Help with Noisy Tannoy Ribbon Mic?

[slor]

I'm working on an old Tannoy ribbon mic, a "Type 1" I believe. With a new ribbon, the mic sounds good, if a tad quiet (transformer measures roughly a 1:18 ratio).

The issue is that there is a fair amount of hum. It's not a grounding issue so far as I can tell; all connections are correct and the case is properly grounded. Flipping the polarity at output doesn't change a thing.

What DOES change the hum--not completely, but noticeably--is rotating the mic so that the case is aligned horizontally rather than vertically.

If it's of use, the mic also features an on-off switch, which subs in a pair of resistors across the output when "off" is selected. Jumpering the connections doesn't make any difference. 

Any thoughts whatsoever would be appreciated!

 

[zebra50]

Hi Seth.

I have replied to you by email but since this is here, I thought it might be useful to post my thoughts in case it helps others. Here goes...

Firstly, when you say "the case is properly grounded", how did you do it? Usually on the cast body of these mics there is no solder tag or grounding post, so one needs to get a little creative, either by clamping a ground wire or drilling and tapping a little hole. I recall that your microphone has a switch so perhaps there is a screw there that you can use to ground the outer body..

Anyway, assuming you found a way to do that, and that the motor assembly is also grounded (there should be a grounding lug near the transformer) then we will move on. Triple check grounding to be sure.

Both the motor and the transformer can pick up hum. The motor consists of a wire in a magnetic field and so behaves like a single coil guitar pickup and is similarly prone to picking up electromagnetic interference. The transformer is an open frame type without a shielding can, and will also pick up hum to a some extent. The hum will be worse for a high impedance model, and if the wires from the ribbon clamps to the transformer are causing resistance - make sure those connections are good and that the wires are thick.

One modification that I see mentioned for these old mics is to run two wires from the top ribbon clamp down each side of the motor to the transformer. This is supposed to be humbucking to an extent and might be worth a try. I haven't found it particularly helpful myself, but you can give it a go.

You could also try some wire mesh front and rear of the ribbon to make an inner faraday cage.

When these mics were made back in the 1930s/1940s, people did not own the plethora of electrical equipment that we do now, and I think we live in an electromagnetic soup! So you are probably stuck with finding the best position for the mic.

Hope that helps!
Stewart