Hissing and whistling in the STe audio output

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sandord
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Re: Hissing and whistling in the STe audio output

Post by sandord » Mon Nov 12, 2018 11:41 pm

exxos wrote:
Mon Nov 12, 2018 11:16 pm
sandord wrote:
Mon Nov 12, 2018 11:12 pm
That's the first picture...
ah ok...

have you measured the power rails on each chip to see if anything spiky there ?

Also, hold down the reset button (it will stop the machine from running) so that would be interesting to test...
I'm sorry, I can't do any more measuring anymore any time soon. I have a lot of stuff to do and will be out for a long weekend. Gotta get some sleep now!

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Re: Hissing and whistling in the STe audio output

Post by sandord » Tue Nov 13, 2018 12:58 pm

I forgot about some left channel measurements I also made, they might be interesting to compare to the right channel measurements.

NOTE: these are all AFTER recap so some filtering might be missing. I can't make measurements of this setup BEFORE recap now because I don't have time.


= DAC ============================================================================================

[L]

dacl.png
dacl.png (32.95 KiB) Viewed 859 times

[R]

dacr.png
dacr.png (33.13 KiB) Viewed 859 times

= U504 ============================================================================================

[L]

after-l-u504-p1.png
after-l-u504-p1.png (27.84 KiB) Viewed 859 times

[R]

after-r-u504-p14.png
after-r-u504-p14.png (27.46 KiB) Viewed 859 times

= U505 ============================================================================================

[L]

after-l-u505-p5.png
after-l-u505-p5.png (24.16 KiB) Viewed 859 times

[R] (U506, the R requivalent of U505)

after-r-u506-p5.png
after-r-u506-p5.png (25.22 KiB) Viewed 859 times

= U507 ============================================================================================

[L]

after-l-u507-p1.png
after-l-u507-p1.png (28.03 KiB) Viewed 859 times

[R]

after-r-u507-p7.png
after-r-u507-p7.png (28.32 KiB) Viewed 859 times

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exxos
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Re: Hissing and whistling in the STe audio output

Post by exxos » Tue Nov 13, 2018 9:36 pm

It looks like the problems start right after the first op-amp ?

In that case, it would have to be getting some frequency from somewhere, looks like it starts off at about 1.3KHz, also at 15.6KHz.

I would have to assume the noise is being introduced by the power rails to the op-amp.

That wouldn't be exactly simple to fix... Really it needs its own little regulated PSU, looks like 8.2V and -5V ?! odd, but I would be tempted to lift up those 2 10R resistors....
111111111.jpg
111111111.jpg (7.84 KiB) Viewed 836 times
... and use a PP3 battery for the 8.2V (would be about that for a rechargeable one) then have another in series with a 7905 regulator to get the -5V rail.... or 3x AA batteries in series instead...

Noise can still get into the op-amps via bad grounding on the MB.. but if that ruled out some noise, then at least we have a proper starting point to what is going on...
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