DENON POA 1001 (Power Amp) buzzing

[bmwz8]

Hi,

Just bought a nice DENON POA 1001 power Amp. I love this 22kg baby
But it is buzzing. Here are the result of investigations :

- The amplifier itself is buzzing, the sound in the speaker is good, there is no significant hum.
- Buzzing is present with or without source/preamp, with or without speakers, tunring the power plug did not change anything.
- Buzzing was already here when I tried the amp at the sellers', so I guess it does not come from the electric installation of my flat.
- I also have his little brother (POA 1003), and the little one is not buzzing, so I guess we cannot say it is "normal".
- There is a relay clicking once few seconds after having switching on (proctection), which is normal. The buzzing is here before and after the relay cliking.
- The amp is a double mono. Cannot identify where it comes from, seems to come from left AND right.
- I heard that It might come from the transformers that could vibrate when getting old. I'm quite surprised because those are very huge, well built, and as I said, there is no buzzing in the POA 1003.

I made a recording of the buzzing (sound is not loud in the tiny video, you may need to increase volume or get headphones)

The funny part is that there is no service manual available, although I bought the one of the POA 1003 (would it help ? )

End of the enigma. Any Idea of where the buzzing may come from and what I could check ?

On a french thread, a guy said he managed to get rid of the buzzing by changing filter capacitors. Surprising, but why not. Unfortunately I don't have the capacitors measuring device and these big cans are expensive so I rather not buy them and see that it did not solve the problem.

I have a multimeter, I can solder and I already repaired few integrated amplifiers.


(sorry if there are some english mistakes, I'm french and french are not good in speaking foreign languages, but we are football world champions )

Thanks a lot !

 

[rothwellaudio]

I heard that It might come from the transformers that could vibrate when getting old.

This is true, but it doesn't really have anything to do with the age of the transformers. As the magnetic field within the transformer rises and falls 50/60 times per second the metal parts have a tendency to repel/attract each other 50/60 times per second. This causes the buzzing. Adjusting the tension on the screws which hold the laminations together can sometimes tune out the buzzing. Dipping the transformer in varnish is standard practice at the manufacturing stage to minimise rattling/buzzing. Maybe it wasn't done thoroughly in this case.
Sometimes adjusting the tension of the screws which hold the transformer in place can help.

 

[bmwz8]

Hi, thanks for your reply.

I forgot to mention that I tried adjusting the tension of the screws that hold the transformer to the chassis. Tried to "make them tight", and also tied to isolate one transformer with tiny silentblocks. Didn't make any change.

Concerning the varnish, does it have to be done by a professional or can I do it myself in my flat ? I read that it needs a lot of devices/machines to be sure that no bubbles remain. I guess that simply pooring varnish in the transformer holes would not be enough ? Laminations are not hold by screws, I think they are soldered

I was thinking of maybe build two temporary boxes made of carton paper to cover and isolate the transformers, just to see if the buzzing really comes from them. Maybe that's the first thing to check...

 

[bmwz8]

Hi,

Made somme additionnal tests, and it appears that when I unplug the PCB with the big 22000uf capacitors, the annoying noise from the transformers disappears. Does it mean that the big capacitors are probably old/out of normal value and that I might "simply" have to change them with new ones ?

Thanks a lot !

 

[rothwellaudio]

Made somme additionnal tests, and it appears that when I unplug the PCB with the big 22000uf capacitors, the annoying noise from the transformers disappears. Does it mean that the big capacitors are probably old/out of normal value and that I might "simply" have to change them with new ones ?

When a transformer has no load on its secondary it isn't passing any energy from primary to secondary and is less likely to buzz. A normal load on the secondary shouldn't cause any buzzing so it's possible that the buzzing you're hearing is caused by too much current being drawn. It's possible that the power supply caps are faulty, though if one had gone short circuit I would expect a fuse to blow. An alternative possibility is that the amp's bias current is set too high.
Btw, it would be unusual for the same fault to develop on both channels of a dual mono setup. Are both transformers buzzing, or just one?

 

[bmwz8]

Both transformers are buzzing, same noise, same frequency, sound is identical. Fully dual mono yes, looks like 2 amps in one case. Power caps are 2*22000uf for each channel.

I guess that if the power supply caps had gone in short circuit, then I would not be able to hear any music, right ? I was thinking of caps getting old and out of their range.
The possibility of too high bias is really good, I will try to check it, thanks a lot for this idea, however the problem is there is no service manual, I may have to look at the one of the POA 1003, and hope that it's similar...

 

[avionic]

Concerning the varnish, does it have to be done by a professional or can I do it myself in my flat ?

Ideally it needs to be in a vacuum to pull any air within the laminates away so the varnish can fill all the voids.

 

[petehall347]

i had some luck with spray lacquer .no idea how long it lasted but it worked at the time .