hum problem

[yaniv]

Hi,

I have a PM-1000 strip.
I connected its output unbalanced to my fireface400 and I have hum :-(
but when I connected it directly to phones I don`t have any hum.

I also look at the output with scope , and I saw that the hum occures only when I connected it to the fireface ... when it connected to phones the scope shows "quiet" line..

all the power supplies are from the same source ....

Can someone help me ?!

[Matt Syson]

hi
It is a 'standard' earth (ground) loop problem. Search the many threads about this and you will find suggestions. You sat a '1000 strip, have you made this yourself? While the power supply metalwork MUST be earthed, the analogue ground used by the module does not have to be tied directly, so you could use a pair of 'back to back' diodes and a small capacitor or a 10 ohm resistor (3W wirewound) to link the analogue ground to mains earth on the strip. If you don't understand this, find someone who does.

Matt S

[yaniv]

I attched a draw of what you describe ... did i understand you right ?

also, about the small capacitor , how small should it be ? and if I understand it shouldn`t be electrolitic because of its polarity ?


thanks A lot !

[Matt Syson]

Hi
Sorry no it is incorrect. You have drawn all in SERIES. They should all be in PARALLEL. The capacitor should be around 0.1 to 1uF polyester. As it is in parallel with the dides the maximum voltage across the capacitor would be about 0.7 Volts.
You could add a resistor in parallel as well say 22 ohms but leave this out unless you have problems.
What this arangement does is tie the audio ground to safety ground within 0.6 volts for safety, and at HF due to the capacitor but at low frequencies there can be up to 0.6 volts difference so preventing ground circulation currents.
Diodes 1N4001 type or 1N5401 (anything power type).
Matt S