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Re: Hissing and whistling in the STe audio output
Posted: 12 Nov 2018 23:41
by sandord
exxos wrote: 12 Nov 2018 23:16
sandord wrote: 12 Nov 2018 23:12
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!
Re: Hissing and whistling in the STe audio output
Posted: 13 Nov 2018 12:58
by sandord
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
[R]
dacr.png
= U504 ============================================================================================
[L]
after-l-u504-p1.png
[R]
after-r-u504-p14.png
= U505 ============================================================================================
[L]
after-l-u505-p5.png
[R] (U506, the R requivalent of U505)
after-r-u506-p5.png
= U507 ============================================================================================
[L]
after-l-u507-p1.png
[R]
after-r-u507-p7.png
Re: Hissing and whistling in the STe audio output
Posted: 13 Nov 2018 21:36
by exxos
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
... 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...
Re: Hissing and whistling in the STe audio output
Posted: 19 Jul 2020 20:12
by Tomswork
Just a question about the -5 supply to op amp were is coming from some of those setups are antenna
Tom
Re: Hissing and whistling in the STe audio output
Posted: 19 Jul 2020 21:12
by exxos
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: 19 Jul 2020 21:55
by PhilC
@Tomswork i deleted your post as the photo gave away your location
Maybe post it again but edit the photo?
Re: Hissing and whistling in the STe audio output
Posted: 19 Jul 2020 21:57
by Tomswork
Thank you I did not see that I'm more than halfway to tunnel vision
Tom
Re: Hissing and whistling in the STe audio output
Posted: 19 Jul 2020 21:59
by Tomswork
This little amp is small and could bi pass the problem
uxcell® Super Mini PAM8403
Screenshot_20200719-135744_Samsung Internet.jpg
Tom
Re: Hissing and whistling in the STe audio output
Posted: 19 Jul 2020 22:15
by JezC
exxos wrote: 19 Jul 2020 21:12
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 :)
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... :lol: :roll:
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. 8-)
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: 19 Jul 2020 22:25
by exxos
JezC wrote: 19 Jul 2020 22:15
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. 8-)
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 :roll:
JezC wrote: 19 Jul 2020 22:15
(and IIRC @exxos you mentioned possibly adding in some upgraded sound system onto a future STF platform...?)
Yep, working on it in the background. 8 DAC channels to play protracker music without needing much CPU time..