Biskit Posted September 17, 2007 Share Posted September 17, 2007 not sure why it goes unstable? I've found that many comms systems go unstable if the mic and earphone ground connections are linked on any heasdet. I discovered this when trying to do what the OP describes. I only had balanced mic cable to hand, so tried wiring pins 1 & 3 to the cable screen at both ends (mic gnd and ear gnd respectively) which I thought would be fine, assuming gnd would be common at the beltpack anyway. I used the two cores seperately for pins 2 and 4 (mic signal and ear signal respectively). My cable was only about 0.5m and it caused oscillation even with volume turned right down. I later re-made it using the 2xunbalanced type cable (figure-8 shaped stuff) keeping the gnds seperate and it works fine. I have since made a second one which is also ok. I can't believe crosstalk between cores would be the issue over such a short length. Notably I didn't wire to the 'chassis' tab of the XLR at either end. If you do this, eg link it to pin 1 (mic gnd), but at the beltpack the XLR chassis is connected to the earphone gnd (or vice-versa) this would link the two gnds and cause the problem I observed. Still not absolutely sure why this happens... probably the accurate impedance matching required for the mic preamp (which is a very high-gain system). It is very easy to tip such circuits into instability with impedance mismatches. I'm not sure which 'gnd' (mic gnd or ear gnd) is tied to the actual system gnd... possibly neither. Ben. Link to comment Share on other sites More sharing options...
tlc Posted September 19, 2007 Share Posted September 19, 2007 Our tecpro wall boxes seem quite happy with a 3 metre extension wired with 4 core individually screened cable.Got the cable from RS Link to comment Share on other sites More sharing options...
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