On the other hand nice work with that amp, BrokenEnglish. As far as I can see it the external PSU doesn't has any puffering/filtering and the amp has the puffer/regulator sections as well. Am I right?
I did a few years back a phone preamp with such an arragement and it worked fine until for some reason I crossed the input cable coming from the pick-up with the cable going from the external PSU. I was welcommed instantly with a strong hum. If I crossed the input cable with the mains I heard nothing. (Crossing means that the two cable lies in each other in some angle). The reason was (of course) that the secondary voltage of the PSU is much lower (if we speak about solid state designes) than the mains voltage. If you pull 200mA for the secondaries and you have 22V DC, then you are pulling 20mA from the 220V (such is the ideal transformator with no loss). The 200mA together with the cable going from the secondary did presented a much higher 50Hz energy emittion as the mains cable. Of course it was with a high level MC pick-up (max. 3mV voltage) and the phone amp has a much higher gain then any headphone amp. But since then I either filter the voltage in the external PSU or I don't use a separate case.
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