A heavyweight from the golden era of Japanese hi-fi. The Yamaha B-4 is a Natural Sound stereo power amplifier from 1978, built as two symmetrical mono amplifiers in one 21kg chassis, with twin transformers, dual independent power supplies and that understated black Yamaha styling. The party trick is a front-panel switch that takes it from a sweet, pure 30 watts per channel in Class A to a muscular 120 watts per channel in Class B, so you can choose delicacy or grunt depending on your speakers and your mood. Clean, neutral, colour-free and seriously overbuilt.

FEATURES & PROS:

• Class A / Class B Switch: 30W per channel of pure Class A, or 120W per channel in Class B, at the flick of a front-panel switch.
• True Dual-Mono: two symmetrical mono amplifiers, twin transformers and independent supplies for low crosstalk and stable power.
• Ro Output-Impedance Control: continuously vary output resistance from +1 to -1 ohm to tune the bass and speaker match.
• Overbuilt: 15,000uF audio capacitors, mylar and styrol caps, metal-clad resistors and heavy-gauge speaker coils.
• Fully Protected: speaker and output-transistor protection circuits guard your system on power-up and under fault.

THE WORD: Vintage hi-fi resources place the B-4 squarely in the golden era of Japanese audio: a beautifully overbuilt dual-mono amplifier that switches between a sweet 30W Class A and a muscular 120W Class B, with owners of its M-4 sibling praising a big, detailed sound and a huge soundstage. Read more at The Vintage Knob.

SPECIFICATIONS

• Released: 1978 (made 1978 to 1981)
• Type: Natural Sound stereo power amplifier, dual-mono construction
• Power: 120W per channel (Class B, 8 ohms), 30W per channel (Class A, 8 ohms), 170W per channel into 4 ohms
• Operation: switchable Class A and Class B
• THD: under 0.004% (Class A), under 0.007% (Class B)
• Frequency response: 10Hz to 100kHz; damping factor 200; signal to noise 118dB
• Output stage: bipolar 3-stage Darlington complementary push-pull OCL (not VFET)
• Extras: Ro output-impedance control (+1 to -1 ohm), speaker and transistor protection
• Weight: 21kg; dimensions 435 x 145.5 x 381mm
• Mains: Japanese 100V; a step-down transformer is required in South Africa
• Condition: used