Tube mic cable trouble

[tmuikku]

Hi,

I had time to finalize my two MK7 tubemics from years back, here's the project
http://groupdiy.com/index.php?topic=30124.0

Pf86 tube, fixed bias, suggested psu. The thread says heater must be very low ripple for no hum. It is good enough since there is  no audible or daw analyzer visible psu hum over background noise.

This no noise scenario is with mogami tube mic cable. For the other mic i had just made new cable with sommer cable octave tube.

The other mic had hum and it took a while to look where psu hum is coming. Much swapping and ruling out things i'm almost sure it is the cable! At least it appears to be. The mics and psus are pretty much identically built and the psu hum appears on the mic with sommer cable.

What should i be looking for to cause this phenomenon? Both should be good cables and are almost identical for electrical specs(?). I triple checked they are properly soldered and the twisted pair is used for audio, heftier wires for heater and ground. Cable shield is not connected at the mic end. Mic body is grounded to the same ground as audio inside the mic.

I tuned the sensitive heater rail in the psu for a given psu + cable + mic combination.  Sommer cable is about two meters shorter so little less resistance, capacitance and inductance at play. Cables are roughly 7m and 5m long.

The hum induced is by my daw analyzer something like 20db louder than with mogami cable. I'm puzzled.  Maybe it was too late last night or what, i going crazy? well now i' heading for another test round. I'll post a reply later.

http://www.mogamicable.com/category/bulk/microphone/tube/

http://shop.sommercable.com/en/Cables/Bulk-Cables-Audio/Mikrofonkabel-SC-Octave-Tube-200-0186.html

 

[Matt Nolan]

Even though you've fine tuned your heater supplies for each combo, did you use the same connectors? Can you do a quick swap of mic / cable / PSU to narrow down if it really is the cable? Could be a mic. Could be a PSU. Try swapping tubes also.

Also, why not connect your cable shield to the mic body (but keep signal ground separate from that at the mic end, connect the two inside the PSU)?

 

[tmuikku]

jeah same connectors, can swap mics and psus and cables and tubes. already tried swapping but the tubes. unless i was too tired

need to get some nylon bolts to separate mic housing from pcb ground. will try, thanks for tips!

 

[tmuikku]

swapped all around and it was the cable! actually not the cable but my assembly, doh moment

cable shield was touching strain relief at the mic end connector effectively grounding the mic housing there as well. resulting ground loop induced the hum. my current grounding in the mic is housing to the audio ground and cable shield connected only at the psu end.

i ordered some nylon bolts to separate the mic housing from audio ground. this will eliminate related problems since mic housing will be extension to the cable shield separate to audio ground.

no blame for sommer cable!