Electrical Considerations

Eargle, Ch. 8(partial)

Phantom Power and Impedance

48V phantom power (P48) has a resistance of 6800 Ω

Phantom Power Do’s/Don’ts

Don’t power ribbon or dynamic mic’s. T12 causes noise and damage in dynamic mic’s

Don’t plug or unplug microphones into cables with phantom power on.

Don’t turn phantom power on or off when signal is bussed to monitor channels and speakers are on. Possible HF transducer burnout

Standard Electrical Values for Mic CableGauge: #24 AWG stranded copper wireResistance/meter: 0.08 ohmsCapacitance/meter: 100 pF

Line Loss

P48 can support long cable runs.P48 has resistance of 6800 Ω.Cable adds 0.08/meter. For 100 m, 8 Ω

per leg, 16 Ω total (2 pins).Negligible increase to 6800 Ω.

High Frequency Loss

High Frequency loss is more of a problem for long cable runs than phantom power problems.

60 m with 200 Ω mic impedance (typical of condenser), will show minimal loss (1 dB) at 20 kHz. Roll-off starts at 5 kHz.

60 m with 600 Ω mic impedance (typical of dynamic mic), will show 2 dB loss at 10 kHz, and 3 dB loss at 20 kHz. Roll-off starts at 1 kHz.

RF Interference

Long cable runs susceptible to RF interference. Lighting systems Radio transmitters, including mobile devices.

Starquad cables most resistant to interference.

Separate electrical and mic cable runs.

Ground Loops

Electronic devices chained together.Ground path in cables and in metal rack

housing gear.Metal rack generates some magnetic flux.Solution: ground chassis, only connect

grounds at output end (no continuity).