System Design: Tips & Tricks
This page provides tips & tricks for system design by experienced Telex Intercom Headsets technicians.
- Cabling
- Usually the intercom system’s specifications are based on the use of 22 AWG microphone cable. Microphone cable of 22 gage measures 3Ω per 100 feet or about 30Ω per 1000 feet (round trip resistance). The wire table says 32Ω per 1000 feet round trip, but the shield resistance is much lower than the wire resistance. The Audiocom system uses both wires and the shield to transport DC so the calculations will be different for DC voltage drop versus distance.
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- Headset Cable Lengths
- The dynamic (low level) headset cable carries signal levels that differ by as much as 34 dB + 52 dB = 86 dB. Ordinarily, there are three types of unwanted coupling possibilities: resistive (through a common ground), capacitive and inductive. Since separate grounds are carried back to the microphone preamplifier and headphone amplifier, the common ground resistive coupling is, in this design, negligible. The capacitive coupling can be made no significant by a 100% shield in the cable. The inductive coupling mode dominates in this design, and can be offset in several ways:
• The distance between the microphone and headphone pairs can be increased, while the mutual inductive coupling is decreased by the use of “ribbed” cable (two cables molded together side-by-side).
• Both the microphone cables and the headphone cables can each be tightly twisted.
• Two or four separate cables can be run. A balancing transformer on the microphone circuit may be used. Estimated, Safe Operating Distances are as follows:
• Single cable, two shielded twisted pair: 10 feet.
• Dual ribbed cable, two shielded twisted pair: 30 feet.
• Separate cables, shielded twisted pair in each: 50 feet and more.
• Balanced microphone input: up to 100 feet depending on cable used.
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- Headphone Impedances
- Low impedance headphones are louder, causing the user station to draw more current from its power source. High impedance headphones are not as loud, drawing less current. Many user stations have a headphone impedance range from 25 - 600Ω. Headphones up to 2,000Ω will function but greatly reduced levels. In a double muff headset such as a Beyer DT-109, there are two 50Ω headphones connected in parallel resulting in an impedance of 25Ω.
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