Hissing and whistling in the STe audio output

[Paulo P]

On the STE one, I do hear (and you can hear in the audio) when it starts doing memory test, you can hear the "dashes" on the screen as the come up..

I need to find me headphone adapter and try that again...

Yes. That's the same as mine STe. I hear doing memory test the same way (my file Color_&_Mono.zip). I also hear the dashes as they come up on the screen. But I hear them on left channel, maybe someone of us have swapped channels during recording.
So it looks like every STe, probably every Mega STe, even TT (by the post above) are doing this.
The question is if your STe DAC Fix would fix this or at least improve this. Or maybe there is another solution to solve this. I know... it's a detail... but...

[exxos]

I tried my machine with the DAC fix.. I think it does help, but sounds still there. Though oddly, when I unplug the mains lead, all that "PC noise" stops.. so probably a grounding issue causing those sounds.

I tried looking on my scope at the noise, but only really see MHz range noise, so tracing the noise with the scope isn't really possible.

Headphones was odd.. when first plugged in, it was audible, but soon faded to next to nothing.

Electrolytics are not good in audio circuits, those should be replaced with ceramics for starters..

I think isolating each section in the DAC circuit and trying the noise output to try and isolate where the noise is being generated from is next step.. not sure I will have time to do that though.

For example, lift those circled parts and see what happens one by one...

cct.JPG (130.2 KiB) Viewed 6169 times

[troed]

I think it's important to separate between random noise added due to memory accesses etc, video circuitry (15kHz, different in mono) and the 2600Hz "STE left channel whine".

The latter likely has a single cause regardless of other activity in the system.

[stephen_usher]

On the STE one, I do hear (and you can hear in the audio) when it starts doing memory test, you can hear the "dashes" on the screen as the come up..

I need to find me headphone adapter and try that again...

I'm pretty sure it's the same noise that the TT produces (though when accessing TT RAM it's far louder)...

https://youtu.be/sSaqNFz4QuE

The volume on the external speakers was turned up high and the internal speaker was turned off.

[exxos]

I'm pretty sure it's the same noise that the TT produces (though when accessing TT RAM it's far louder)...

The volume on the external speakers was turned up high and the internal speaker was turned off.

Does sound similar indeed. I think the whole DAC circuit is almost identical ?

Question is to isolate the DAC from the op-amps to see if the sound changes or not...

[stephen_usher]

Are you sure that it's not RF leakage? It sounds very similar to the noise most old, poorly shielded computers (e.g. ZX81, ZX Spectrum) make on the radio. A stock ST has this interference on the modulator output too.

[exxos]

Are you sure that it's not RF leakage? It sounds very similar to the noise most old, poorly shielded computers (e.g. ZX81, ZX Spectrum) make on the radio. A stock ST has this interference on the modulator output too.

Could well be.. would need to screen off the whole audio circuit to see if it helps or not. RF can be coupled onto the supply rails, audio ins and outs, anywhere.

The DAC part shouldn't really be affected by RF, but the op-amps likely are.. this is why I suggest isolating stuff to see if its on any particular part of the audio system or not.

[sandord]

I got my DMA sound fix kit today and measured the results.

Here's the stock STE analysis (recapped PSU though):

ste-stock-mon-off.png (27.02 KiB) Viewed 5784 times

Here's the stock analysis after soldering the kit:

ste-dac-fix-mon-off.png (26.62 KiB) Viewed 5784 times

Quite a big difference!

By the way, my monitor was disconnected during both recordings. Here's what you get when you leave the monitor on and connected (measured with fix installed):

ste-dac-fix-mon-on.png (26.46 KiB) Viewed 5784 times

That should be someting to take into account when recording.

[troed]

The 2600Hz "STE left channel whine" strikes again.

[exxos]

It does need more investigation still...