Having worked on Kenwood’s top of line L07C and L08C preamps, both of which excelled in their performance, particularly the L08C, I decided to see what would be possible with their smaller relation the Kenwood Basic C2.
The C2 is well built with a solid front panel and has the usual selection of controls, including both MC and MM phono preamps. The tone controls can be taken out of circuit and have selectable center frequencies for both bass and treble. There is comprehensive tape monitoring functions which was typical for preamps of this vintage (mid 1980’s). There is a single main output.
The preamp consists of 5 main PCB’s, the main PCB, headphone amp board, control board, tone board and balance/loudness board.
One of the main distinguishing features of the L08C is the power supply, with multiple supplies each with their reservoir capacitor bank. The C2 power supply has a similar approach, though with less capacitors in parallel. The phono stage and flat amp stage both have their own individual power supply comprising a FET/BJT voltage regulator and a zener diode, delivering +/-18.6v. Originally the zeners were typical 5-10% 18v types, however these were replaced with 2% 18v types, giving a consistent 17.76v, which is a better match for modern IC’s that use 18v rails. The tone and control boards are connected to the main power supply directly.
The phono stage is built around an IC, with two FET’s and 2 BJT’s and the input is direct coupled, with the RIAA equalization i the feedback loop. Originally the IC was an NJM 4560D, though this was replaced by a socketed LM4562. There is a signal coupling cap which was replaced with a 10uf MUSE UES bipolar type. The power supply for the phono stage had C65..C85 which were 100uf replaced with low impedance 220uf and C63/C64 were replaced with 470uf 50v low impedance. The zeners were replaced with 2% 18v.
The flat amp includes a pair of dual FET’S which feed an IC, which was an NJM4021D which was replaced with a LM4562 in a socket. The input comes from the volume and balance control. These is a 47uf capacitor in the signal path and this was replaced with a MUSE UES Bipolar of the same value. The 560pf and 330pf ceramic were replaced with polypropylene film. As with the phono stage power supply all capacitors were replaced with low impedance types. The flat amp is supplied from the main power supply, which uses the same combination of FET, Zener diodes and BJT transistors for a regulated output. The main power supply capacitors (C77/78) were upgraded from 1000uf to 2200uf, C73/C74 were replaced by 220uf and C71/C72 by 470uf all 50v rating and all low impedance and the zeners were replaced with 2% 18v versions. The rectifier diodes were replaced with ultrafast types (UF4007)
There is a further power supply for the relay providing 12v for the rlay drive and power to the power and output LED’s. All the electrolytic capacitors were replaced with low impedance types of the same value. The relay was replaced with an OMRON signal relay.
The control PCB includes the mute circuitry and the tone amp section, which used NJM 4560, which was replaced by LM4562. The signal caps, C9..C12 were originally electrolytic and these were replaced with film capacitors, though space dictated the choice. The output signal capacitors were replaced with MUSE UES Bipolar of the same 47uf value. The mute electrolytic was replaced with a low impedance cap of the same value. The 10pf feedback capacitors on the IC were replaced with C0G versions of the same value.
The filter PCB also used a NJM4560 which was replaced by an LM4562, the feedback capacitors of which (33pf) were replaced by WIMA FKP polypropylene of the same value. The 1uf input signal capacitors were replaced with 1uf polypropylene and the 33uf output signal capacitors were replaced with 47uf MUSE UES Bipolar. C35/C36 were also replaced with WIMA FKP types.
The headphone amp uses an M5218L IC, which was left in place, the 10uf electrolytic caps were replaced with Nichicon FG of the same value and all the ceramic were replaced with WIMA FKP film types of the same value.
The results were very satisfying with the preamp measuring very well, with a 3db improvement in signal to noise and very low THD and THD+N. Sonically this preamp is very smooth with good detail, not in the class of the L08C or L07C, but not that far away either.