Sony ST-A6B (1977-1980)

A high-performance FM/AM stereo tuner with PLL synthesis and 7-gang varicap tuning, designed for balanced reception and audio quality.

Overview

The Sony ST-A6B is an FM/AM stereo tuner made by Sony Corporation in Tokyo. It was released in 1977 and produced through 1980. Sony built it to balance reception performance, audio fidelity, and ease of use. It was a more affordable alternative to the ST-A7B and shares core technology with its higher-end sibling. Its performance holds up against many mid-tier 'super' tuners of its era. It was overlooked at the time, but it has gained recognition for its engineering and build.

Specifications

Tuning range	87.5 - 108 MHz
Practical/Usable sensitivity (FM)	10.3 dBF (1.8 µV)
50 dB quieting (mono)	15.9 dBf (3.4 µV)
50 dB quieting (stereo)	37.1 dBf (39 µV)
Signal-to-noise ratio (FM)	70 dB
Capture ratio	1.5 dB
Selectivity (IHF)	Normal: 35 dB, Narrow: 80 dB
Alternate channel selectivity	70 dB
AM suppression	50 dB
Image rejection	80 dB
IF rejection	90 dB
Spurious response rejection	90 dB
RF intermodulation	80 dB
Subcarrier suppression	70 dB
Distortion Factor/THD (FM)	0.3%
THD (mono, normal)	0.08%
THD (mono, narrow)	0.2%
THD (stereo, normal)	0.15%
THD (stereo, narrow)	0.4%
IM Distortion (mono, normal)	0.08%
IM Distortion (mono, narrow)	0.2%
IM Distortion (stereo, normal)	0.15%
IM Distortion (stereo, narrow)	0.4%
Stereo separation (1kHz)	35 dB
Frequency response (FM)	30 - 15,000 Hz, +0.5 -1.5 dB
Fixed outputs	650 mV, 2.2 kΩ
Variable output	1.2 V max. / 200 Ohm
Power consumption	18W
External dimensions	430 x 145 x 335 mm
Weight	8.5 kg
Power Requirements	AC 100, 120, 220, 240 V, 50/60 Hz
Usable Sensitivity (IHF): AM	300 µV/m
Signal-to-Noise Ratio: AM	45 dB
Total Harmonic Distortion: AM	0.8%
Frequency Response: AM	50 - 3,000 Hz

Design

The ST-A6B uses a PLL (Phase Locked Loop) synthesizer system for tuning and a Pulse Count Detector (PCD) system for FM detection. The front end has a dual gate MOS-FET RF amplifier, a dual gate MOS-FET mixer, and a FET-buffered local oscillator, with a 7-gang varicap and a 4-gang variable capacitor. The tuner has a quad loop tune circuit with a 7-row CG varicon and a quadruple-tuned antenna circuit. The IF stage uses a Sony Uni-Phase uniphase filter, and the MPX section uses a PLL IC. There is a high-blend circuit, automatic switching between Normal and Narrow modes based on signal strength, and a muting circuit that uses a reed relay. It also has a built-in 400 Hz Cal Tone for setting recording levels, Low Cut and High Cut filters, and a signal/multipath meter. Audio outputs are both fixed and variable, and the antenna connector is an F-type 75 Ω coaxial.

Context

The ST-A6B was likely meant as a more affordable alternative to the ST-A7B. It shares the same basic design with the Japan-only ST-A5, which has a similar 7-gang varicap and a silver finish but was not sold outside Japan.