Low-Impedance Connection

Author: N.H. Crowhurst

Although electric pickup is not important to the low-impedance circuit itself, it can be transferred from the primary to the secondary of the transformer by the capacitance between windings. Use of an outside sheath as an electric shield will help prevent this.

The principal problem with a low-impedance (50-ohm or lower) line for long-distance connection is the resistance of the wire itself. Unless a very heavy gage is used, the line will have a resistance of about one ohm for every 50 feet. (A 5000-foot length would have a resistance of 100 ohms.)

Resistance losses deteriorate performance

If the microphone impedance is 50 ohms and the resistance of the connecting wire is 100 ohms, the total input resistance to the amplifier will be 150 ohms. This means the input transformer can step up only from 150 ohms instead of from 50 ohms, as it could if the line were quite short. If all of this 150-ohm impedance were due to the microphone itself, the microphone would produce a correspondingly higher audio voltage; however, this is not the case. What we actually have is a 150-ohm source providing only the audio voltage that would be provided by a 50-ohm source.