. Whenever I shake the AVR or press on its top cover, it's either the left or right headphone channel that either totally mutes or becomes very faint. Sometimes, pressing against the top cover, causes the headphone audio to be out of phase, indicating that return ground continuity problem.

I really don't understand what can cause such issues. I've disconnected all RCA speakers and HDMI cables, and now the headphones seem to work and no longer respond to moving/shaking the AVR. I did try 3 different sets of headphones and cables. Nothing changes. I think it's some sort of a ground-related issue. As if the loose connection happens only with the AVR's ground touches the ground of another device in my system.

Meanwhile, by looking at the service manual of the X6400H, the headphone's output is simply a joke. It is being fed via a relay and a resister directly from the front power amp. I'm attaching a screenshot.

Now for the biggest mystery, if the speakers' "minus" (black) terminal is floating from the signal's ground, how the H can the high-level to headphone bridging work because the headphones jack ground is the same as the signal ground?


Are the speakers' black terminals the same as the chassis ground? According to my DMM, they are not. So how does the headphone high-level speaker feed work? Is there an audio transformer in the link that I'm missing from the diagram?

Thanks