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.

Pioneer stated the C-21 was mainly assembled by hand using the finest grade circuit parts. These included Nichrome-vaporized, metal-coated resistors, metallized polypropylene capacitors, metallized polyester film capacitors, super-low noise electrolytic capacitors, and a large power supply transformer having an oriented core with separate secondary windings for the left and right channels with additional shielding to further reduce crosstalk. Even the contact points inside each gain pot were silver palladium treated and resistors were palladium and silver alloy. The total number of semiconductors included 36 transistors and 47 diodes. The quality of the electronic parts that went into the C-21 made for a wonderfully transparent and accurate preamplifier.

Circuit system:

Equalizer-amplifier Circuit System: 2 steps of differentials current mirror amplification Class-A SEPP

Control-amplifier Circuit System: 2 steps of differentials current mirror amplification Class-A SEPP

Input terminal:

Phono: 2.5mV/100 ohm, 10kohm, 25kohm, 50kohm, 75kohm, 100kohm

Cartridge load capacity: 100pF, 150pF, 200pF, 300pF, 400pF, 500pF

AUX1, 2, Tape Play:150mV /, 50kohm

Phono maximum permissible input: 300mV (1kHz, THD 0.01%)

Output-terminal:

Tape Rec: 150mV / 1kohm

Output (RL: 50kohm): 1V/450 ohm (rate), 20V/150 ohm (MAX)

THD (20 Hz - 20 kHz):

Phono: 0.006% (at the time of 1V output Volume -20 dB, gain-control 0 dB)

AUX: 0.005% (at the time of 1V output gain-control 0 dB)

Frequency :

Phono: 20 Hz-20 kHz ±0.2 dB

AUX: 10 Hz-100 kHz +0 -0.2 dB, 3 Hz - 300 kHz +0 -1.0 dB

SN ratio (IHF, A network) Short circuit:

Phono: 80 dB (output 1 V:00)

AUX: 100 dB (output 1 V:00)

Phono Subsonic Filter: 15 Hz, 6 dB / oct

Semiconductor: Transistor: 36 pieces, Diode, other: 47 piece

Power supply: 100V - 220V, 50Hz/60Hz

Power consumption: 10W

Dimensions (W x H x D): 420 x 81 x 357mm

Weight: 6,3 kg