I am trying to get a good understanding on sources of 50 Hz “ground hum” interference in audio equipment and good practices for shielding and grounding by making hum on purpose and taking steps to remove it.
I have been doing some experiments using a EMU 0404 external soundcard with 1 Mohm ¼” jack unbalanced line inputs. I am using it as a pre-amp/ headphone amplifier (not attached to a computer).
Here are my observations:
The EMU has an unearthed 5V dc ”wall wart” power supply. I noticed that there is a stable 100V ac (50Hz) and 0.2V dc potential difference between the 0V connection and earth. However, if I touch 0V and earth at the same time, I do not receive an electric shock. Why is there this large (presumably high impedance) common mode voltage? Why is it stable, if it is a “floating” power supply?
If I connect an unterminated 2 meter unbalanced ¼” jack cable to one of the inputs, I can hear lots of 50 Hz hum on the output. This is not surprising as the cable is acting as an antenna and the 0404 has high input impedance. However, when I connect the sleeve part of the jack to earth, most of the hum disappears. I can still increase the hum significantly by touching the tip of the jack with my finger.
If I remove the earth connection and insert the jack into a shielded but unearthed box, I can no longer detect the hum with my ears.
Can someone explain these observations, in particular the effect of the large common mode potential difference to earth on 50Hz hum?
So I was surprised about the behaviour of the power supply and also that it would be used for audio equipment. Presumably the common mode voltage is stable because neutral is attached to earth in the uk. Could someone point me to a schematic of a switched mode power supply which behaves like this?
What I conclude from 2. Is that most of the hum is caused by the large common mode ac voltage of the sleeve/shield (which is connected to signal ground), which I find surprising. Surely the tip connection also has this common mode voltage at the input stage of the soundcard, so overall it should not make any difference?
When I touch the tip, I am turning the cable into a better dipole antenna, so this increased hum does not surprise me.
Observation 3. tells me that when everything is fully shielded, the common mode voltage no longer seems to matter.