help me figure out this noise

help me figure out this noise (solved)

hi all. I'm afraid I do not possess all the necessary knowledge to wrap my head around this, so maybe someone could point me in the right direction?

A pair of old Genelec 1030a, they were issue-free until a couple of months ago, when gradually, one then in two weeks the other, both began exhibiting a loud hiss and a bit of buzz when first switched on, or simply when the room is colder.

I know my way around audio, so I've ruled out noisy source, interference, ground loops, bad cables, etc. I checked the PCBs for visibly faulty components but could not find any. I suspected the filter capacitors on the power supply at first, so I replaced them, but that made no difference.

In each speaker, the amplifiers are mounted on a large metallic backplate which acts as a heatsink for a pair of STK404-100. When the plate is cold, the speaker box produces a lot of self noise. If I blow on the plate from the back with a hairdryer and get it a few degrees warmer, the noise goes away. When cold, the noise is in both the woofer and tweeter paths. I tried getting individual components on the board hot or cold but nothing makes any difference until I warm or cool the metal plate itself (which in turn warms or cools the STK ICs, they prop the PCB up from the plate). Taking care not to warm the PCB itself, just the plate, still makes the noise go away.

Both speakers have the same issue.

I'm super puzzled. That room temperature of 18-19C or so can be too cold for these amps to sit stable - what a bewildering idea. Like all things Genelec, schematics are hard to find (1030a and 1031a are quite different). edit: I understand class AB amps (like the 404) have thermal "specifics", that's where I stop comprehending.

I'd be very grateful if someone had the expertise and patience to explain what this might be? STK 404-100 datasheet is here, and here's a photo of the treble part of the board. Any help or ideas much appreciated!
 
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I'm thinking: which one is more likely 4 ICs to develop the same issue at the same time, or something else common to both in each unit to not work nominally, so that when the ICs aren't warm, some inherent characteristic of theirs becomes noticeable. After those 2x4700µF/35V I changed, are two voltage regulators, with more electrolytic capacitors just after (roughly the same arrangement as in the 1031a except electrolytic). I tried to measure these ±15V rails with a voltmeter. As soon as I touch the -15V one the hiss intensifies noticeably, especially when cold. There's a minute difference between the voltages - two tenths of a volt or so, so that's extra to investigate. I'll first take a closer look at those rail-gnd caps - if such high resistance connected across (i.e. digital voltmeter) is enough to trigger the noise issue, things seems to be at a boundary of sorts when cold.
 
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Figured I'd post an update.. These voltage regulators - 7815 and 7915 - each have a filter capacitor afterwards between the rail and ground. I don't have any SMDs but replaced them with traditional electrolytic ones - 2 x 4.7µF/50V in one and 2 x 10µF/63V in the other box. Cooled the backplates to a point that would definitely cause noise before, and switched the speakers on. No noise. They're dead quiet. I'll check again tomorrow morning but so far I'm liking this.
Beats me where all this noise would come from but as per datasheet -Vcc goes to pin 5, which looks like the pair of biasing diodes inside the STK404-100. -Vcc rail capacitor must have dried out after all these years, so until the IC components warm up a little, the poor filtering would make for a loud noise floor. Hope this is of use to someone one day :)
 
sorry to resurrect this post, but I am facing the same issue with a couple of my 8240A speakers. They are both powering on but with lots of hum. I am investigating the same circuit areas around the 7815 and 7915 regulator. Most of the Caps on the PSU/Amp board are all SMD :-( I have replaced the filter caps on both of the PSU areas (there's a +/- 45V PSU for the amp and a +/- 15v psu for the preamp. Not sure yet which is the problem but the 7815/7915 is obs. the preamp.
 
sorry to resurrect this post, but I am facing the same issue with a couple of my 8240A speakers. They are both powering on but with lots of hum. I am investigating the same circuit areas around the 7815 and 7915 regulator. Most of the Caps on the PSU/Amp board are all SMD :-( I have replaced the filter caps on both of the PSU areas (there's a +/- 45V PSU for the amp and a +/- 15v psu for the preamp. Not sure yet which is the problem but the 7815/7915 is obs. the preamp.
Hello edwar64896
Did you find a solution to this?
I also have a pair of Genelec 8240A with similar problems. These are normally left on 24h and I never had any problems with them until I switched them off for a holiday. When I powered them up both of them had a the same terrible hiss/buzz noise you are describing. Listen to attached file Genelec noise.mp3

The noise disappeared after leaving them on for a while, but one of the speakers get still this noise occasionally when left on.
Any idea?
 

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