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English: The RCA Radiola AR-812 radio receiver, the first consumer superheterodyne receiver. The superheterodyne circuit, invented by US inventor Edwin Armstrong in 1918 during World War 1 when he was working for the US Army Signal Corps, is the circuit used in the vast majority of radio receivers today. The rights were purchased by RCA, and the AR-812 medium wave receiver was released March 4, 1924. It used 6 UV-199 triodes: a mixer, a local oscillator, two IF and two audio amplifiers in a push-pull output stage, with an IF of 45 kHz, and was priced at $289 with tubes and horn speaker, and $220 without. It was built to be semi-portable, with compartments for the batteries in back and a handle on top, although it weighed 30 lbs. without batteries. The two large knobs are the input and local oscillator tuning, they had to be adjusted in tandem. They had blank carboard dials, so users could mark the positions of stations on them. The small knobs adjust the filament current. Its superior sensitivity and selectivity compared to competing receivers made it a commercial success. There are many reports of transcontinental and transoceanic reception.