English: The first Neutrodyne radio receiver, built by Louis Hazeltine at Stevens Institute of Technology and presented at the March 2, 1923 meeting of the Radio Society of America. The Neutrodyne was a modification of the tuned radio frequency (TRF) receiver which neutralized feedback in the triode vacuum tube, preventing oscillations which caused howling and squealing noises which plagued this type of receiver, and also radiated interfering radio signals which could interfere with other receivers. The oscillations were caused by the large capacitance between the grid and plate electrodes in the triode, which could feed energy back from the output to the input. The Neutrodyne circuit prevented oscillation by a second feedback path which fed some of the plate signal back to the grid with opposite phase, to cancel the interelectrode feedback. This prototype had five vacuum tubes: two stages of tuned radio frequency amplification, a detector, and two stages of audio amplification. The three interstage coupling coils are visible, mounted at an angle to minimize magnetic coupling which could also cause feedback and oscillations. The neutralizing signal for each stage is taken from a reverse-phase winding on each coil. The neutrodyne was widely used until the 1930s, when it was replaced by the superheterodyne receiver

Caption: "The new Hazeltine circuit receiver. Five tubes are used and the circuit is incapable of regeneration or oscillating, which is the cause of much interference in the present-day receivers, especially when many are located in a small district, such as New York.