Kenwood L-07C

Name: Kenwood L-07C
Brand: Kenwood

A control amplifier with two separate phono front ends and an output stage built to drive a monoblock power amp. Kenwood's L-series separates were not designed to be mixed and matched.

Overview

Kenwood's manual is blunt about what the L-07C is for. It is a stereo control amplifier designed to be used with the model L-07M, a monaural DC power amplifier. The two were engineered together, and the manual recommends what Kenwood calls the Direct Drive Amplifier System - the L-07C with two L-07M monoblocks - to get the best out of both. That is a narrower mandate than most preamps of the period carried. This one has a partner, and it expects you to bring it.

The L-07C doesn't treat moving-magnet and moving-coil cartridges as the same problem. It carries two separate equalizer amplifiers: one with 2 mV input sensitivity for MM-type cartridges, and a second with 0.2 mV input sensitivity for MC-type cartridges. Kenwood's reasoning, laid out in the manual, is that running an MC cartridge into a head amp and then into an MM equalizer degrades the sound, because the output level of an MM cartridge is only a few millivolts to begin with. So instead of a head amp, the MC cartridge gets its own dedicated equalizer stage. The PHONO 1 and PHONO 2 pin jacks are gold-plated.

The MC stage is where the engineering gets into the weeds. Kenwood documents the noise problem in detail: FET-based MC stages suffer thermal noise from channel resistance, shot noise from drain-gate leakage current, and 1/f noise that rises as frequency falls. The countermeasures are specific. Thermal noise drops when transconductance rises, so Kenwood paralleled FETs. The manual notes that gm nearly doubles and thermal noise falls by a factor of the square root of two. Shot noise is handled with a cascode circuit that holds the drain-source voltage of the input FETs down to 8V despite a high supply rail, so leakage current stays low. The 1/f noise is addressed by choosing a low-noise FET, the 2SK68A. The supply runs at ±47V to keep dynamic range high. The cascode connection also widens bandwidth by reducing the effect of gate-drain capacitance. Kenwood notes the technique originated in triode preamps for ultra-shortwave receivers.

All of that feeds a buffer amplifier with one job: lowering output impedance. Kenwood's argument is that a conventional control amp uses a volume control at the output, which makes a low output impedance hard to achieve. The L-07C instead uses a 100% negative-feedback buffer as an impedance converter, and the manual claims this makes an ultra-low output impedance possible. The buffer's first stage is a differential amplifier built around the µPA-63H, a dual FET diffused on a single chip and chosen for stability against temperature change. A constant-current circuit improves common-mode rejection. The second stage is a class A amplifier, and the final stage is a SEPP output. Gain is 0 dB, because the entire output signal is fed back to the input stage. The manual notes that one of the feedback resistors, normally inserted between the gate of the input IC and ground, is eliminated here. The output signal is fed back from both before and behind a resistor (Rm261) to reduce that resistor's influence.

Volume control is a ladder-type attenuator rather than a conventional pot. The name comes from the layout: a group of pi-pads connected in tandem, like a ladder laid on its side. Input and output impedance stay constant across the entire attenuation range. That keeps the signal source impedance seen by the buffer amplifier constant, which keeps distortion low. The manual also notes that this arrangement is what makes the subsonic filter perform well. It has a cost: the ladder configuration has a fixed insertion loss of 6 dB. Kenwood sets the master volume so that ordinary listening lands around the −20 dB position, and the gain attenuator sits after the master volume specifically to reduce residual noise.

The subsonic filter cuts below 18 Hz at 12 dB per octave. It is implemented by feeding the buffer's output signal back to the input stage through a capacitor and resistor network. The power supply uses positive and negative voltage-regulator circuits plus a relay driving circuit, and the manual describes the positive and negative halves as identical.

Period coverage of this generation of Kenwood separates is thin, and what survives is often fragmentary. A 1980 component roundup in High Fidelity (1980, p.61) listed the L-07C MkII alongside the L-07M Mark Two mono power amplifier. It quoted overload at 120 mV on phono and signal-to-noise at 75 dB phono and 90 dB aux, IHF A-weighted. That is a MkII, not the original, and the listing is a single line in a crowded table rather than a full review.

One detail for anyone comparing manual figures against bench tests: Kenwood states that the specifications in the manual are measured at the tip of a special 12-meter audio cable provided with the unit, at its connection to the L-07C's output terminal. That is an unusually specific measurement condition. Numbers quoted from the manual may not line up exactly with what a bench test on a different cable produces.

The L-07C was built to feed the L-07M, and the manual says so outright. Run it with the wrong power amplifier and you are not hearing what Kenwood built. It has two equalizer amplifiers, gold-plated jacks, and a buffer stage with one job.