Center of the Series 20 and a japanese favorite of many a budding audiophile who either couldn't afford an Exclusive C3 or fancied the no-frills design.
Inside is however plenty of frills :
Class A SEPP two-stage current mirror differential circuits for both phono and flat amp sections, ultra parts with ±1% tolerance polyproylene caps and ±100ppm /C° metal film resistors, ±37V phono stage power supply and two constant voltage main power supplies.
Even more important is the volume attenuator which 2 gangs of 32 steps made of silver printed contacts - same as that of a Sony TA-F7B. Both were sourced from ALPS.
Neither are ZP-1 ALPS as those of the Sony TA-E88B, but the latter cost three times as much as a C-21...
The gain/balance controls are also stepped attenuators with 22 contacts made of silver palladium quite probably sourced from ALPS as well.
Reviewers of the Audio Accessory japanese magazine very often used the C-21 as center of their test rigs well into the 1980s.