I’d previously simply connected half of the balanced XLR mic output to the mic input of the 847; I was aware this wasn’t ideal (and I’d lose the benefit of a balanced mic cable!) but it worked and I’m ultimately lazy. See my post Upgrade From Yaesu MH-31 to Studio Mic for details.
1:1 600 Ohm Transformers
Whilst trying to eliminate a buzz on TX (and exhausting all other options!) I figured I should maybe properly transform the mic output to an unbalanced connection. Only, commercial transformers a) often add an impedance transformer and b) cost anywhere from £15 (ebay special) to £80 (quality brand), to £200+ for devices with active pre-amps and equalisers.
So, ebay 1:1 600 ohm transformers to the rescue! I bought two for £5 including delivery. I then snipped off one end of my XLR cable, soldering the wires to a small section of stripboard. A 1:1 transformer was then added and a shielded mic cable exiting to a 1/4 inch jack (to match my customised adapter/wiring for the 847 mic input). The mic side of the transformer is left floating and one side of the radio half is pulled to ground. No other components are required; see below.
Balanced to Unbalanced Mic Schematic
I’m unsure of the difference between chassis ground and mic ground on the 847 and can’t remember which I connected. I know I tried both with no difference to my buzz.
The cause of my TX issue was eventually traced to internal 847 cabling running directly over the internal fan. This caused a ‘buzz’ whilst the fan was spinning up to speed; re-routing cables has essentially solved the issue. However, I’m still unclear how internal routing of cables can interact with a specific external mic. I’d expect the buzz to be either present or not. Whilst the problem is resolved the mystery is still ongoing.
At least I can now rule out poor balanced/unbalanced mic cabling from future issues 🙂
