Yamaha C-2a/M-2 Pre/Power Amplifier Lab Report

Lab Report

As our inside shot illustrates, the M-2 is both densely packed and dominated by a huge power supply, the latter conferring a substantial 2x290W/8ohm and 2x490W/4ohm output that's comfortably in excess of its 240W/8ohm rating. There's headroom too, sufficient to support 342W, 635W and 655W into 8, 4 and 2ohm loads under dynamic conditions while current-limiting/protection restricts the figure to just 35W/1ohm [see Graph 1, below]. Even in the 1970s, and despite the compact layout, Yamaha's trademark low-noise amplification was reflected in the M-2's 96dB, and the C-2a's 97dB, A-wtd S/N ratio(s) (re. 0dBW and 0dBV, respectively). Distortion was very low indeed too, just 0.0002-0.001% across the 20Hz-20kHz audioband at 0dBW [see Graph 2, below], increasing with power to 0.0004%/10W, 0.0017%/100W and 0.0023% at the rated 240W/8ohm (all at 1kHz). The C-2a preamp's distortion varies with gain rather than level, falling as low as 0.0005%/1kHz at full gain (+22.5dB), 0.0009% at +12dB gain, 0.007% at +6dB gain and 0.027% at 0dB (unity) gain – all measured at the same 0dBV preamp output. Incidentally, the C-2a's maximum output is a high 18.5V (unbalanced).

Frequency responses are very flat and extended, the C-2a reaching out to within ±0.3dB from 7Hz-100kHz (the subsonic filter acts below 20Hz to –6.7dB/10Hz and –20dB/5Hz) and the M-2 from 1Hz/0dB to 100kHz/–1.3dB via its DC-coupled inputs. The output impedance(s) are respectably flat at 0.05ohm from 20Hz-20kHz via the M-2 (rising to 0.22ohm/100kHz) and 175 ohm from 200Hz-100kHz via the C-2a (rising to 390ohm/25Hz). DC offsets are low and stereo separation >60dB midband. PM


Dynamic power output vs. distortion up to 1% THD into 8ohm (black trace), 4ohm (red), 2ohm (cyan) and 1ohm (green) speaker loads. Max current is 18.1A


Distortion versus extended frequency from 5Hz-40kHz (black, 0dBV preamp; red, 0dBW power)

Power output (<1% THD, 8/4ohm) 290W / 490W
Dynamic power (<1% THD, 8/4/2/1ohm) 342W / 635W / 655W / 35W
Output imp. (20Hz–20kHz/100kHz) 0.050–0.053ohm / 175ohm
Freq. resp. (20Hz–20kHz/100kHz, 0dBV) +0dB to –0.0dB/–0.3dB (C-2a)
Freq. resp. (20Hz–20kHz/100kHz, 0dBW) +0dB to –0.1dB/–1.3dB (M-2)
A-wtd S/N ratio (re. 0dBV/0dBW) 96.7dB (C-2a) / 95.5dB (M-2)
Dist. (20Hz-20kHz, 0dBV Pre/0dBW Pow.) 0.0001–0.002%/0.0002–0.001%
Power consumption (Idle/rated output) 95W / 825W (31W, C-2a)
Dimensions (WHD) / Weight 435x183x362mm / 21kg