Converting unbalanced mic to balanced output
I'm getting RFI into my Flex 5000A audio lines because the ATR30 mic has an unbalanced output - I'm told. I have this problem only with one antenna that is too closel. The Flex has provisions for a balanced or unbalanced input so I took the easy way and used what came with the mic.
I had assumed that it would be necessary to buy a small isolation transformer and rewire with the transformer converting from unbalanced to balanced output. However, today I just read several mic wiring articles, one from Heil indicating that it is only necessary to rewire, using two shielded wires for the plus and minus signal of the mike and using the other two wires outside of the shield for switching, etc. The article from Heil further stated that the shield be grounded only at one end, presumably at the transmitter end.
I just happen to have a roll of Heil four conductor (two#18 wire inside silver coated wire shield). I'd appreciate any comments on this approach - I'd hate to do the work and find it inadequate... soldering those minature connecter pins are a big job for my old hands.
The important thing is to use a cable that has a shield. The link has a pretty good discussion of balance vs. unbalanced microphones. A fine point of cable shielding that many people don't understand is that by connecting the shield at one end only, you prevent ground loops from developing.
[added] Let me clarify that balanced vs unbalanced has nothing to do with the cable. It's not the same as coax vs. window wire feeding our antennas. A balanced mic has a transformer that converts the impedance to Hi-Z or high impedance. An unbalanced mic does not and is Lo-Z.
Good luck. Bill.
Last edited by KB4QAA; 11-15-2009 at 04:01 PM.
I appreciate the link and read it with interest.
Originally Posted by KB4QAA
I understand the transformer impedance matching, but for some reason I had tied the two together.
I found in three other articles, including one from Heil, that stated a 'unbalanced' mic, which uses a signal line and a ground line for the return, can be converted to to a 'balanced' line by rewiring so that you send the plus and minus signal inside a shielded cable and use the shield on the input end only - as you said.
I have some of the good Heil shielded "pair-plus two" cable and ordered some connectors today. This mic is going into a preamp with 20db gain so I am not going to worry about mismatch yet. Besides, the audio was fine on a couple other antennas, but not when using the new, closer one that has an SWR of 1.0:1 through most of 20M voice band. It's just too close.
Thanks for your help.
Did you try wrapping the mike cable (near the rig) a few turns through and around a ferrite core, as an RF choke?
You might also try the "Questions & Answers" forum. I don't have a flex 5000 so I won't offer any advice.
Originally Posted by W4ZDI
It's easy to offer advice when RF problems occur in one transmitter and not in others: It's a design inadequacy because inputs are not sufficiently decoupled from the influences of RF.
Originally Posted by AC0FP
I have no idea why anyone designing a "high performance" rig would not take precautions against RF ingest via all ports. It costs almost nothing and should be routine. But, for some reason, they don't.
I don't think wiring your rig for a balanced mic input is going to solve your problem, as the RF field is simply too high. You need to reduce the RF getting into the rig, and the only ways of doing this are using a choke (already suggested), bypassing the RF to 'ground' using small disc ceramics or simply reducing the mic gain. Do some experimenting with the capacitors, as too high a value will affect the audio - use the highest values you can consistent with them having a minimal affect on the audio highs.