Pioneer QX-8000 A
Four channels, 34 pounds of it, and a manual that tells you which of the three versions you actually own.
Overview
Read the back panel of a Pioneer QX-8000 A before you buy it. Pioneer built it in three versions, and the manual is blunt about the differences. The KUW model runs on 120V only and carries UL approval for the U.S. market. The FSAW model has a 5-position voltage selector and carries SABS approval for South Africa. The FW model is the general export unit, also with a 5-position selector, but it adds a de-emphasis switch. That switch toggles between 75µs and 50µs, which is the difference between correct FM tonal balance in North America and in Europe. Buyers who end up with the wrong version for their region will hear it in the high frequencies, and there is no workaround short of outboard correction.
It weighs 34 lb 5 oz without packaging. The chassis measures 20 inches wide, 6-3/4 inches tall, and 15-5/8 inches deep, so it is a full-size receiver and needs a proper shelf rather than a cramped cabinet. The manual's own table of contents lists the wooden case as a serviceable assembly. The cabinet comes off, the bottom plate comes off, and the internal units are laid out with alignment and repair in mind.
Functionally, the QX-8000 A is a four-channel receiver, and the power amplifier section is built around quadraphonic operation. Pioneer rates it at 180W music power into 4Ω and 130W into 8Ω on the IHF standard, but the continuous ratings tell the more honest story. With all four channels driven at 1kHz into 4Ω, each channel delivers 43W. Into 8Ω, that drops to 30W per channel across four channels. With only two channels driven, the stereo case, the numbers improve to 36W + 36W per side into 4Ω and 27W + 27W per side into 8Ω. With two channels driven across the full 20Hz to 20kHz band at 8Ω with harmonic distortion under 1%, Pioneer rates it at 21W + 21W per side.
Those figures are not impressive next to Pioneer's own two-channel receivers from the same period. Quadraphonic designs of this generation spread their power supply across four channels rather than two. The result is a receiver that will drive efficient speakers comfortably but will struggle with low-impedance, low-sensitivity loads, particularly when all four channels are active. Owners report that the 4Ω continuous rating of 43W per channel is the number to pay attention to, because it reflects what the amplifier can sustain rather than what it can momentarily peak at.
The distortion numbers are more interesting than the power numbers. At full continuous power output, Pioneer specifies harmonic distortion at less than 1% and intermodulation distortion at less than 1%. At 14W + 14W/14W + 14W into 8Ω, roughly half the continuous rating, harmonic distortion drops below 0.05% and IM distortion below 0.1%. The amplifier is happiest when it is not pushed to its limits. The power bandwidth is specified as 10Hz to 100kHz at 8Ω with harmonic distortion under 1%, and the frequency response is quoted as 5Hz to 100kHz, ±3dB. Those are wide figures, though the ±3dB tolerance is generous compared to what a modern spec sheet would claim.
The preamplifier section shows its age. Output voltage is rated at 400mV with a maximum of 2.5V, and harmonic distortion through the preamp is less than 0.5%, noticeably higher than the power amp section's best-case figure. The frequency response through the preamp is 10Hz to 40kHz, ±3dB, which is narrower than the power amp's quoted range. Input sensitivities are 8mV at 120kΩ for the microphone input, 170mV at 120kΩ for AUX 1 and 2, and 170mV at 130kΩ for the two tape monitor inputs. Recording output on both tape record jacks is 170mV.
The tone controls have a wide range. Bass adjustment spans -12dB to +11dB at 100Hz, and treble spans -8.5dB to +10.5dB at 10kHz. The low filter rolls off at -6.5dB per 50Hz with a 6dB/octave slope, and the high filter cuts -11.5dB at 10kHz, also at 6dB/octave. The loudness contour adds +11.5dB at 100Hz and +7dB at 10kHz when the volume control is set at the -40dB position. Hum and noise figures are strong for the era: more than 75dB on phono with the IHF short-circuit A-weighted standard, and more than 90dB on AUX.
The tuner sections hold up better than the power ratings might suggest. The FM front end uses a field-effect transistor at Q1 for RF amplification, with a bridge circuit that balances feedback capacitance against a neutralizing capacitor to keep the stage stable. Frequency conversion is handled by Q3, and the local oscillator is a Colpitts type built around Q2. The IF strip runs four stages - Q13, Q4, Q5, and Q6 - with ratio detection on the secondary of T7. The multiplex decoder extracts the 19kHz pilot through T14 and generates the 38kHz subcarrier using D11, D12, Q11, and T15, feeding switching diodes D13 through D16. Q12 drives the FM stereo indicator lamp, and Q9 handles both muting and the FM AUTO switching function. When the muting switch is on, Q9's collector connects to Q10's base. When it is off, it connects to Q11's base. With a weak antenna signal, Q9 conducts and pulls down the base potential of Q10 or Q11, blocking the signal. That is why a marginal FM signal can go completely silent rather than fading to noise.
The AM section is simpler: four transistors - Q7, Q8, Q5, and Q6. Q7 and Q8 handle conversion to the 455kHz intermediate frequency, and Q5 and Q6 provide IF amplification. D10 on the secondary of T11 handles detection, and AGC is applied from the detected signal back to Q8's base. D7 and D8 protect against excessive input. The AM tuner uses a built-in ferrite loopstick antenna, so no external AM aerial is required for local reception.
FM performance figures are respectable for the period. Usable sensitivity is 2.2µV on the IHF standard, capture ratio 3dB, selectivity better than 40dB, image rejection better than 50dB at 98MHz, IF rejection better than 85dB at 90MHz, and spurious rejection better than 80dB at 98MHz. AM suppression is 50dB and signal-to-noise ratio is 60dB. Harmonic distortion is less than 0.6% in mono and less than 0.8% in stereo at 100% modulation. Stereo separation is better than 40dB at 1kHz, and subcarrier suppression is better than 32dB. The antenna input accepts both 300Ω balanced and 75Ω unbalanced connections.
The AM tuner is less impressive, as AM tuners of this era generally were. Usable sensitivity is 15µV, selectivity better than 25dB, image rejection better than 45dB at 1,000kHz, IF rejection better than 30dB, and signal-to-noise ratio better than 50dB. That is adequate for local AM reception but not for long-distance DX work.
The control amplifier uses NPN and PNP transistors in a direct-coupled configuration with a single-transistor negative feedback tone control stage. Pioneer applied a large amount of negative feedback in the direct-coupled amplifier to maintain stability and high input impedance, and the tone controls work by varying the degree of feedback through a potentiometer. The main amplifier is a quasi-complementary circuit, and the +B fuse protects the power transistors. A blown output stage may take the fuse with it rather than destroying the transistors outright, which also means the fuse is a consumable worth keeping on hand.
The QX-8000 A draws 250W maximum from the wall and provides one switched and one unswitched AC outlet on the back panel. The unswitched outlet suits a turntable or other device that needs power whether or not the receiver is on. The unit ships with an FM T-type antenna, four pin plugs, four speaker plugs, a polishing cloth, and the operating instructions.
The practical concerns for a QX-8000 A today are the same as for any quadraphonic receiver of this vintage. Verify the voltage configuration and de-emphasis setting before you buy. A KUW model in Europe or an FW model in North America will need external voltage conversion or correction. The 34 lb 5 oz weight means shipping costs will be significant, and the wooden case is vulnerable to damage in transit. The modest continuous power output means speaker matching matters more than it would with a higher-powered two-channel receiver. The quadraphonic format has limited software support compared to stereo, so many of these receivers end up used in two-channel mode, where their power output is more competitive.
The manual's alignment procedure for the MPX decoder is detailed and requires an FM signal generator, an FM MPX modulator, an oscilloscope, and a VTVM. You adjust T13, T14, and T15 for maximum 19kHz output, then adjust a semi-fixed potentiometer on the CR unit for maximum L or R signal output at the tape record jacks. This is not a job for a hobbyist without proper test equipment. It becomes necessary when the stereo decoder drifts, which in a receiver of this age it eventually will.
The QX-8000 A is a full-size receiver built around four channels. It is not the most powerful receiver Pioneer ever made, and it is not the simplest to maintain. It is a genuine four-channel design with a tuner section that still holds up, rather than a two-channel receiver later adapted for four-channel operation.
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