order to obtain stable closed-loop operation. With feedback the amplifier is driven over the linear region with a 60cps signal of less than 1.0 mv (millivolts). The voltage gain is about 15,000 and the power gain is 4XI0 8 • Considering the increased input impedance, the amplifier is driven over the linear range with 10-9 watts to produce full output. The use of high-supply frequency and capacitive coupling extends a-c magnetic-amplifier sensitivity by several orders of magnitude. In addition, a capacitively coupled amplifier possesses high speed, phase sensitivity, excellent stability, and good linearity. Low-level d-e signals also can be amplified and these features retained if a powerfrequency chopper is used on the input of the amplifier.T H E INCREASE in demand for communication service throughout the world has exceeded the rate of growth in population and the tremendous expansion of industrial activities.' Both civilian and military requirements have grown beyond the expectations of those associated with the communication field. Along with this need for increased capacities in communication networks, the need has developed for more reliable, higher quality communication service. Military operations and the defense of a country are dependent on the ability to communicate rapidly and accurately. Likewise the operations of large industrial organizations are controlled by signals transmitted over long distances. With these factors in view the M M-600 radio relay system described in this paper was developed. It is intended for use in major communication networks to transmit large numbers of signals from one central location to another over distances up to 4,000 miles. It was designed for common-carrier-type applications where reliability, excellence of performance, and ease of maintenance are of paramount importance. New components such as Conclusions A high-performance a-c magnetic amplifier can be constructed by using a highcarrier frequency and capacitively coupling the signal-frequency power to the load. . This technique realizes the advantages of a true a-c amplifier which include freedom from the effects of d-e drift, consequent increased sensitivity, elimination of an input demodulator, inherent phase sensitivity, and accurate reproduction of the input waveform. A capacitively coupled magnetic amplifier possesses excellent stability, and when amplifying power-frequency signals, can achieve very high voltage and power gains. A 3-stage amplifier has been built which has an equivalent noise level of 5J.Lv, or