Sony STR-6550 (1971)

The STR-6550 weighs 8.5kg. It is a precision piece from Sony's solid-state era, not a casual receiver.

Overview

The amplifier section, built around silicon transistors developed specifically for audio applications, aims to reduce the kind of distortion that clouds detail and muddies transients. While rated output is modest by modern standards—25W per channel under IHF conditions into 8Ω—the damping factor of 25 suggests a level of control over speaker cones that many contemporaries couldn’t match. It’s not brute force; it’s precision.

The tuner is the best part of the STR-6550. FM signal-to-noise ratio is 68dB and capture ratio is just 1.5dB, so it was built for fringe-area reception and pulls in marginal stereo stations cleanly. FM selectivity is 70dB (IHF) and image rejection is 55dB, which points to a well-shielded, tightly tuned front end. Sony replaced the traditional LC filters with proprietary solid-state filters in both the FM and AM sections. That cut drift and component aging and kept performance consistent. A novel gain allocation strategy also mutes the tuner while you sweep, which gets rid of the harsh scraping noise you get between stations on lesser tuners.

The receiver is 434mm wide, 345mm deep and 144mm tall, in a metal chassis that weighs 8.5kg. The mass helps with stability and shielding. The front panel has a speaker selector switch for toggling between two sets of speakers. The loudness contour, active at -30dB, applies a +6dB boost at 50Hz and +4dB at 10kHz to make up for the ear's reduced sensitivity at low volumes. A high-filter switch rolls off everything above 5kHz at 6dB/octave, which helps with noise from worn tapes or weak broadcasts. The preamplifier and power amplifier sections can be separated through dedicated input and output jacks, which allows a later upgrade to a dual-mono or bi-amped setup. That was a rare thing to find in a 1971 receiver.

Specifications

Key Features

MOS-Type FET Front End in Tuner Section

The front end of the tuner uses a single MOS-type FET. Unlike bipolar transistors, FETs have high input impedance and low noise, so they amplify weak RF signals without hurting the signal-to-noise ratio. That choice helps the receiver reach 1.8 μV sensitivity under IHF standards, and distant FM stations come in with stereo separation exceeding 35dB at 1kHz.

Sony Proprietary Solid-State Filters

Swapping the conventional LC filters for solid-state equivalents in both the FM and AM sections was a significant departure for the time. These filters are more stable over temperature and time, so they need less alignment and drift less. The receiver holds its selectivity (70dB IHF) and image rejection (55dB) without the upkeep of coil-and-capacitor networks. It also supports the clean muting during tuning, since the gain allocation can be controlled more precisely without the resonant peaks and phase shifts of LC designs.

Quasi Complementary Service SEPP OTL Circuit

The amplifier is a quasi-complementary SEPP (Single-Ended Push-Pull) OTL (Output TransformerLess) design, so there are no output transformers adding coloration. It uses silicon transistors developed specifically for audio. Harmonic distortion is 0.8% or less at rated output, and cross-modulation distortion is low too. The damping factor is 25 into 8Ω, which gives tight control of the speaker drivers, most noticeably in the bass.

Modular Preamplifier and Power Amplifier Sections

You can bypass the internal preamp and use the power amplifier on its own. The preamplifier output (0.5V / 5kΩ) and power amplifier input (0.5V / 100kΩ) jacks let owners run the STR-6550 in a multi-amplifier system, drive different speaker pairs with separate amps, or move to a higher-end preamp and keep the 54W dynamic output. That flexibility was rare in integrated receivers of the era.

Dynamic Muting and Gain Allocation

Tuning between stations on most receivers of the early 1970s produced loud, unpleasant bursts of noise. The STR-6550 avoids this with a “new concept of gain allocation” that adjusts the amplification stages during tuning and mutes smoothly. It isn't a simple switch. Gain is reduced across multiple stages, so the silence between stations comes without abrupt cutoffs or residual pops.

High-Filter Switch for Noise Reduction

The high-filter switch rolls off frequencies above 5kHz at 6dB per octave. It deals with high-frequency noise from aging tapes, weak FM signals or AM broadcasts. Unlike crude treble cuts, it keeps midrange clarity while removing hiss and interference. It works well alongside the loudness switch, which boosts bass and treble at low listening levels. Using both at once gives a balanced sound for late-night listening.

Collectibility & Value

Current market listings on Bunjang indicate a value of approximately $350 - $355 for the Sony STR-6550, suggesting steady demand among collectors of vintage Japanese audio gear. No official data on common failures or maintenance requirements is available. It is solid-state with no vacuum tubes, so long-term reliability is likely high, assuming the capacitors have been refreshed.

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