June 2003
Wyetech Labs
Topaz 572B Stereo Amplifier: Measurements
All amplifier measurements are performed
independently by BHK Labs. Please click to learn
more about how we test amplifiers there. All measurement data and graphical
information displayed below are the property of SoundStage! and Schneider
Publishing Inc. Reproduction in any format is not permitted.
 Measurements were made with 120V AC line voltage.
 Power output and distortion plotted with both channels
driven (worse of two channels shown).
 Gain: 20.8x, 26.4dB.
 Output noise, 8ohm load, unbalanced input, 1kohm input
termination: wideband 1.0mV, 69.0dBW; A weighted 0.15mV, 85.5dBW.
 AC line current draw in "operate" mode: 3.6A.
 AC line current draw in "standby" mode: 0.89A.
 Output impedance at 50Hz: 2.86 ohms.
 This amplifier does not invert polarity.
Power output with 1kHz test signal
 8ohm load at 1% THD: 4.5W
 8ohm load at 10% THD: 25W
 4ohm load at 1% THD: 1W
 4ohm load at 10% THD: 30W
 16ohm load at 1% THD: 8.5W
 16ohm load at 10% THD: 20W
General
The Wyetech Labs Topaz 572B SET (singleended triode)
amplifier measured here is typical of a design utilizing the Sovtek SV 572 10 output tube.
This tube replaces the 211 used in the earlier version of this amplifier and allows for
higher classA output power.
Frequency response, as seen in Chart 1, rolls off at both
ends of the audio range, and there is some pronounced ultrasonic aberrations likely due to
the characteristics of the output transformer. The lowfrequency response holds up down to
20Hz  better than most of the low or nofeedback SET designs. Output impedance is
moderate, giving an approximate plusorminus 2dB frequencyresponse variation on the NHT
dummy speaker load. Total harmonic distortion plus noise as a function of power output and
load on the 8ohm output for a test frequency of 1kHz is plotted in Chart 2. Also shown in
this chart is the SMPTE IM distortion for an 8ohm load on the 8ohm output. It shows that
the amount of distortion at the 1kHz test frequency is reasonably low, at less than 2% up
to 10W output for the 8ohm load (and measures similarly for a 4ohm load on the 4ohm
output). Raising the load to 16 ohms increases the distortion somewhat, whereas decreasing
the load to 4 ohms reduces the amount of distortion up to about 1W. This is because the
dominant second harmonic is partially canceled by this operating condition. Total harmonic
distortion plus noise as a function of frequency at several power levels is plotted in
Chart 3. Amount of distortion over the main midrange energy band is less than 2% for power
outputs of 10W or less. Admirable is the relatively low amount of distortion increase at
the higher frequencies. However, distortion does rise considerably at the low end of the
audio range. Damping factor vs. frequency, shown in Chart 4, is a modest 2.6 over the mid
range of frequencies, and actually increases a bit at the frequency extremes. As is usual,
on the 4ohm output, the damping factor was about double. In the spectral plot of
distortion plus noise for a 10W 1kHz signal into an 8ohm load on the 8ohm output, the
signal distortion components are dominated by the second harmonic with the third, fourth,
and fifth harmonics being about equal at 1/20 of the amount of the second harmonic. The
amount of AC line hum components adding up to about 1mV would likely be audible on
speakers with efficiencies of 90dB or more.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
Magenta line: open circuit
Red line: 8ohm load
Blue line: 4ohm load
Cyan line: NHT dummyspeaker load
Chart 2  Distortion as a Function
of Power Output and Output Loading 
(line up at 2W to determine lines)
Top line: 8ohm SMPTE IM
Second line: 4ohm THD+N
Third line: 8ohm THD+N
Bottom line: 16ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
8ohm output loading
Cyan line: 15W
Blue line: 10W
Magenta line: 5W
Red line: 1W
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 8ohm load
