Hmm, so the mike cable isn't the problem, it's the cable from the preamp to the modulator.
I can't believe that any commercial audio gear would have such high output impedance that fifteen feet of cable capacitance would make much difference. And I don't think that the difference between fifteen feet of RF cable and fifteen feet of audio cable would make much difference at all. Now, musicians with high impedance guitar pickups can often tell the difference between guitar cables that are not much longer than 15 feet... and it does affect the highs. But a guitar pickup is a tricky, high impedance, highly inductive signal source. It seems more likely to me that maybe the preamp has a floating balanced output, and only one side is connected to the shielded line, and the shield is grounded. Output in that situation would be very quirky, and dependent on the load impedance and any bypass capacitance. If you had, say, 100pF to ground on either side at the preamp balanced output, and 100pF to ground at the modulator input (and I assume that the modulator input is single-ended), then it would act like a capacitive divider again, with 200pF effective capacitance to a low-z audio source. The resistive load would affect the low end, and varying cable capacitance would affect the level and the low frequency corner as well. However, if the bypass caps were 4700 pf or .01 uF, the effect of cable capacitance would be slight. For a few hundred pF of cable capacitance to affect the top end much, there would have to be only a very small amount of extra bypass capacitance. Maybe someone made a passive low-pass filter at the preamp output, and the extra cable capacitance is having a big effect on an unintended resonance. (Doubtful.) Or maybe there is some RF getting in that is getting rectified and causing an audio resonance when you transmit. Different cable, even a different cable path, can affect RF leakage. Do the RG cable and the Radio Shack cable have their own connectors, or did you replace the RG with the Radio Shack cable using the same connectors? And does the preamp use those 3-pin XLR connectors? That would suggest a probable balanced audio output. I really suspect something like a balanced output (and/or input) that is not connected right. To use a floating balanced output for a single ended system, just ground one side and take output from the other. Balanced windings are usually not center-tapped, but they might be set up for phantom power somehow... which is another possible source of odd effects if the winding is not connected right. The other likely possibility is RF rectification. I would run a variable oscillator through the preamp and measure response and output at various points with a VTVM or a modern digital meter, and see where the weird stuff is coming from. That's the best thing. Or you could feed some music source into the preamp, put the transmitter on a dummy load, and listen with a receiver while you change stuff. The differences you saw were large, and you should be able to track them down. Also, just try it as-is, but first with the transmitter on an antenna, and again on a dummy load, and see if that makes a difference. Could be RF rectification. Bacon, WA3WDR
