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Notes: The Cyrus Stereo 200 power amplifier was measured at 120V AC line voltage, both channels driven, using its balanced inputs. Measurements were taken for both channels, but unless noted otherwise, the results reported below are for the left channel only. Because the Stereo 200 is a switching amplifier, measurements were made with the Audio Precision AUX-0025’s low-pass filter, except as noted.
- Power output at 1% THD+N: 174.3W @ 8 ohms, 262.4W @ 4 ohms
- Power output at 10% THD+N: 220.0W @ 8 ohms, 345.0W @ 4 ohms
- Input/output polarity: noninverting
- AC-line current draw at idle: 26.3W, 0.30A, 0.74PF
- Gain: output voltage divided by input voltage, 8-ohm load (Lch/Rch): 26.4X/26.2X, 28.4dB/28.4dB
- Input sensitivity for 1W output into 8 ohms (Lch/Rch): 107mV/107mV
- Output impedance @ 50Hz: 0.028 ohm
- Input impedance @ 1kHz (Lch/Rch): 62.8k ohms/60.1k ohms
- Output noise, 8-ohm load, balanced inputs, termination 600 ohms (Lch/Rch)
- Wideband without AUX-0025: 1.86V/1.92mV, -3.64dBW/-3.36dBW
- Wideband with AUX-0025: 3.94mV/4.0mV, -57.1dBW/-57.0dBW
- A weighted: 0.114mV/0.124mV, -87.9dBW/-87.1dBW
The Stereo 200’s switching-amplifier output circuit is powered by a conventional nonswitching power supply. It has a unique circuit for adjusting the output filter to the optimal conditions for the speaker load used.
Chart 1 shows the frequency response of the Stereo 200 with varying loads. This was done by going through the output-filter startup program with 8-ohm resistive loads. As can be seen, the curve for the 8-ohm load is the flattest, with more rolloff for a 4-ohm load, and some pretty bad peaking with an open-circuit load. The NHT dummy speaker load is nicely contained within about +/-0.8dB throughout the audioband.
Chart 2 illustrates how the Stereo 200’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE intermodulation (IM) test signals and amplifier output for loads of 8 and 4 ohms. The THD+N curves have some strange kinks on the way up to clipping. The IM curves show significant increases in the amount of distortion in the 5-30W range; these are worse for the 4-ohm load.
THD+N as a function of frequency at several different power levels is plotted in Chart 3. The Cyrus Stereo 200 had some trouble producing high-frequency power. I had to modify the Audio Precision’s settling routines from the normal exponential settling to no settling to get the measurements I did get. The amount of HF rise is pretty substantial; I felt I had to stop at a 30W power level to avoid damaging the review sample.
Chart 4 plots the Stereo 200’s damping factor vs. frequency. Although it seems typical of many amplifiers -- high at low frequencies, then declining over the audioband -- it begins to decrease at 50Hz, which is quite a bit lower than the norm, and declines to a very low value at 20kHz.
A spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal is plotted in Chart 5. The magnitudes of the AC line harmonics are about as low as I have seen in any amplifier, showing no really identifiable line frequency harmonics. The signal harmonics are also low, and consist mainly of a declining series of odd harmonics.
Chart 1 - Frequency response of output voltage as a function of output loading
Red line = open circuit
Magenta line = 8-ohm load
Blue line = 4-ohm load
Cyan line = NHT dummy-speaker load
Chart 2 - Distortion as a function of power output and output loading
(Line up at 50W to determine lines)
Top line = 4-ohm SMPTE IM distortion
Second line = 8-ohm SMPTE IM distortion
Third line = 4-ohm THD+N
Bottom line = 8-ohm THD+N
Chart 3 - Distortion as a function of power output and frequency
Red line = 1W
Magenta line = 5W
Blue line = 10W
Cyan line = 30W
Chart 4 - Damping factor as a function of frequency
Damping factor = output impedance divided into 8
Chart 5 - Distortion and noise spectrum
1kHz signal at 10W into an 8-ohm load