Humming / 50Hz Buzz with new Electroware C64 PSU

[Rio]

Hello,

I've been using my own MBSIDv2 with an original C64 PSU for years. I use the following power circuit from Thorsten (mbhp_8xsid_c64_psu_optimized.pdf) - I have no noise, hum on the audio outputs. Everything is going great, so far so well.

Since the C64 PSUs could be prone to overvoltage (after 40 years), I wanted to switch to the alternative. So I decided to use this: 

Electroware - C64 PSU

With this power supply I have the mains hum on the audio outputs. Another Electroware PSU will behave the same way, so the PSUs themselfes are not defective. I mix 6x SIDs on 2 channels (stereo) via 3 Channel-audio-mixer and supply the mix circuit there with + 14V, coming from of the 8xsid_c64_psu_optimized circuit - approved before. 

This is my current schematic:

I ran through the following scenarios (please read carefully before answering):

1. The buzz comes when I use the new PSU and connect everything as shown above.

2. I have no buzz when I directly connect SID Pair 1 to "line in" without any extension (NO: CORE3/SID3LR/CORE4/SID4LR/MIXER)

3. The hum also comes when I directly connect SID Pair 1 to "line in" and do not connect the mixer circuit (no power and no audio), but the CORE3/SID3LR/CORE4/SID4LR are powered.

4. When everything is connected as shown, the humming disappears immediately, e.g. by disconnecting +14V power from one of the SID3 boards (6581 - L or R). 5x SID chips sound free of hum!!

5. The humming disappears immediately, e.g. by disconnecting +14V power from both SID1 boards, but everything else is connected. The 4 rear SID chips (board LR 3 and 4) sound free of hum.

 

Note: It seems to me to be a general ground loop somewhere between the CORE / SID boards cabling, caused by the newer PSU. The length of the data cable (tested with very short different cables) or the mixer itself doesn't seem to be really responsible for the hum.

What could I wire and test differently? I am grateful for any suggestion. I would like to switch to the newer PSU, because it works under one or the other of the conditions.

Thanks and Greetings,

rio

Edited December 3, 2020 by Rio