Sony STR-7055 (1972–1976)
At 15.3 kg, it lands in the rack like a declaration—this is not your living room’s background player, but a full-strength, no-compromise receiver built to anchor a serious system.
Overview
The Sony STR-7055 isn’t just another receiver from the early '70s—it’s a statement piece disguised as a workhorse. Marketed as a full-fledged FM/AM stereo receiver developed on the same design philosophy as Sony’s elite ES Series, it was engineered to function as a preamplifier and power amplifier in one, blurring the line between integrated and separates. Released in 1973 and produced through at least 1976, it carried a premium price of ¥79,800 in Japan, or 3,295 French Francs in European markets, positioning it as a high-end contender in Sony’s lineup. At a time when many manufacturers were still transitioning from tube to solid-state designs, the STR-7055 arrived fully committed: all-silicon transistor construction, DC-coupled amplifier stages, and a quasi-complementary symmetry output stage that delivered clean, stable power without the bulk of traditional output transformers.
Owners report it was built for audiophiles who refused to compromise—featuring a robust 100 VAC power supply, dual power rails in the amplifier section, and a preamp stage utilizing specially developed high-voltage, low-noise ICs. The front panel layout is dense, almost intimidating, with dedicated switches for tape monitoring, filter routing, speaker selection, and a center-zero tuning meter that gives it the look of test equipment. This isn’t a receiver designed for casual listening; it’s for those who wanted control, precision, and the ability to integrate multiple sources—including two tape decks with full tape-to-tape copying capability—without lifting a finger.
Specifications
| Manufacturer | Sony |
| Model | STR-7055 |
| Type | FM/AM stereo receiver |
| Year of Production | 1972–1976 |
| FM Receiving Frequency | 76 MHz to 90 MHz |
| FM Sensitivity | 2 μV (IHF); 1.6 μV (S/N: 30 dB) |
| FM Stereo Separation | 38 dB or more (400 Hz) |
| AM Receiving Frequency | 530 kHz to 1605 kHz |
| Effective Output (20 Hz – 20 kHz, both channels) | 35W + 35W (8 Ω) |
| Harmonic Distortion Factor | 0.2% or less (rated output) |
| Frequency Response (at 1W output) | 10 Hz to 100 kHz, +0 -1 dB |
| Phono Input Sensitivity / Impedance | 2 mV / 50 kΩ |
| Aux/Tape Input Sensitivity / Impedance | 150 mV / 50 kΩ |
| Tone Control Range | Bass: ±10 dB at 100 Hz; Treble: ±10 dB at 10 kHz |
| Filter Response | Low Filter: 50 Hz, 12 dB/oct; High Filter: 9 kHz, 12 dB/oct |
| Power Supply | 100 VAC, 50/60 Hz |
| Power Consumption | 120 W |
| Dimensions (W×H×D) | 471 × 157 × 375 mm |
| Weight | 15.3 kg |
Key Features
FET-Charged Front End with Linear Varicon Tuning
The FM tuner’s high-frequency amplifier circuit uses a newly developed junction FET paired with a high-performance linear frequency varicon, a design choice that minimizes noise and drift while maximizing stability. This isn’t just marketing fluff—owners note the tuning feels unusually smooth and precise, especially when compared to contemporary receivers using standard variable capacitors. The FET-based front end contributes directly to the receiver’s excellent sensitivity of 2 μV (IHF), allowing it to pull in weak stations with clarity. Combined with a stable local oscillator and FET mixer circuit, the front end sets a high bar for signal integrity before the signal even reaches the IF stage.
Two-Element Solid-State IF Filter with IC Integration
In the intermediate frequency (IF) section, Sony implemented a two-element solid-state filter backed by IC processing—a relatively advanced approach for 1973. This design improves selectivity (70 dB IHF) and image rejection (70 dB), reducing interference from adjacent and out-of-band signals. The use of ICs here also ensures consistent performance over time, avoiding the alignment drift common in coil-based IFTs. The AM section takes this further with a triple-tuned IFT and high-integration IC solid-state filter, enhancing AM reception clarity in an era when many receivers treated AM as an afterthought.
MPX Stereo Decoder with Maintenance-Free Coil Unit
The MPX stereo separation circuit relies on a newly developed IC and a coil unit specifically designed to resist degradation from heat, moisture, and aging. Unlike many receivers of the period that required periodic adjustment of the MPX coil, the STR-7055’s unit is sealed and non-adjustable, a deliberate move to ensure long-term reliability. Separation is rated at 38 dB or more at 400 Hz, which was competitive for the time, and users report stable stereo imaging even on marginal signals—thanks in part to the built-in high-blend circuit that links with the High and Both filter switches to improve weak-signal reception.
Dual Power Supply with DC-Coupled Complementary Amplifier
The power amplifier section is where the STR-7055 truly separates itself. It uses a differential amplifier input stage, dual independent power supplies, and a pure complimentary symmetry (SEPP) output configuration—all directly coupled with no interstage capacitors. This DC amplifier design eliminates phase shifts and transient distortions associated with coupling caps, contributing to the receiver’s flat frequency response from 10 Hz to 100 kHz (±1 dB at 1W). The effective output of 35W + 35W into 8 Ω across the full 20 Hz–20 kHz bandwidth (at 0.2% THD) is modest by today’s standards, but in 1973, it represented serious fidelity-focused power. The damping factor of 50 or more (at 1 kHz, 8 Ω) ensures tight control over speakers, especially important with the era’s less efficient designs.
High-Voltage Low-Noise ICs in Preamplifier Section
Sony’s in-house semiconductor team developed specialized high-voltage, low-noise ICs for use in the preamp, equalizer, and flat amplifier stages. These ICs—combined with a differential amplifier and NFB-type tone control circuit—are credited with the receiver’s excellent linearity and low distortion. The tone controls themselves are designed to preserve midrange naturalness, a common pitfall in receivers that over-emphasize bass and treble at the expense of vocal clarity. The loudness contour switch adds +10 dB at 50 Hz and +4 dB at 10 kHz, making it effective at low volumes without becoming boomy or harsh.
Three-Speaker System with Five-Way Output Switching
The receiver includes a speaker change-over switch that supports a Route 3 speaker system, allowing selection between five configurations: Speaker A, B, or C alone, or combinations A+B and A+C. This level of flexibility was rare in integrated receivers and suggests Sony anticipated use in larger listening environments or multi-room setups. The inclusion of a headphone output (rated for 8 Ω to 10 kΩ loads) and a speaker protection circuit—complete with a dedicated circuit breaker—further underscores its serious engineering pedigree.
Two Tape Decks with Full Tape-to-Tape Copying
With two full tape input/output pairs (Tape 1 and Tape 2), the STR-7055 allows two tape decks to be connected simultaneously. The tape monitor system is smartly implemented: Tape Monitor 1 takes precedence, allowing one-touch monitoring of Tape 1 even when Tape 2 is selected. More impressively, it supports direct tape-to-tape copying between decks via internal switching—no need to re-patch cables. This feature, combined with Rec Out and REC/PB input/output sections, made it a favorite among users who recorded from radio broadcasts or LPs.
Historical Context
The STR-7055 occupied a near-flagship position in Sony’s 1970s receiver lineup, often described as the spiritual successor to the STR-6120 and STR-6200F from the previous decade. According to collector consensus, it was Sony’s top-tier model during its production run, though it was eventually succeeded by the rare ES-II-inspired STR-7800SD and its more accessible sibling, the STR-6800SD. It shared its core design language with the ES Series, particularly in its use of DC amplification, dual power supplies, and modular IC-based signal processing. While not officially branded as ES, it carried the same engineering rigor—making it a "premain" in both concept and execution.
Collectibility & Value
Market examples have sold for as low as $254.15, though condition and region of sale heavily influence price. There is no official collectibility rating in the documentation, but its reputation among vintage audio enthusiasts is strong—described in forums as “one under rated receiver” and “sings a mouthful of warmth and musicality.” Failures reported by owners include erratic behavior, noise generation, and power issues—often traced to aging electrolytic capacitors in the amplifier section, which commonly require recapping during restoration. Lamp replacement and control cleaning are routine maintenance steps, as the tuning meter and switches are prone to dimming and crackling over time. Despite its complexity, the availability of service manuals (including one hosted on archive.org) makes it a feasible project for skilled restorers.
eBay Listings
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.
Related Models
- Sony STR 6065 (1971)
- Kenwood KR 3200 (1973-1975)
- Kenwood KR 3400 (1977-1979)
- Kenwood KR 4200 (1975-1977)
- Kenwood KR 5150 (1977-1979)
- Marantz 1515 (1976-1978)
- Marantz 4220 (1974-1976)
- Marantz 4230 (1974-1976)
- Marantz HD 440 (1976-1977)
- McIntosh MX112 (1971-1975)