Kenwood KA-4004 (1972–1976)

At 9.3 kg, it carries the heft of an era when amplifiers were built to last.

Overview

The Kenwood KA-4004 was originally branded the Trio KA-4004, back when the company was moving from the Trio name to Kenwood. It is a solid-state stereo integrated amplifier, released in the early 1970s, between 1972 and 1976. It belongs to Kenwood's Hi-ATC (Advanced Technology Circuit) series, a line engineered for extended performance into the ultra-low frequency range. The focus is stability and deep bass control rather than raw power. At 18 watts per channel (rated continuous, 20Hz-20kHz into 8Ω), it wasn't built to shake walls. It was built for clean, detailed sound with low measured distortion and a wide power bandwidth of 10Hz to 50kHz (IHF). That bandwidth is far beyond the audible range, which hints at a circuit designed for transient accuracy and phase coherence.

The rated output is modest, but the KA-4004 lists several power measurements. At 1kHz with both channels driven, it delivers 19W into 8Ω and 26W into 4Ω. That is slightly higher than the continuous 20Hz-20kHz spec, as expected. Single-channel output climbs to 25W (8Ω) and 36W (4Ω), which indicates headroom for dynamic peaks. The Music Power rating is 56W (8Ω) and 95W (4Ω) per channel. It comes from the IHF burst-testing method, so it is useful for comparing with other gear of the era but is not a continuous rating. The damping factor is 32, which suggests moderate control over speaker cones, adequate for many bookshelf and floorstanding designs of the time.

The preamp section includes tape record outputs via both pin and DIN connectors, with levels of 160mV and 40mV respectively, and a preamp out at 1V. Frequency response is flat across line inputs (20Hz–50kHz ±1dB), with phono response tightly adhering to RIAA equalization (±0.3dB deviation). The tone controls offer a full ±10dB boost or cut at 100Hz and 10kHz, backed by a loudness contour that adds +8dB at 100Hz and +3dB at 10kHz when set to -30dB, useful for low-volume listening. Subsonic (80Hz, 12dB/octave) and high-frequency (7kHz, 6dB/octave) filters provide further tonal shaping, alongside muting and tone defeat switches.

Housed in a silver-colored metal case and made in Japan, the KA-4004 measures 435 x 153 x 300 mm and weighs 9.3 kg—a substantial presence on any rack. It runs on 100VAC or 117V, 50/60Hz, consuming 66W, and includes one switched and one unswitched AC outlet for powering source components. The headphone output and speaker terminals rated for 4Ω–16Ω loads add to its versatility in a mid-tier 1970s system.

Specifications

Key Features

All-Stage Direct-Coupled Circuit with Dual Power Supply

The KA-4004 uses a direct-coupled design across all stages, so there are no coupling capacitors between amplifier stages. Combined with a pure complementary symmetry circuit and a dual (±) power supply, this reduces phase shift and transient distortion, particularly in the low end. It fits the Hi-ATC series goal of extended ultra-low frequency performance, with less bass roll-off and better transient response. The topology is not unique to Kenwood, but it was still relatively advanced for a mid-tier integrated amplifier in the early 1970s.

Metal-Can Transistors in Critical Stages

Metal-can transistors are used in the first amplification stage and the phono equalizer circuits, where long-term reliability and low noise are critical. These TO-5 or TO-12-style packages were known for better thermal stability and lower microphonics than plastic transistors of the era. Their use here suggests a deliberate choice for durability and sonic integrity, especially in the sensitive phono preamp.

Low-Noise Nichrome Metal-Film Resistors

The first stage, equalizer and tone control circuits use low-noise nichrome metal-film resistors. These are less prone to thermal drift and aging than carbon composition types, which helps keep performance consistent over decades. That supports the amplifier's high signal-to-noise ratio (110dB closed circuit) and low residual noise (0.3mV).

Block Board Switch Mounting

The tone and filter switches are mounted directly to a block board, so no lead wires are needed between the switches and the circuit board. This reduces potential noise pickup and contact resistance, which improves signal integrity and reduces hum. It is a small engineering detail aimed at preserving the S/N ratio in a unit with many user-accessible controls.

Proprietary Shielded Wire Design

Kenwood used a proprietary shielded wire layout to minimize degradation caused by soldering heat. Shielding the sensitive internal wiring is meant to preserve signal quality during assembly and over time, and to reduce the risk of micro-cracks or insulation breakdown near solder joints. The exact implementation isn't detailed. The intent is to protect the signal path from manufacturing-induced stress.

Comprehensive Filter and Control Set

Besides the standard tone controls, the KA-4004 has a subsonic filter (80Hz, 12dB/octave) to remove turntable rumble and a high-frequency filter (7kHz, 6dB/octave) to tame overly bright recordings or tweeters. The muting switch allows silent source switching, and the tone defeat function bypasses all EQ circuitry for a purer signal path.

Historical Context

The KA-4004 was part of Kenwood's Hi-ATC series, which emphasized extended low-frequency performance and stable operation into difficult speaker loads. It sat within the broader KA series of integrated amplifiers, alongside models like the KA-6004 and KA-8004. It was not the flagship. Kenwood reserved that role for later models like the KA-9100. The KA-4004 was a well-engineered middle ground, with high measured performance and solid build quality without the premium price tag.

Collectibility & Value

The KA-4004 is not a high-value collector's item, but vintage audio enthusiasts restoring period-correct systems still want it. A second-hand listing once cited $160 as a budget-friendly price, and recent owner opinions suggest $200 is fair for a fully functional unit, with most units trading under $250. Common issues include "pop" noise on power-up and seized or noisy potentiometers, particularly the treble control. Restoration often involves a full recap (replacing electrolytic capacitors) and servicing of the 2SC1000 and 2SC1345 transistors, which are reportedly prone to failure. Service documentation, including schematic sheets and a kit assembly manual, is occasionally available through enthusiast networks.

eBay Listings

Original kenwood ka 4004 Speaker/Mode/Bass/Treble/Input/Tape

$11.99

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