McIntosh 50W-1 (1949–1950)

A vacuum tube stereo power amplifier, and McIntosh Laboratory's first product.

Overview

The McIntosh 50W-1 is the company's first product. Founder Frank McIntosh got fed up with the distortion and limited power of existing amplifiers, so he built his own. It came out in 1949 and set the key design principles for the brand. It is marketed as a stereo amplifier, and it delivers 50 watts per channel in stereo mode and up to 150 watts in mono. The output transformer is a Unity Coupled Circuit design. It delivers full power across the entire audio spectrum, keeps distortion low and protects the connected speakers.

Specifications

Rated output stereo (8 Ω)	50W + 50W
Rated output mono (8 Ω)	150W
Output load impedance	Stereo : 2 Ω, 4 Ω, 8 Ω, 16 Ω; Mono : 1 Ω, 2 Ω, 4 Ω, 8 Ω
Rated output band	20 Hz to 20 kHz
Total harmonic distortion	Not more than 0.002%
Cross modulation distortion factor	0.1% or Less (at Rated Output)
Frequency response (at 1W output)	20 Hz ~ 20 kHz + 0 -0.25 dB; 10 Hz ~ 100 kHz + 0 -3.0 dB
Signal-to-noise ratio	95 dB or more (at rated output)
Damping factor	Stereo : 16 (2 Ω), 50 (4 Ω), 20 (8 Ω), 14 (16 Ω); Mono : 16 (1 Ω), 50 (2 Ω), 20 (4 Ω), 14 (8 Ω)
Input Sensitivity / Impedance	0.75 V, 2.5V/75k Ω (Switched by level control)
Power	120 VAC, 50Hz/60Hz
Power consumption	50W (no signal); 460W (at maximum rated output)
External dimensions	Width 445x Height 152x Depth 467 mm
Weight	25.9 kg (net)

Design

The 50W-1 uses a patented "Unity Coupled Circuit" output transformer. It holds power delivery consistent across all frequencies and speaker impedances and keeps distortion down. The transformer also protects the speakers if the amplifier fails. There is a Power Guard system too, with a waveform comparison circuit that watches the difference between the input and output signals. At 0.5% distortion the Limit (red) lamp lights. At 1% the system kicks in to prevent clipping and speaker damage, and the amplifier still holds its rated output. The input stage is differential and uses low-noise tubes. The output is a single-ended para-push circuit with six complementary service power tubes.