Sansui QRX-6500
A quadraphonic receiver built around its decoder, with a continuous power rating that people still argue about fifty years later.
Overview
Flip a QRX-6500 around and count the speaker terminals. Sansui built this receiver to drive up to three pairs of front speakers and two pairs of rear speakers - five pairs total, enough to run four-channel in one room and stereo in another off the same chassis. The people who designed it assumed you were actually going to use all four channels.
Sansui positioned it as the top 4-channel receiver of its line, and it was one of the company's early contributions to the quadraphonic genre. One circuit block does two jobs. It decodes encoded 4-channel program sources back into four channels, and it synthesizes conventional 2-channel sources into four. The second job is the one collectors care about, because it means the QRX-6500 was built to handle a competing matrix format, not just Sansui's own QS system.
The power numbers need untangling first. Sansui's own manual describes a "4-channel, 280-watt integrated amplifier section," and 280W is the figure that ended up on the front of the brochure. That is music power, a peak figure, not a continuous one. The continuous rating is more modest and less consistent. Sources report roughly 33 to 37 watts per channel depending on which page of the documentation you trust: 33 watts per channel into 8 ohms with all four channels driven, and 37 watts per channel with each channel driven singly. Treat the 280W as the marketing figure and the low-to-mid 30s as the real continuous number. For a quad receiver of this vintage that is respectable. It is enough to drive most four-channel speaker systems of the period, and not enough to make inefficient modern towers sing.
The tuner section is not an afterthought. Tuning range covers FM and MW (AM), and the front panel carries two tuning meters and an extra-large tuning flywheel. Frequency response is quoted at 30Hz to 30kHz overall, with the phono stage tighter at 30Hz to 15kHz, ±1 dB. Total harmonic distortion figures conflict across sources — 0.5% in some documentation, an implausibly low <0.05% in others — so the honest read is that the 0.5% figure is the one to trust, and the lower number is likely a typo or a different measurement condition. Damping factor is listed at 10 at 8 ohms, which is low by the standards of the better stereo receivers of the era and worth knowing if you plan to drive speakers with difficult impedance curves.
Sansui's advertising for this receiver leaned hard on the Vario-Matrix claim. Period advertisements in Audio (May 1973, p.19) described the circuit as a technological extension of the QS Regular Matrix that delivered front-to-back separation to a degree never before possible with matrix technology. The ad copy claimed the separation was so pronounced that engineers had taken to calling it the "discrete matrix." The same language ran in High Fidelity (April 1973, p.81), which noted that the decoder included two synthesizer positions for creating four-channel sound from two-channel recordings, with both the synthesizer and the Regular Matrix decoder offering Hall and Surround settings to vary the effect. Owners have argued about whether the "discrete matrix" framing was fair for fifty years, but the underlying circuit was more sophisticated than most of what the competition was shipping.
Key Features
QS Vario-Matrix Decoder with Hall and Surround Modes
The QS Vario-Matrix decoder is a five-position rotary control. It selects between two QS Synthesizer modes (Surround and Hall), two QS Regular Matrix modes (Surround and Hall), and an additional Phase Matrix mode for decoding SQ records. That last position is the interesting one, because it means the QRX-6500 was built to handle a competing matrix format, not just Sansui's own. Owners who have spent time with both QS and SQ material report that the Vario-Matrix logic gives the QS side a real separation advantage, while the SQ position is functional but not the reason to buy the receiver. The Hall and Surround settings are not subtle. Hall is the more diffuse of the two, Surround the more aggressive, and the effect varies considerably depending on the source material.
Four-Channel Control Center with Independent Per-Channel Tone
The QRX-6500 is a control center in the fullest sense. Bass and treble responses of each of the four channels can be adjusted by individual controls on the front panel, so it is not a ganged two-channel tone stack pretending to be quad. There are individual front/rear high and low filters, individual loudness controls, separate speaker selection controls for front and rear, separate left/right balance controls for the front as well as the rear speaker systems, and a main front/rear balance control on top of all that. A seven-position Mode switch keeps the front sound up front, and a seven-position Selector handles PHONO 1, PHONO 2, FM AUTO, FM MONO, AM, AUX-2-CH, and DISCRETE 4-CH. Pushbutton tape monitor switches handle up to three tape decks, one 4-channel and two 2-channel. Two sets of two stereo headphone jacks are provided, one set for front or rear channels, plus inputs for tape recording and playback. It is a lot of switching.
Speaker Switching for Five Pairs
The QRX-6500 connects up to three pairs of speaker systems for the front channels and two pairs for the rear. Any pair may be driven independently or a combination of two pairs may be driven, as selected by the front and rear speaker switches. Each connected speaker system must have an impedance of from 4 to 16 ohms. If you want to drive two pairs simultaneously, they should all be 8 ohms or more. The rear-panel terminals use Sansui's one-touch push-button design: push the button, insert the lead, release. It is quick but not as secure as a binding post, and worth checking for corrosion on any unit that has spent time in a damp climate.
Semi-Complementary SEPP-OTL Output Stage
The power amplifier stage is the semi-complementary SEPP-OTL design, built around four TO-3 output transistor sockets. Sansui selected silicon transistors and advanced circuit design to keep total harmonic and intermodulation distortion below 0.5% and to achieve a power bandwidth of 20Hz to 30,000Hz. The TO-3 sockets are a serviceability advantage, since output transistors can be replaced without desoldering from the board. They are also a potential failure point on a fifty-year-old unit, and any technician servicing the receiver should check them for oxidation and poor contact.
Multi-Voltage Power Supply with Selector Plugs
The QRX-6500 was built for international sale, with voltage selector plugs that can be set for 100, 110, 117, 127, 220, 230, 240, or 250 volts. The fuse must be changed to match: a 5-ampere fuse for 100-127 volt operation, a 3-ampere fuse for 220-250 volts. A unit that has been moved between regions without the fuse being swapped may have been running on the wrong protection. This is the single most common piece of neglected maintenance on these receivers, and it is worth checking before powering up any example with an unknown history.
Historical Context
Production years for the QRX-6500 are not settled. Sources place it variously at 1973 until about 1975, 1972 to 1974, or 1972 to 1973. The conflict is unresolved, and the safest statement is that the receiver was built in the early 1970s during the first serious push of quadraphonic hardware into the home market. It was one of Sansui's early contributions to the quadraphonic genre, and it sat at the top of the company's 4-channel receiver line.
The QRX-5500, QRX-3500, and QRX-3000 appear in documentation alongside the QRX-6500 as related models, though no explicit hierarchy or shared-design relationship is documented. The QRX-6500 was the flagship of whatever that lineup was, and it retained a following while most of its quadraphonic contemporaries have been forgotten.
Collectibility & Value
Market prices for the QRX-6500 are all over the map. Examples have been observed at $150 on the low end, advertised at $800 and up on eBay for working units, and listed by specialist dealers at $1,199 for a well-preserved example with a one-year warranty. A "new price" of $622.33 has been recorded for a used unit in excellent condition. The spread reflects the usual variables: whether the unit has been serviced, whether all four channels work, and whether the cosmetics, such as a clean faceplate and cabinet wear consistent with age, are acceptable. Replacement front LED lamp kits sell for around $34.
These are not receivers to hand to a general-purpose electronics shop. The quadraphonic signal path, the Vario-Matrix logic, and the multi-voltage power supply all require a technician who understands the period and the format. Service manuals and Regular Matrix operating instructions are available, which helps, but the receiver is complex enough that a shop unfamiliar with quad Sansui gear is likely to do more harm than good. Units that have been "parted out" appear regularly on the secondary market, which is a source of spare parts.
Buying advice depends on what you intend to feed it. If you have a shelf of QS-encoded quadraphonic records and a pair of rear speakers doing nothing, the Vario-Matrix decoder is the whole point. If you are looking for a two-channel amplifier with a quad badge, you are paying for a lot of circuitry you will never engage, and a conventional stereo receiver of similar continuous power will serve you better for less money.
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