mystery whine

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ed
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#1 mystery whine

Post by ed »

not really a project but I struggled to find a better place for it....

for the last couple of weeks I've noticed a whine coming from the stats when the system is idle. The whine is audible in the room but not loud. I say idle because the breeze class d is switched to standby but the pi and the dac are always on.

So I made an effort to trace the whine a week or so back. I switched everything off at the wall socket(not the stats charge) and the whine persisted.
I switched everything off individually(pi, dac and amp) and the whine persisted.
I removed the I/Cs individually and the whine persisted.
I removed the speaker cables and the whine ceased.

so I've no idea which component is causing the whine...yet.

I'm thinking that the amp has residual so switching it off might not yield an immediate result.

today I heard the whine while passing the music room so it was like a red rag to the bull.....and I got the scope out.

as soon as I connected the scope the whine stopped...but the scope still measured a signal between 350hz and 500hz at 5mv. The frequency was stable but varied between the 2 figures at about 1/2 second intervals. A pedant might say that that is not stable...Pah!

I scratched my head!
I disconnected the scope and all was silent for about 2, maybe 3 minutes....and then the whine came back.

I paid close attention while connecting the scope back...and caught the little rascal red handed...as I connected the probe ground to the the black speaker socket there was a brief spark....and the whine disappeared.

The whole system is smps driven: pi, dac and class d amp, so this is the first time the system has seen a real ground...
my only certainty(so far) is that connecting the scope stops the whine.

anybody got anything to suggest?

edit..with the scope switched off but the probe ground connected to the black speaker socket there is silence...
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Nick
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#2 Re: mystery whine

Post by Nick »

The problem is that class D amps normally drive both +ve and -ve rails, so you can't just hang a ground on the speaker as one day you will plug a grounded source in and short out half the amps output stage (or you may have done that to get the spark). I would consider hanging a ground wire on the output of the dac or the input to the amp. Assuming RCA's
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IslandPink
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#3 Re: mystery whine

Post by IslandPink »

Sounds like the mystery hum of Holmfield, which I read about again a few days ago !
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#4 Re: mystery whine

Post by ed »

Nick wrote: Thu Oct 21, 2021 2:27 pm The problem is that class D amps normally drive both +ve and -ve rails, so you can't just hang a ground on the speaker as one day you will plug a grounded source in and short out half the amps output stage (or you may have done that to get the spark). I would consider hanging a ground wire on the output of the dac or the input to the amp. Assuming RCA's
spot on!

I disconnected the ground of the probe and turned the system on...music came out of the right hand side, left was silent for 2 to 3 mins then gradually came up to level...all good.

switched the system off(to standby) and the whine was obvious.
connected the ground of the probe(scope not powered on) to the fins on the headphone amp(not powered on but in circuit via I/Cs) and the whine disappeared, complete silence.
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#5 Re: mystery whine

Post by ed »

moving on......

left the scope ground probe connected to the fin on the headphone amp...no whine
disconnected the I/Cs going into the amp and the whine immediately came back...
so, to my mind the culprit is deffo the class d amp but it doesn't produce a whine if the ground upstream is connected to earth...
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