Re: Hissing and whistling in the STe audio output
Posted: Mon Nov 12, 2018 11:41 pm
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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Hissing and whistling in the STe audio output
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Re: Hissing and whistling in the STe audio output
Posted: 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]
[R]
= U504 ============================================================================================
[L]
[R]
= U505 ============================================================================================
[L]
[R] (U506, the R requivalent of U505)
= U507 ============================================================================================
[L]
[R]
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 (32.95 KiB) Viewed 5664 times
[R]
- dacr.png (33.13 KiB) Viewed 5664 times
= U504 ============================================================================================
[L]
- after-l-u504-p1.png (27.84 KiB) Viewed 5664 times
[R]
- after-r-u504-p14.png (27.46 KiB) Viewed 5664 times
= U505 ============================================================================================
[L]
- after-l-u505-p5.png (24.16 KiB) Viewed 5664 times
[R] (U506, the R requivalent of U505)
- after-r-u506-p5.png (25.22 KiB) Viewed 5664 times
= U507 ============================================================================================
[L]
- after-l-u507-p1.png (28.03 KiB) Viewed 5664 times
[R]
- after-r-u507-p7.png (28.32 KiB) Viewed 5664 times
Re: Hissing and whistling in the STe audio output
Posted: 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....
... 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...
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 (7.84 KiB) Viewed 5641 times
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...
Re: Hissing and whistling in the STe audio output
Posted: Sun Jul 19, 2020 8:12 pm
Just a question about the -5 supply to op amp were is coming from some of those setups are antenna
Tom
Tom
Re: Hissing and whistling in the STe audio output
Posted: Sun Jul 19, 2020 9:12 pm
The whole PCB & parts is an antenna and the audio circuit has bad grounding in the mix. The only likely solution would be to rebuild the whole audio circuit on a new PCB and enclosed in a metal case. I suggested such but nobody is willing to build and try it 
Re: Hissing and whistling in the STe audio output
Posted: Sun Jul 19, 2020 9:55 pm
@Tomswork i deleted your post as the photo gave away your location
Maybe post it again but edit the photo?
Maybe post it again but edit the photo?
Re: Hissing and whistling in the STe audio output
Posted: Sun Jul 19, 2020 9:57 pm
Thank you I did not see that I'm more than halfway to tunnel vision
Tom
Tom
Re: Hissing and whistling in the STe audio output
Posted: Sun Jul 19, 2020 9:59 pm
This little amp is small and could bi pass the problem
uxcell® Super Mini PAM8403
Tom
uxcell® Super Mini PAM8403
- Screenshot_20200719-135744_Samsung Internet.jpg (176.33 KiB) Viewed 4262 times
Re: Hissing and whistling in the STe audio output
Posted: Sun Jul 19, 2020 10:15 pm
I didn't have an STE when you first posted this...things change
Now I have 2 working STE's & would be interested to do this...just the two usual constraints lack of time & lack of money...
I have a colleague at work who is very into his audio & analogue designs (including audio amps) so I might manage to 'volunteer' at least some of his expertise to assist.
Anyway, probably one for the back-burner but definitely one of real interest at some point (and IIRC @exxos you mentioned possibly adding in some upgraded sound system onto a future STF platform...?)
Re: Hissing and whistling in the STe audio output
Posted: Sun Jul 19, 2020 10:25 pm
I am a audio repair and design engineer
, built and repaired amps for 30 years, hobby and work. I mostly did digital amps towards the end, before I got swamped with all things Atari related. I still need to finish my power amp one day Yep, working on it in the background. 8 DAC channels to play protracker music without needing much CPU time..