TPA-30 Mk II Mono Amplifiers: 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 one channel driven
(this is a mono amplifier).
- Output-tube bias set per instruction manual.
- Gain: 16.9x, 24.6dB.
- Output noise, 8-ohm load, unbalanced input, 1k-ohm input
termination: wideband 0.149mV, -85.6dBW; A weighted 0.046mV, -95.8dBW.
- AC line current draw at idle: 0.72A.
- Output impedance at 50Hz: 1.3 ohms.
- This amplifier does not invert polarity.
Power output with 1kHz test signal
- 8-ohm load at 1% THD: 9W
- 8-ohm load at 10% THD: 40W
- 4-ohm load at 1% THD: 2.7W
- 4-ohm load at 10% THD: 42W
The Thor Audio TPA-30 Mk II is an unusually shaped tube
power amplifier with a meter for setting output-tube bias. Unusually, the manual states
that the bias settings for each output tube for this particular amplifier are not the
same, suggesting that the bias is a measure of the bias voltage rather than the actual
Chart 1 shows the frequency response of the amp with
varying loads. The spacing of the curves indicates a reasonably low output impedance
leading to a moderately high damping factor. The frequency response variation with the NHT
dummy load is about +/-1dB.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. The
results indicate that the amplifier puts out about the same power into either 4- or 8-ohm
loads, but, of course, with higher distortion with the 4-ohm loading. Output clipping
starts at about 35W; a typical result for a pair of EL-34 output tubes.
Total harmonic distortion plus noise as a function of
frequency at several different power levels is plotted in Chart 3. Amount of rise in
distortion at high frequencies is moderate -- a good thing. Distortion does rise at low
frequencies due to the intrinsic nature of output transformers.
Damping factor vs. frequency is shown in Chart 4. A
spectrum of the harmonic distortion and noise residue is plotted in Chart 5. The magnitude
of the AC-line harmonics is unusually low in this design. As seen in a number of amplifier
designs, there is some modulation products of the line harmonics with the nulled-out
test-frequency fundamental tone at 1kHz. The spectrum of the test-signal-related harmonics
is rather complex and extensive, with many harmonics of appreciable amplitude above the
second and third.
- 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 10W to determine lines)
Top line: 4-ohm SMPTE IM
Second line: 8-ohm SMPTE IM
Third line: 4-ohm THD+N
Bottom line: 8-ohm THD+N
|Chart 3 - Distortion
as a Function of Power Output and Frequency
8-ohm output loading
Cyan line: 30W
Blue line: 15W
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 a 8-ohm load