Hissing and whistling in the STe audio output

[sandord]

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!

[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

[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...

[Tomswork]

Just a question about the -5 supply to op amp were is coming from some of those setups are antenna

Tom

[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 :)

[PhilC]

@Tomswork i deleted your post as the photo gave away your location

Maybe post it again but edit the photo?

[Tomswork]

Thank you I did not see that I'm more than halfway to tunnel vision

Tom

[Tomswork]

This little amp is small and could bi pass the problem

uxcell® Super Mini PAM8403

Screenshot_20200719-135744_Samsung Internet.jpg

Tom

[JezC]

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...?)

[exxos]

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:

(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..