McIntosh C200 preamplifier Measurements
Looking first at the C200's phono stage, this offered a respectable 40dB of gain set to MM and 63.3dB set to MC (assessed at the balanced tape output jacks). Despite this, the background noise level was low. The MM signal/noise ratio was 69.4dB unweighted (ref. 5mV input at 1kHz), while the MC figure was 56.4dB (ref. 500µV at 1kHz). A-weighted, these ratios improved to 83.4dB and 64.1dB, respectively.
The phono input offered very low distortion. Via the MM input, I measured just 0.0013% THD (true sum of the harmonics, ignoring the contribution of noise) with a 10mV, 1kHz input signal. The highest-level harmonic was the second, at just -100.6dB. Correlating with this superb linearity, the overload margin was a superb 25.6dB across most of the audioband for the MM input, increasing slightly to 26.5dB at 20Hz. The high- and mid-frequency margins dropped to a still good 19.4dB for the MC input, while the low-frequency margin remained excellent at 23.8dB. Assuming an overload point of 1% THD, all these figures were equivalent to a maximum output from the balanced tape-out jack of almost 10V!
The RIAA error was relatively low, at ±0.25dB (a little better from the left channel), but the shape of the response will be just audible, with a very slightly shelved-down treble (fig.1). In itself, this will not be responsible for PB's finding the MC input to sound dark. However, Dynavector recommends a load greater than 30 ohms for Paul's preferred cartridge, which has a source impedance of 6 ohms and 18µH. The C200's MC input impedance measured 36 ohms at 1kHz, 32 ohms at 20kHz, so perhaps this is indeed a little on the low side for this cartridge. The MM input impedance measured 48k ohms at 1kHz. Both inputs preserved absolute polarity; ie, they were noninverting.
Fig.1 McIntosh C200, MM input, RIAA error at 5mV input at 1kHz (0.5dB/vertical div., right channel dashed).
The line input also preserved signal polarity through both the unbalanced and balanced inputs—the latter are wired with pin 2 positive—and featured a usefully high input impedance at 1kHz of 34k ohms unbalanced, 50k ohms balanced. The volume control operated in accurate 0.5dB steps, with the maximum setting on the display of "15" equivalent to a gain of 14.9dB in both balanced and unbalanced modes (balanced input to balanced output, unbalanced input to unbalanced output). Corresponding with this maximum gain, the unity-gain setting ("0.0") of the control gave a slight insertion loss of -0.1dB.
The unbalanced output impedance measured 57 ohms at 1kHz, this exactly doubling from the balanced jacks, as expected. However, while this figure rose inconsequentially at 20Hz, to 64 ohms from the unbalanced jacks, the rise from the balanced XLRs was much larger, to 1130 ohms. While this will not be significant with power amplifiers having moderately high input impedances, it leads to a prematurely rolled-off bass with low impedances.
This can be seen in fig.2. The top pair of traces were taken into a high 100k ohms; the C200's response is flat from 20Hz to well above the audioband. But into 600 ohms, the bass rolls off to -3dB at 45Hz. The C200 is unlikely to see a load as low as 600 ohms in practice—the Jeff Rowland Design Group amplifiers are the only ones I know of that feature an input impedance this low—but C200 owners should make sure their power amplifiers have an input impedance of at least 15k ohms if the bass is not to sound a little lightweight. McIntosh's own MC501, reviewed by PB in August, measured 18.3k ohms, which will be just high enough.
Fig.2 McIntosh C200, line input, balanced frequency response, volume control max, at 1V into 100k ohms (top) and 600 ohms (bottom). (0.5dB/vertical div., right channel dashed.)
At the other end of the spectrum, the C200's wide bandwidth was not affected to any significant extent by loading or the setting of the volume control. It was also identical for balanced and unbalanced throughput. Channel separation was good rather than excellent, with the crosstalk level increasing with rising frequency, presumably due to capacitive coupling. This can be seen in fig.3, which also shows that the separation is a few dB better across the band with balanced operation.
Fig.3 McIntosh C200, line stage, balanced channel separation (bottom), unbalanced separation (top). (10dB/vertical div., R-L dashed.)
Like the phono stage, the C200's line stage was extremely linear. Fig.4 shows how the THD+noise percentage changes with frequency, taken at a 1V output level into both 100k ohms and 600 ohms for balanced (bottom traces) and unbalanced (top traces) operation. Balanced operation clearly gets the best from the circuitry, though a slight but inconsequential rise in distortion and noise can be seen at the low end into 600 ohms, this tying in with the rolled-off bass into this load.
Fig.4 McIntosh C200 line stage, THD+N (%) vs frequency at 1V into 100k ohms and 600 ohms, balanced (bottom) and unbalanced (top). (Right channel dashed.)
Fig.5 is a spectrum of the C200's unbalanced output driving a 1kHz tone into a lowish 8k ohm load. (Ignore the low-level, low-frequency spuriae, which are due to a ground loop between the C200 and my measurement computer that I couldn't eliminate.) The measured distortion level was just 0.0008% (RMS sum of the harmonics), and the highest component, as with the phono stage, was the second harmonic at -103dB, with no harmonics above the fourth visible above the low noise floor. Dropping the test load to 600 ohms didn't compromise this superb performance. Intermodulation distortion was also very low (fig.6) and, again, unaffected by the load impedance.
Fig.5 McIntosh C200, unbalanced line stage, spectrum of 1kHz sinewave, DC-10kHz, at 1V into 8k ohms (linear frequency scale).
Fig.6 McIntosh C200, unbalanced line stage, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 1V into 8k ohms (linear frequency scale).
Finally, fig.7 plots the THD+noise percentage against balanced output voltage into 100k and 600 ohms. The maximum output voltage into 100k ohms was a high 29.5V, this dropping only slightly, to 24V, into the demanding 600 ohm load. Note that below 10V output, the downward slope of the traces implies that the measured percentage is actually dominated by noise, not distortion. The unbalanced behavior was identical, with half the voltage available, as expected. Peculiarly, the voltage limiting appears to take place before the volume control, as lowering the volume setting moved the traces in fig.7 to the left.
Fig.7 McIntosh C200, line stage, THD+N (%) vs balanced output voltage into 100k ohms (bottom) and 600 ohms (top).
Overall, this is superb measured performance, in the McIntosh tradition.—John Atkinson