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, 8-ohm load, unbalanced input, 1k-ohm 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
- 8-ohm load at 1% THD: 4.5W
- 8-ohm load at 10% THD: 25W
- 4-ohm load at 1% THD: 1W
- 4-ohm load at 10% THD: 30W
- 16-ohm load at 1% THD: 8.5W
- 16-ohm load at 10% THD: 20W
The Wyetech Labs Topaz 572B SET (single-ended 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 class-A 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 low-frequency response holds up down to
20Hz -- better than most of the low- or no-feedback SET designs. Output impedance is
moderate, giving an approximate plus-or-minus 2dB frequency-response variation on the NHT
dummy speaker load. Total harmonic distortion plus noise as a function of power output and
load on the 8-ohm output for a test frequency of 1kHz is plotted in Chart 2. Also shown in
this chart is the SMPTE IM distortion for an 8-ohm load on the 8-ohm 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 8-ohm load (and measures similarly for a 4-ohm load on the 4-ohm
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 4-ohm output, the damping factor was about double. In the spectral plot of
distortion plus noise for a 10W 1kHz signal into an 8-ohm load on the 8-ohm 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.
- Frequency Response of Output Voltage as a Function of Output Loading
Magenta line: open circuit
Red 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 2W to determine lines)
Top line: 8-ohm SMPTE IM
Second line: 4-ohm THD+N
Third line: 8-ohm THD+N
Bottom line: 16-ohm THD+N
|Chart 3 - Distortion
as a Function of Power Output and Frequency
8-ohm 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
1kHz signal at 10W into an 8-ohm load