Pioneer introduced a line of true audiophile components in the late 1970s known as Series 20. These components used the latest engineering and displayed the finest quality parts Pioneer had ever manufactured. The components included the C-21 Preamplifier, the M-22 and M-25 Amplifier, the D-23 Crossover, the F-26 and F-28 tuner, the A-27 Integrated Amplifier, the PLC-590 Turntable and PA-1000 Tonearm. There was also a U-24 Program Selector used primarily by Pioneer dealers. All but the turntable and tonearm are covered in this section.
Pioneer C-21
Pioneer designed the C-21 Preamplifier to work in conjunction with the M-22 or the M-25 amplifier in the Series 20 line. The control layout was simple and uncluttered. Front left to right, they were Power, Gain (left and right), Tape monitor, Volume, Cartridge Load (adjustable for resistance and capacitance), and selector knob for Phono, Subsonic Phono, Aux1 and Aux 2.Specifications were impressive, to say the least. Frequency response in the Phono section was 20 to 20,000 Hz +/- 0.2dB; it was 10 to 100,000 Hz +0 dB, -0.2 dB on Aux1 and 3 to 300,000 Hz +0 dB, -1.0dB on Aux2 ! Total harmonic distortion was a mere 0.01%. This was most definitely in the audiophile arena of performance.
Pioneer M-22
The M-22 was a pure Class A amplifier that was rated at 30 watts per channel. Pioneer stated in its Series 20 brochure: "Class-A circuits are used throughout to reduce distortion. If operated in Class-B the M-22 would deliver no less than 150 watts per channel--five times as much--with so-so distortion. The most sophisticated circuit construction yet developed by Series Twenty is reflected here, along with truly first-class parts and components to asssure you're getting the real advantages of such design."
And this is what Pioneer had to say specifically about the M-22's performance: "Not to be overlooked is the extremely wide power bandwidth offered in the M-22: 5Hz to 100kHz, both channels driven, 0.01% THD. Note also that the frequency response is an amazing 2Hz to 150kHz, + 0dB/1 dB at 1 watt, and that the signal-to-noise ratio is a high 106 dB (IHF short-circuited A network." At its full power output of 30 watts into 8 ohms, this amp had no more than 0.01% THD between 10 Hz and 30kHz! Class A indeed!
Pioneer D-23
The D-23 was a multi-amp electronic crossover network to permit bi-amping or tri-amping. The crossover frequencies were divided into low, mid-low, mid-high and high. The range of the crossover frequencies could be varied within these four groups with the upper row of control knobs. The four larger knobs in the center were the level controls. The six smaller knobs at the bottom of the panel were to adjust the slope.
The D-23 had impressive specifications, like all the Series 20 components. Its total harmonic distortion was a mere 0.005% between 20 Hz and 20 kHz. The signal to noise ratio was 100 dB IHF, A-rated.
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Pioneer U-24 Program Selector
When Pioneer designed the Series 20 line, the C-21 pre-amp had one input for phone and tape, plus two auxiliary inputs. Pioneer saw the need for a program source selector for those audiophiles--or Pioneer dealers--that required more inputs for multiple sources. The U-24 Program Selector was the result.
The U-24 had two AUX inputs, three phono inputs, four tape inputs and outputs different tape decks (cassette or reel to reel) with tape to tape dubbing facilities, and three power amplifier outputs. It measured 16.53 inches wide, 3.18 inches high and 12.97 inches deep.
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Pioneer M-25
The M-25 was rated at 120 watts continuous, per channel with both channels driven into 8 ohms from 5 Hz to 30,000 Hz, with no more than 0.01% total harmonic distortion. In fact, this amp could reproduce ultrasonic frequencies up to 80 kHz with only 0.05% THD! Why have such capability beyond the range of human hearing? Pioneer stated: "It is essential to the high quality of signals within the hearable range that those beyond that range are handled cleanly in order to avoid harmonic distortion and other harful irregularities. And this the M-25 does with impressive accuracy, at full power."Pioneer M-25To do this, Pioneer employed newly-developed Ring Emitter Transistors (RETs) instead of Field Effect Transistors (FETs). The RETs had better high frequency characteristics. It employed all the high-grade semiconductors as those used in the M-22, and the same 22,000 microfarad power capacitors. It naturally had higher output power supplies.
The F-26 tuner, like the other components in the Series 20 line, received carefully design engineering to push the technological envelope on tuner design. In true audiophile fashion, it featured FM only. It was a higher performance, lower profile design compared to the F28 tuner. Pioneer employed newfeatures and circuitry having: Quartz-Locked Touch Sensor Tuning, an elaborate "Clean Pilot" system for cleaner high-frequency, and a Parallel Balanced Linear Detector( PBLD) for vastly improved signal-to-noise ratio and detection efficiency.
This tuner also featured automatic wide/narrow IF bandwidth slection. The F-26 was design with a large power supply and two 6000 microfarad power supply capacitors, a muting level control on the rear panel, adjustable stereo/mono signal switchover, adjustable muting control, and gold plated terminals.
Pioneer A-27
The A-27 Integrated Amplifier was the most expensive component in the Series 20 line. Pioneer clearly intended to incorporate much of the circuit design from the M-type amplifiers, but have a greater degree of control than the C-21 pre-amp. It used the "Magni-Wide" DC power amplifier section, but also used a total of three other all-DC-configured amplifier stages: a DC phono equalizer amp, a DC flat amp and DC "head amp" for Moving Coil (MC) type phono cartridges. Like the M-25, the A-27 used RETs in the output stage. It also used automatic Class AB switching, operating in Class A mode until the sudden audio peaks automatically switched to Class B.
The A-27 used two large power supply transformers, one for each channel, positioned on the left of the chassis, and four electrolytic capacitors. The circuitry employed 206 semi-conductors: four FETs, 122 transistors and 80 diodes.
The A-27 was rated at 120 watts per channel into 8 ohms, from 5 hz to 30 kHz with no more than 0.012% total harmonic distortion. Between 20 Hz and 20 kHz, toal harmonic distortion was a mere 0.008%--virtually non-existant distortion! Intermodulation distortion was only 0.006%. The A-27 entire frequency response ranged from 5 Hz to 200 kHz.
Pioneer F-28
The other tuner in the Series 20 line was the F-28. It differed in appearance and price (lower) from the F-26, but was still a superb performer. It employed Pioneer's Photo Logic Synthesizer/Quartz Locked system and "Clean Pilot" circuitry. It also used the Parallel Balanced Linear Detector and featured selectable Wide/Narrow IF Bandwidth. The circuitry used a 5-gang variable capacitor for higher tuning accuracy to improve image response, spurious response and IF response ratios, all having 120 dB or better.Its signal to noise ratio in mono was 84 dB and in stereo 81 dB.The F-28 employed 10 FETs, 21 ICs, 63 transistors and 56 Diodes. It measured 16.56 inches wide, 6.12 inches high and 14.21 inches deep. It weighed 19 pounds, 13 ounces and had an M.S. R. P. of $690.00.
