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noise with dac+ pro -> stax headphone

Hi!

I recently purchased a dac+ pro which I like VERY much so far. So much actually that I thought about using it with my old, wonderful Stax headphones.

But there I got a weird noise problem.

I really lack the knowledge to understand what is happening here, so I just tried to collect some information.


This is what happens:

Hifiberry into power amp (power section af an old linn majik integrated amp, going into naim intro2): perfect.

Hifiberry into cheap active desktop speaker (Scansonic S3): perfect.

Hifiberry into Headphone amp: hum (50Hz?) and strong unpleasant noise (I'd describe it as kind of "ripple"). Headphone is an electrostat (Stax), with the amp providing polarisation voltage and a significant amount of amplification (up to 60dB, resulting in 200Vrms). You need a significant voltage to get anything out of these headphones, though I am not sure there is actually any current running (certainly not in the headphones themselves). The amps input impedance is supposed to be 50kOhm (power amp: 10k)

I get the same noise when I connect the headphone amp to the raspis own audio out (which is deactivated). I doublechecked with a second raspi, similar problem.

The noise is highly dependent of the power supply. Strangely enough, it is stronger with a supposedly better ifi power. It seems to be related to raspis activity as well - it got stronger when I forced some disk IO, though I could not detect any change by forcing (constant) cpu load.

Defectice headphone amp, you think? I checked with other sources, it is dead silent.

Even more weird: I get the sound even when I switch off the headphone amp. Of course there is still polarisation voltage left, but there ought to be no amplification. The noise does not change at all before gradually fading out (takes some minutes).

Another strange experiment: hifiberry connected to power amp. Headphone amp connected to preamp. There ought to be no connection between power and pre other than the common power supply. At least I thought so. Same noise.

Grounding? The only device that ought to be grounded is the power amp (it isn't though - no grounding on these wall outlets unfortunately).

Ethernet? No, I use wifi only with raspi3.

Touching any exposed metallic part (be it a rca plug, the raspis ethernet port, the headphone amps metal case, the metal case of the linn amp (basically anything with an electrical connetion to the raspi) dampens the noise. Grip it tightly, and there is silence.

And another scenario: hifiberry to linn amp, raspi audio to stax amp: strong noise. Pull the power plug for the linn amp: even stronger noise. Touch the linn amp: silence.


Please don't laugh at my theory: By touching a metal part I add a significant amount of capacity, compensating for the noise. Else it is the headphone taking that role, audibly so.


Questions:

any Ideas how the source of the noise could be silenced?

should I try and give the analog section its own power supply? Seeing that it looks like any indirect connection with the "dirty" system is enough to produce the problem, I must say I am not too optimistic.

And (OT) any Idea why the ifi power is worse than the standard raspi wall wart?


Regards
David

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