VINTAGE TECHNOLOGY ARCHIVE / AUDIO / SONY / STR 7055

Sony STR-7055 (1972–1976)

At 15.3 kg, it is a heavy receiver. This is not a background player for the living room. It is a full-strength receiver built to anchor a serious system.

Overview

The Sony STR-7055 gets overlooked at the swap meet and then embarrasses the flashier gear on the bench. It's an FM/AM stereo receiver built around the same design concept Sony used for its ES Series. The company pitched it as a "premain" - a preamplifier and power amplifier in one chassis. Collector consensus puts it at the top of Sony's receiver lineup during its run, though the model number and a modest power rating convinced a lot of buyers it was mid-tier. It wasn't.

Prices split by market: ¥79,800 in Japan and 3,295 French Francs in Europe, both quoted for 1973. That was real money.

The amplifier section delivers 35 watts per channel into 8 ohms across the full 20 Hz - 20 kHz band at 0.2% THD. Sony's own service documentation lists 45 + 45 watts into 8 ohms and 55 + 55 into 4 ohms at 1 kHz. Dynamic power on the IHF constant-supply method is quoted at 136 watts total into 8 ohms and 200 watts into 4. That's useful headroom with inefficient speakers. It's a DC-coupled, quasi-complementary SEPP design with a differential input stage and dual independent power supplies, and no interstage capacitors in the signal path. The damping factor is 50 or more at 1 kHz into 8 ohms, which keeps the bass composed rather than loose.

The tuner is no afterthought. FM sensitivity is 2 μV IHF (1.6 μV at 30 dB S/N), selectivity 70 dB IHF, image rejection 70 dB, IF rejection 100 dB, capture ratio 1.0 dB, and stereo separation better than 38 dB at 400 Hz. Owners report the tuning feels unusually smooth and precise next to contemporary receivers using ordinary variable capacitors, and that stereo imaging holds together even on marginal signals. The front end pairs a junction FET with a linear-frequency varicon. The IF uses a two-element solid-state filter with IC processing. The MPX decoder uses a sealed, non-adjustable coil unit designed to shrug off heat, moisture, and age. Five ICs, 49 transistors (four of them FETs), and 29 diodes live inside.

Key Features

DC-Coupled SEPP Output with Dual Power Supplies

The output stage is pure complementary symmetry, directly coupled with no interstage capacitors, fed by two independent supplies and a differential amplifier up front. That buys bandwidth and stability. Frequency response at 1 watt runs 10 Hz to 100 kHz, +0/−1 dB, with power bandwidth quoted at 15 Hz to 35 kHz. Distortion sits at 0.2% or less at rated output and 0.1% or less at 1 watt, with intermodulation distortion matching those figures. The speaker protection circuit includes a dedicated circuit breaker for the amplifier itself.

Junction-FET Front End and Sealed MPX Coil

Sony's front end combines a newly developed junction FET high-frequency amplifier with a high-performance linear-frequency varicon, a FET mixer, and a stable local oscillator. Downstream, the MPX stereo circuit uses a new IC and a coil unit with no adjustment at all. It's sealed on purpose so characteristics don't drift with aging, heat, or humidity. A built-in high-blend circuit ties into the High and Both filter positions to clean up weak stereo reception. It's the kind of tuner that pulls a distant station out of the hash and keeps it there.

Four-Position Active Filter and NFB Tone Controls

The filter section is a four-position active design running from High-OFF (flat) through to Low-Both (bandpass). The low filter rolls off below 50 Hz at 12 dB/octave. The high filter rolls off above 9 kHz at 12 dB/octave. Tone controls are NFB-type, ±10 dB at 100 Hz for bass and ±10 dB at 10 kHz for treble. The loudness contour adds +10 dB at 50 Hz and +4 dB at 10 kHz, which is useful at low volumes without turning boomy. Inputs are 2 mV / 47 kΩ on phono and 150 mV / 50 kΩ on line. Signal-to-noise is 70 dB on phono, 90 dB on line, and 100 dB at the main amp. Rec out is 250 mV / 10 kΩ and pre out is 1 V / 4.7 kΩ, and the pre/main loop is separable.

Three Speaker Pairs, Two Tape Decks, Full Dubbing

The speaker change-over supports three pairs in five configurations - A, B, or C alone, plus A+B and A+C. That's more routing than most integrated receivers of the period offered. Two tape input/output pairs let two decks run simultaneously, and a tape-to-tape mechanism handles copying deck to deck without re-patching. Tape Monitor 1 takes precedence when both are engaged. A center-zero tuning meter and FET-switch muting round out the front panel, alongside a switched AC outlet rated at 300 watts.

Chassis and Power

Dimensions are 471 × 157 × 375 mm, weight 15.3 kg (18.6 kg boxed). Power consumption is 300 watts from a 100 VAC, 50/60 Hz supply. The styling is clean and classic. One period description praises the front panel as incredibly fine-tuned and classy. The wood case is the one most surviving examples wear.

Historical Context

The STR-7055 is the 1970s near-equivalent of the 1960s STR-6120 and STR-6200F: same near-topper status in the lineup, same quality, but a very different internal structure. The late-'60s ES units were lavishly simple inside, with clean cable routing and board placement. The 7055 arrived in a more hectic era of board-to-stage connections. It was eventually replaced by the very rare ES-II-inspired STR-7800SD and its more common STR-6800SD sibling.

The model above it in the line, the STR-7065, added more power, a third speaker output, microphone mixing in place of the AUX input, and a supplementary FM signal meter — the rest of the recipe was the same. Despite the higher list prices across the range, Sony sold a lot of STR-7055s, which is why they still turn up. The period press treated the receiver line as a system-matching question above all: High Fidelity (February 1974, p.39) ran an advertisement arguing that a tuner and amplifier should be mated more carefully than a husband and wife, and that the STR-7065's tuner had the sensitivity to reach distant fringe signals while retaining the discrimination to reject what it shouldn't hear. The 7055 sits in that same family logic, and the same magazine's November 1974 test coverage (p.124) carried the STR-7055's measured spec block alongside its bigger sibling.

Collectibility & Value

There's no official collectibility rating, and the market is quiet. One example has sold for as little as $254.15 - undated - while European and Russian listings have appeared around €220 and 39,000 rubles respectively. Parts listings turn up routinely: a chassis-stays-and-screws set at $10, a back panel at $23.99, and LED lamp upgrade kits around $24 for anyone tired of replacing bulbs.

The known trouble spots are the usual ones for a receiver of this age. Owners report erratic behavior, noise, and power issues traced to aging electrolytic capacitors in the amplifier section, which commonly need recapping during restoration. Lamp replacement and control cleaning are routine, and the tuning meter and switches are prone to dimming and crackling over time. Service manuals are available, including one hosted on archive.org, which makes it a feasible project for a competent restorer. One long-term owner has run an STR-7055a since 1974 with nothing worse than two cleanings. Forum shorthand calls it an underrated receiver that sings with warmth and musicality.

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VINTAGE TECHNOLOGY ARCHIVE / 10-21-2026 / FEEDBACK