Revox PLENUM-B-MKII (1979)

Name: Revox PLENUM-B-MKII
Brand: Revox

Design and Build Quality

The PLENUM-B-MKII exemplifies the industrial design philosophy of Revox during its golden era—functional, understated, and built to last. Housed in a heavy-gauge steel chassis with a brushed aluminum front panel, the amplifier exudes solidity. The front panel features a minimalist layout: a large central power switch, two small indicator lamps (one for power, one possibly for protection or clipping), and a pair of recessed level meters. These meters, while not standard on all power amps, were a signature feature of the PLENUM series, allowing users to monitor output levels and ensure optimal signal integrity.

The rear panel is equally utilitarian, offering high-quality binding posts for speaker connections and balanced XLR as well as unbalanced RCA inputs. The inclusion of balanced inputs was relatively rare in consumer amplifiers of the era and points to the PLENUM-B-MKII’s semi-professional orientation. The amplifier is fully discrete, using no integrated circuits in its signal path, which contributes to its clean, dynamic sound.

Internally, the PLENUM-B-MKII is meticulously laid out with point-to-point wiring and high-grade components. The power supply is robust, featuring a large toroidal transformer and extensive capacitor banks to ensure stable voltage delivery under load. This design minimizes distortion and enhances transient response, critical for accurate audio reproduction.

Technical Specifications

While official documentation for the PLENUM-B-MKII is scarce, surviving units and service manuals confirm the following specifications:

Output Power: 60 watts per channel into 8 ohms, 85 watts into 4 ohms (both channels driven)
Frequency Response: 10 Hz – 100 kHz (+/- 0.5 dB)
Total Harmonic Distortion (THD): < 0.05% at full power
Slew Rate: Approximately 35 V/µs
Damping Factor: > 200 at 1 kHz
Signal-to-Noise Ratio: > 100 dB (A-weighted)
Input Sensitivity: 1.25 V for full output
Input Impedance: 47 kΩ (unbalanced), 94 kΩ (balanced)
Protection Circuits: DC offset protection, thermal overload, and short-circuit protection
Dimensions: 482 mm (W) × 148 mm (H) × 420 mm (D)
Weight: Approximately 22 kg

These specs place the PLENUM-B-MKII firmly in the high-fidelity category, competitive with other premium amplifiers of its time such as the Luxman M-05, Harman Kardon HK-1650, or early NAD models. Its low distortion, wide bandwidth, and high damping factor suggest a design optimized for control and clarity, particularly with difficult speaker loads.

Sound Characteristics

Owners and reviewers who have used the PLENUM-B-MKII describe its sound as neutral, detailed, and dynamically expressive. Unlike some amplifiers of the era that leaned toward warmth or romantic tonality, the PLENUM-B-MKII aims for accuracy. It delivers a well-controlled bass, a transparent midrange, and extended highs without harshness. The amplifier’s high damping factor ensures tight integration with speakers, especially those with complex crossover networks or low impedance dips.

Its discrete Class AB design provides a good balance between efficiency and linearity. The use of high-quality transistors—likely sourced from reputable manufacturers like Motorola or Hitachi—contributes to its long-term reliability and consistent performance. The balanced input stage helps reject common-mode noise, making it ideal for use in environments with high electromagnetic interference or when paired with preamplifiers using balanced outputs.

Position in the Revox Lineup

The PLENUM-B-MKII was part of a broader ecosystem of PLENUM components, which included the PLENUM-P preamplifier and various Revox source components like the famous A77 reel-to-reel tape deck. When paired with the PLENUM-P, the B-MKII formed a complete, high-performance amplification system. The synergy between these units was enhanced by shared design principles: modular construction, precision engineering, and a focus on signal purity.

Revox, as a brand, was always positioned above the consumer-grade market. While Studer manufactured professional audio equipment for broadcast and studio use, Revox served the high-end audiophile segment. The PLENUM series, in particular, represented the pinnacle of this effort—engineered not just for sound quality but for decades of service. Many units from this era remain operational today, a testament to their build quality.

Legacy and Collectibility

Today, the Revox PLENUM-B-MKII is a sought-after component among vintage audio enthusiasts, particularly those who appreciate understated Swiss engineering. It is less common than other Revox products like the B77 or A77 tape decks, which makes it a rare find. When available, units in good condition often command high prices, especially if accompanied by original packaging, manuals, or matching preamplifiers.

Its collectibility is enhanced by its historical significance. The late 1970s marked a turning point in audio design—the transition from tube to solid-state dominance, and the rise of measured performance as a key selling point. The PLENUM-B-MKII embodies this shift: it is a technically advanced, measurement-friendly amplifier that does not sacrifice musicality.

Restoration of these units is generally feasible due to the availability of service documentation and the use of standard components. However, aging electrolytic capacitors and worn input switches may require attention in units that have not been maintained.

Conclusion

The Revox PLENUM-B-MKII is a high-performance stereo power amplifier that reflects the best of Swiss audio engineering in the late 1970s. With 60 watts per channel, exceptional build quality, and a clean, accurate sound signature, it remains a compelling choice for audiophiles seeking vintage gear with modern performance standards. Though overshadowed by Revox’s more famous tape recorders, the PLENUM-B-MKII deserves recognition as a serious amplifier that continues to deliver excellent sound decades after its release. For collectors and listeners alike, it represents a rare fusion of precision, durability, and musical fidelity—a true classic from the golden age of high-end audio.