Bozak B-300 Concerto IV (Late 1960s–Early 1970s)

At 41 kg, it doesn’t just sit in the room—it commands it, with a presence as bold as its 30 cm woofer’s low-end reach.

Overview

The Bozak B-300 Concerto IV was the flagship of the TRIO-BOZAK lineup. It is a floor-standing system, 41 kg and nearly 77 cm tall. The weight goes into serious driver engineering, a sealed enclosure and a crossover built to handle a dual-tweeter array. Some of its contemporaries chased efficiency or compact size. The B-300 went for control, with a flat, extended response from 40 Hz to 20 kHz and minimal coloration.

The exterior is an arched frame with a matte walnut oil finish. The grille and iron decorations follow a Greco-Roman motif, which is ornate, even baroque, by today's standards. Underneath is a plain acoustic suspension design, with a hermetically sealed enclosure. That avoids the tuning quirks of ported cabinets and gives tighter, more predictable bass. There are three drivers in a two-way configuration: a single 30 cm (12 inch) B-199A cone woofer for lows and mids, and two 5 cm (2 inch) B-200Y cone tweeters for high-frequency dispersion. The tweeters are mounted on a foam rubber platform to dampen spurious vibrations and smooth out response peaks. Bozak clearly paid attention to resonance control.

The terminal section has a multi-amplifier selector switch and terminals, which suggests the B-300 was designed with future expansion in mind, possibly bi-amping or integration into more complex systems. The configuration echoes the B-312, but the cabinet volume is larger, allowing deeper bass extension and improved low-frequency headroom. According to Audio magazine in 1971, the B-300 Concerto IV was “by far the finest in its class.”

Specifications

Manufacturer	BOZAK
Model	B-300 Concerto IV
Product type	Two-way speaker system, floor type
Configuration	2-Way, 3-Speaker, Sealed (Bass Reflex) Type, Floor Type
Units Used	Woofer: 30 cm cone type (B-199A); Tweeter: 5 cm cone type x2 (B-200Y)
Frequency response	40 Hz to 20 kHz
Impedance	8 Ω
Rated input	20W
Maximum input	50W (MUSIC)
Power Handling Capacity	20 watts RMS (reportedly)
Crossover frequency	2.5 kHz, 6dB/oct.
External dimensions	Width 600 x Height 765 x Depth 500 mm
Weight	41 kg
Sensitivity	91.5 dB (reportedly)

Key Features

30cm Cone Woofer with Felted Paper and Wool Composite

The B-199A woofer cone is a felted paper composite with wool mixed into the pulp. The formulation was chosen for a balance of light weight, structural hardness and high compliance. This blend reduces breakup modes and lowers distortion, especially in the critical midbass region. The cone also uses a variable density system, denser at the center and gradually decreasing toward the edge. That helps maintain piston-like behavior across its range and prevents internal interference that could muddy the response.

Dual 5cm Cone Tweeters on Vibration-Damping Platform

Instead of a single tweeter, the B-300 uses two B-200Y 5 cm cone units. Each has a diaphragm formed with a special metal damping technique to suppress resonant peaks. They are mounted together on a foam rubber platform, which mechanically isolates them from the main cabinet structure. The dual arrangement improves high-frequency dispersion and power handling, while the platform absorbs cabinet-born vibrations that could otherwise color the treble. It addresses two common flaws in large speakers: inconsistent off-axis response and tweeter fatigue under load.

Hermetically Sealed Acoustic Suspension Enclosure

The cabinet is a fully sealed acoustic suspension design, which avoids the tuning complexities and potential port noise of bass reflex systems. This choice favors time-domain accuracy and tighter bass control over raw efficiency. The sealed volume lets the 30 cm woofer work with greater linearity at low frequencies, which contributes to the 40 Hz lower limit. It is less efficient than a ported design, but the trade-off is improved transient response and reduced group delay.

Multi-Amplifier Selector Switch and Terminal Section

The rear panel has a multi-amplifier selector switch alongside standard terminals, which suggests the B-300 was engineered for flexibility. The switch may let users toggle between single-amp and bi-amp configurations, or select between different input sources without external switching. That was a rare feature for the era, and it indicates Bozak anticipated advanced system integration and possibly professional or semi-pro use.

Rebuildable Crossover with Driver Balance Adjustment Potential

The original crossover operates at 2.5 kHz with a 6 dB/octave slope. Owners note it can be “completely rebuilt” and modified, and some have added potentiometers to dial in driver balance. That helps restorers, since original capacitors may degrade over time. Film capacitor upgrades are a common refinement, improving clarity and longevity.

Historical Context

The B-300 Concerto IV was developed as the standard-bearer of the TRIO-BOZAK lineup. Its design shares a lot with the B-312, but the larger enclosure enhances bass performance. It was part of a broader family that included the B-302A, B-305, B-310A and B-400, all aimed at different segments of the high-fidelity market. It was produced from the late 1960s into the early 1970s, when brands like AR, KLH and Advent were also pushing acoustic suspension and driver innovation.

Collectibility & Value

Current market prices for restored units range from $400 for refoamed pairs to $590 and $685 for those with upgraded components like film capacitors. Refoaming the woofer surrounds is a common maintenance task, and one owner reported success after refoaming two years prior. Crossover rebuilding is also routine, often with sensitivity adjustments, though one restorer noted an “expected reduction in sensitivity” after the repair. Most feedback is positive: “They’re beautiful speakers,” “in perfect shape in every way.” Some owners report issues with newly acquired pairs, so careful inspection is needed.