Re: Dynaco ST70 Interconnectivity
To get earth loop hum you need two things - first an earth loop (you probably have that as I assume both units are safety earthed via their mains leads and they might well both have signal ground connected to the safety earth too) and second a source of electromagnetic field to induce some 50Hz current in the earth loop. It could be that the field source is leakage flux (magnetic) from the M3's mains transformer or some stray electric field from its internal mains wiring, or possibly 100Hz from the rectifier/reservoir capacitor loop. These fields will only be present when the M3 is switched on, of course.
Cheers,
GJ
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