Power supply

[simon]

Standard ATX supplies are a very good option as they are very cheap. If we would go with a standard format PCB, we would solve this issue as well as the case question.

Yesterday I ordered some parts to build an LC-filter. When I test this, I will test an ATX supply (without extra filter circuit) as well.


Simon

[lee]

I fear a pontential ripple noise disturbing audio and video signals.

A lot of motherboards have linear regulated supplies just for their audio chip analog voltages. I just looked at five boards from the top of my pile of boards and three have regulators and all have local supply filtering.

Lee.

[simon]

Ok, so we should plan with this as well. The 9V supply for the SIDs is linear regulated already. But I assume the noise comes from the power supply as it only occures after a while when the supply is warm.

Simon

[lee]

The 9V supply for the SIDs is linear regulated already. But I assume the noise comes from the power supply

The audio output followers are powered by the +12V rail, they should be powered by the SID +9V. Also the +9V could probably do with some more decoupling, say a 100uF to 470uF, low ESR electrolytic.

Lee.

[simon]

The 9V supply for the SIDs is linear regulated already. But I assume the noise comes from the power supply

The audio output followers are powered by the +12V rail, they should be powered by the SID +9V. Also the +9V could probably do with some more decoupling, say a 100uF to 470uF, low ESR electrolytic.

You mean the SID and the output followers should be powered with the same source? Or just use the 9V for the output amp as it is linear regulated locally? For the first it would need a bit more modification as the circuit is switchable between 6581 and 8580 with different supply voltages.
Anyway the disturbance can be seen on the screen as well. This has led me to overthink the main power supply and power input rail (adding filter).

Simon

[lee]

You mean the SID and the output followers should be powered with the same source?

Yes. When you go to the trouble of making a quiet supply rail it should be used for all the downstream analog stages. It will probably help if you avoid digital signals in the 0V path as well.

Anyway the disturbance can be seen on the screen as well.

You need to do the same for the analog secctions of the video circuit. From the analog stages of the V9990 through the colour encoder and any other following circuits. You should again avoid digital signals returning through the analog 0V.

Lee.

[simon]

You mean the SID and the output followers should be powered with the same source?

Yes. When you go to the trouble of making a quiet supply rail it should be used for all the downstream analog stages. It will probably help if you avoid digital signals in the 0V path as well.

That will definately be a challange. At least when the PCB has two layers only (what I would prefer).

Anyway the disturbance can be seen on the screen as well.

You need to do the same for the analog secctions of the video circuit. From the analog stages of the V9990 through the colour encoder and any other following circuits. You should again avoid digital signals returning through the analog 0V.

Maybe re-arrange parts and move all analog parts nearer towards the power plug/switch to keep these traces short.

I will be a bit short on time the next two weekends as my girlfriend moves.

Simon

[lee]

Maybe re-arrange parts and move all analog parts nearer towards the power plug/switch to keep these traces short.

Actually no, the opposite would be better. Move the analog sections to the other end of the board and have only two points where the analog supply rails meet the rest of the board. If I get some time I'll grab the package footprints make an image of how the layout should be split.

Lee.

[simon]

Maybe re-arrange parts and move all analog parts nearer towards the power plug/switch to keep these traces short.

Actually no, the opposite would be better. Move the analog sections to the other end of the board and have only two points where the analog supply rails meet the rest of the board.

Really? Why is the opposite the better solution?

If I get some time I'll grab the package footprints make an image of how the layout should be split.

That would be great. I hope we have enough space to route these extra isolated traces across/around the board.

Simon