Shure Model 578 Omnidyne Probe omnidirectional dynamic microphone
General The Shure Model 578 is a slender dynamic microphone designed to provide wide-range reproduction of music and voice. This omni-directional dual-impedance mic is ideal for high-quality public address, theater-stage sound systems, and recording applications. It is also recommended for interview type use where “hand-a-bility” of the microphone is important.
The Model 578 is dual impedance for connection to mic inputs rated at 19 to 300 ohms or to high-impedance microphone inputs. The microphone is supplied with an adjustable swivel adapter, permitting it to be tilted through 90° from vertical to horizontal.
Uniform frequency response from 50 to 15,000 Hz
Slender design, light weight
Built-in On-Off switch
Convenient impedance selection
Versatility—for use in the hand or on a stand— for use indoors or outdoors
Dependability under all operating conditions
Ruggedness—will operate for years without deviation from original standards
Frequency Response: 50 to 15,000 Hz (see Figure 1).
Polar Pattern: Omni-directional.
Impedance: Dual. Microphone rating impedance is 150 ohms (250 ohms actual) for connection to microphone inputs rated at 19 to 300 ohms, and High for connection to high-impedance inputs. See Impedance Selection.
Output Level (at 1,000 Hz): Low Impedance Open Circuit Voltage = –80.0 dB* (.1 mV). Power Level = –60.0 dB**. High Impedance Open Circuit Voltage = –59.0 dB (1.12 mV). *0 dB = 1 volt per microbar. **0 dB = 1 milliwatt per 10 microbars.
Phase: Positive pressure on the diaphragm produces positive voltage on the black lead.
Switch: Built-in On-Off switch with lockplate installed. To lock switch in On position, loosen screw on lockplate and turn lockplate 180°. Re-tighten screw.
Cable: Non-detachable, 4.6 meter (15 feet), three-conductor, shielded.
Case: Satin-chrome plated with stainless steel grille.
Swivel Adapter: Positive action swivel adjustable through 90° from vertical to horizontal, to fit ⅝"-27 stand thread.
Net Weight: 454 grams (1 pound).
Impedance Selection Selection of either high or low impedance is accomplished by the choice of leads connected at the equipment end of the cable.
Low impedance—The black and white cable leads are the “hot” conductors for balanced line connections; the shield is connected to the amplifier or chassis ground (see Figure 2).
High impedance—The red cable lead is the “hot” conductor for unbalanced line connections; the black cable lead and the shield are connected to the amplifier or chassis ground (see Figure 2).
The low-impedance connection is recommended where long cables are required or under conditions of severe hum disturbance. The permissible cable length is practically unlimited, since neither response nor level is appreciably affected by long cables. Shure Model A95 Series Line Matching Transformers are available for use in those cases where a low-impedance microphone line is desirable, but the associated amplifier has a high-impedance input. These transformers provide a proper impedance match between a 19 to 300-ohm microphone line and a high-impedance input, and are available with various input and output connectors.
Text and illustrations are from the Shure Brothers 1984 Publication No. 27A2062 (DA).
Yet another 578 model, this one silver, showing its switch.