The UM 900 is the first switchable pattern, valve condenser microphone which can be connected to industry standard 48 volt phantom powering using a normal 3-pin XLR microphone cable. This is made possible by an advanced circuit design for the amplifier and internal power supply. The pickup capsule uses a traditional, proven design with two large, gold plated plastic membranes.

Three switches are located around the body of the UM 900. The switch in the front selects the polar pattern: omni, wide cardioid, cardioid, hyper cardioid and figure 8. Located 120 degrees to each side are switches for gain adjust and low frequency roll-off. The unusual shape of the microphone's body is especially designed to minimize undesirable reflections of sound toward the pickup element.

Acoustic influences which could result from the housing resonance and/or mechanical vibration are reduced by the robust construction and by a special fixture within the microphone's housing which dampens vibration. The high sensitivity and low noise of the UM 900 make it especially suitable for vocal and speech pickup in professional sound recording studios, scoring stages and broadcast studios. The 5 pickup patterns allow the microphone to be tailored to a wide variety of applications. For example, the microphone can be used as an overall pickup for a large orchestra or it can be used as a solo or spot mike for individual instruments. It may also serve as an announce microphone where its proximity effect may be desirable.

The direction of maximum sensitivity is radial to the microphone body, i.e. side addressed. The front of the microphone is indicated by the model number and the pattern selector switch. The pick-up capsule is mounted in a large, acoustically open headgrill which prevents mechanical vibration interference. This gives the microphone a clean, colorless sound characteristic.

The internal vacuum tube amplifier of the UM 900 can be powered by any external 48 volt phantom supply which can provide at least 4 mA of current. The amplifier uses a specially selected vacuum tube. An internal transverter provides all the necessary voltages. The low impedance output is implemented with an operational amplifier and balanced output transformer.