TRIO KA-5500 (1975)

At 10.5kg, it sits on the rack like a declaration—this is serious engineering, not showpiece nostalgia.

Specifications

Key Features

First Stage FET Differential-Amplified ICL Equalizer Stage

The phono stage isn’t an afterthought—it’s engineered with a first-stage FET differential amplification ICL configuration, designed to tighten phase and transient response. This isn’t just about gain; it’s about preserving the leading edge of a snare hit or the decay of a piano note. The ±2 power supply system isolates the equalizer stage, reducing crosstalk and noise modulation. The RIAA curve holds within ±0.3 dB, and the low distortion at 1W output (0.06%) suggests a transparent hand in signal shaping.

Select NFB Type Tone Control with Buffer Bypass

Kenwood didn’t just slap on a tone stack—they built one that disappears when not needed. The tone control uses a negative feedback (NFB) design with CR elements embedded in the feedback loop, minimizing signal path degradation. More importantly, in the defeat position, those CR elements are disengaged entirely, turning the stage into a buffer amplifier with superior DC stability and phase coherence. This isn’t a passive lift or cut; it’s a circuit that knows when to get out of the way. The ±10 dB adjustment at 100 Hz and 10 kHz gives meaningful control without veering into audiophile gimmickry.

All Stages Directly Connected Pure Complimentary OCL Power Amplifier

No capacitors in the signal path—this is a fully direct-coupled design from input to output, using a pure complementary symmetry OCL (Output Capacitor-Less) configuration. The first stage uses differential amplification, and the entire signal chain runs on a ±2 power supply system to minimize drift and improve stability. The result is a claimed damping factor of 30, which, while not class-leading, suggests respectable control over speaker cones. With 55W continuous into 8Ω across the full 20Hz–20kHz band, and 60W at 1kHz, this amp can drive all but the most inefficient speakers. The all-stage direct coupling contributes to a seamless midband—no capacitor-induced smearing or phase lag.

Left and Right Independent Direct-Reading Power Meters

Few amps from this era offered true output monitoring, but the KA-5500 includes dual meters that read from 0.01W to 60W, switchable between 60W and 3W ranges. These aren’t decorative LEDs—they’re analog meters providing real-time feedback on channel balance and output levels. At a glance, users can spot clipping, mismatched gain, or a failing channel. this was a diagnostic tool as much as a visual flourish. The 41-click detent volume control pairs well with this precision, offering repeatable, tactile adjustments without channel drift.

Double Protection Circuit with ASO Limiter and Relay

Blown outputs were a real risk in 1970s solid-state amps, but Kenwood equipped the KA-5500 with a dual-layer defense: an ASO (Abnormal Signal Output) detection-type limiter and a relay-based shutdown system. The ASO circuit monitors for DC offset or excessive current, triggering the relay to disconnect the speakers before damage occurs. This isn’t just a fuse—it’s active protection that can prevent a catastrophic failure from turning into a costly repair. Combined with the stable ±2 power supply and constant-current load in the class A stage, the design prioritizes longevity as much as performance.

Subsonic Filter and -20dB Attenuator

Two practical tools for real-world use: a fixed 18 Hz, 12dB/octave subsonic filter to eliminate turntable rumble and port noise from bass reflex cabinets, and a -20dB input attenuator for handling hot line sources. The filter is especially useful with older cartridges or warped records, preventing low-frequency energy from wasting amplifier headroom or over-excursing woofers. The attenuator makes it easier to match levels with modern preamps or high-output tuners, avoiding clipping in the input stage.

41-Click Detent Volume Control

Precision matters. The 41 distinct detents on the volume knob ensure repeatable settings and balanced left/right tracking. This isn’t a smooth, vague potentiometer—it’s a stepped attenuator with mechanical indexing, reducing channel imbalance and wear over time. Combined with the direct-coupled circuitry, it ensures that volume changes don’t introduce distortion or channel drift. It’s a small detail, but one that speaks to the overall engineering philosophy: control, accuracy, and longevity.

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