In the early to mid 1970's Quadraphonic sound was just coming to the forefront. Sansui was one of the leaders of the 4-channel movement with their QR line of quadraphonic receivers. Around 1974-1975 they introduced the QRX line which included this beauty, the Sansui QRX-7500. The QRX-7500 was the top of the line at the time in terms of power output at about 30 watts per channel (4 channels into four 8 ohm speakers).
The receiver features a built in CD-4 album decoder, SQ decoder (phase matrix), QS decoder (regular matrix), two phono inputs, and outputs for two sets of four speakers.
What's the difference between the QRX-7500 and QRX7500A? Well, it all has to do with the decoding.
The QRX-7500A was a "hybrid" unit as Sansui moved from the discrete component vario-matrix decoder circuits to their custom Vario-matrix IC's that were used in the QRX-7001/8001/9001. There were two distinct ICs. The 'decoder' IC and the 'control' IC which enhanced the separation and decoding ability. The QRX-7500A has the decoder IC but not the control IC. The later receivers had both.
In all there are three variations of decoders built into the Sansui QR/QRX series over their entire production period. The QRX-7500 utilizes the basic QS decoder as originally designed. It's sometimes called an "RM" or regular matrix decoder. The basic decoder is capable of 3-12db of inter-channel separation. The QR series receivers, the QRX-4500, QRX-5500 and QRX-6500 also have this early decoder design.
With the QRX-7500A Sansui introduced the partially developed vario-matrix decoder circuit containing two of the three custom ICs required for full Vario-Matrix decoding. The circuit performs similarly to the 'basic' decoder mentioned above because the missing 'control' IC is the biggest factor in increasing inter-channel separation and this design doesn't have it. This decoder is capable of 3-12db of inter-channel separation. So, the QRX-7500A has the newer IC designed circuit but lacks the full Vario-Matrix capability of the later design. The Sansui QRX-5500A and QRX-5001 have the same design.
The fully developed Vario-Matrix Type A design with all three IC's was used in the QRX-6001, QRX-7001, QRX-8001, QRX-9001, QRX-777 and QRX-999. The QSD-1/2/1000 decoders also have the Type A decoder. The Vario-Matrix Type A decoder was limited by design by Sansui to 20db of inter-channel separation, though, in theory, Sansui claims that it is capable of infinite inter-channel separation.
The slightly later QRX-7001 is the improved version of the QRX-7500 with the better decoder circuit as well as a beefier amp section. The QRX-7001 produced 35 watts per channel in Quad mode.
The Sansui QRX-7500 is a great quad receiver. It has its limitations given the early decoder design and some issues with solder joints on the circuit boards but overall it is a good performing unit. The QRX-7500A may be a better choice since it has the IC decoder design but if you really want a great decoder then you'll want the QRX-7001, QRX-8001 or QRX-9001. Expect to pay a premium for those however. Any of these units would most likely need a restoration and perhaps some modification which isn't cheap. Once done you would have an exceptional performing receiver though. For a full restoration check out QRXRestore.
QRX-7500's sell for around $300+ while the A version sells for around $400+. The QRX-7001 is around $500 and the QRX-9001 around $800-$1000.