Dynaco ST70 Interconnectivity

[Bookman]

Having foolishly sold my Dynaco ST70 previously and with a bit of time on my hands I bought a brand new one; albeit in kit form.
It was an enjoyable task if not relatively straight forward. Testing was also similarly relatively straight forward with only a few odd moments caused by a dodgy GZ-34. Indeed, after sorting out everything I noticed how quiet the item was upon switching on and sorting the bias. In fact there was deadly silence but in a nice way.
We now come to the reason for posting this query. In this I had always intended on using the preamp output of my Musical Fidelity M3Si integrated amp which is where the problem emanates.
With all interconnects in place and all power leads in their sockets in a switched on state and with the Dynaco on and nicely warmed up there is still no distortion whatsoever. However, as soon as you switch on the M3 there is a slight hum albeit not annoying so. In fact the sound it produces was well worth the time and money.

I cannot believe it is an earth loop issue as it applies only when the M3 power is switched on at the unit?
I would appreciate any comments especially since I tried the line in/out functionality from a radio cassette I have that functions as a preamp but there was no hum; just a bit of a hiss.

Regards
Joe

[GrimJosef]

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

[Bookman]

Thanks GJ,

The situation described is under “no input” conditions which would suggest minimal induced magnetic fields derived from the transformer magnetising current plus the valve heater tap and the GZ-34 conductance. Moreover, there is no hum produced by these micro functions without the M3 connected as described below.

I have produced a sketch that shows a common supply rail derived within the twin wall socket.
With plug B and the Dynaco switched on there is no hum. However, if I then switch on plug A but leave the M3 in the off position that is when the 50Hz hum starts.
I would imagine the M3 is switched on the internal transformer primary side however there is a power on amber LED (implies rectification?) that connects to the protection circuit. When the on switch is pressed the protection relay picks up after a couple of seconds and a blue light then shows.

Apologies for quality of sketch
Joe