Panasonic SA-5500
Seventy watts of music power, and the output transformer left on the drawing board.
Overview
Panasonic's 1972 advertising pitch was blunt: your next receiver should have three things missing. The input transformer, the output transformer, and the output capacitor. Pull those out of the signal path and the amplifier circuit couples directly to the speaker terminals. Panasonic called this "direct coupling," and the thing you cut down on is distortion. The SA-5500 was part of that argument, in a receiver line that ran up through the SA-5800 at 100 watts, the SA-6200 at 150, and the SA-6500 at 200. The SA-5200 at 46 watts was the newest and least expensive model.
The SA-5500 was rated at 70 watts of music power, IHF. That sits above the SA-5200 at the bottom of the family, but it got the same direct-coupled topology and a good chunk of the feature set from the models above it. Panasonic's advertising for the period ran in Audio (March 1972, p.7), High Fidelity (May 1972, p.35), and HiFi-Stereo (February 1972, p.37), among others. It framed the whole line around that missing-transformer story, and the SA-5500 carried it, with the SA-5200 available for less money.
The tuner used two 4-pole MOS FET transistors, and Panasonic rated FM sensitivity at 1.8 µV - enough, per the ad copy, "to pull in stations you thought were out of reach." It also had an FM muting switch to cut interstation noise, a linear dial scale that spread the FM band out for easier separation of stations, and a signal-strength tuning meter. There were dual tuning controls as well, for what the advertising called custom-blended sound.
The amplifier side gets less airtime in the ad copy, but the numbers are there. Frequency response ran 20 to 50,000 Hz, ±1 dB. Distortion was quoted at less than 0.5% in the March 1972 advertising, though later versions of the same campaign in Audio (September 1972) and elsewhere cited under 0.8%. Treat the exact figure as approximate, since it depends on which printing you're reading. Direct coupling was the selling point behind both numbers. Panasonic claimed it improved transient response and damping, the argument being that cymbals should clash rather than pop and a high C shouldn't sound like a screech.
On the front panel, the SA-5500 had a high-filter switch and a loudness switch, features Panasonic pointedly noted were borrowed from its more expensive models.
The SA-5500 was the floor of a four-model range, and Panasonic's advertising treated it as the gateway: buy the 70-watt version, and if that isn't enough, move up to the SA-5800, the SA-6200, or the SA-6500. It was built to a price, and that shows most in the power rating. Seventy watts IHF in 1972 was respectable for a budget receiver but not a heavyweight, and anyone driving inefficient speakers should expect to work the volume knob harder than they would with its bigger siblings.
The rest is straightforward early-solid-state receiver construction.
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