When there's a transmitter in the area, there's only one GOOD circuit
for connecting balanced to unbalanced audio. That's a shielded
transformer. Any semiconductor circuit will have a tendency to act as a
detector causing RF feedback. I made the mistake of using a commercial
PA amplifier that achieved the balanced input with transistors once. One
evening at the church I heard a ham more than a mile away calling "CQ
160" on AM. I tried low pass filters, but they didn't work. I installed
transformers, they worked. Since then I buy no PA amplifiers without
transformers for the balanced inputs. Radio Shack sells a balanced to
unbalanced transformer in a plug for just such purposes.
You need shielding with balanced cable because the microphone input is
at such low level compared to the pickup of unshielded wire, even
twisted pairs. Ordinary CAT 5 isn't shielded.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.
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