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Equipment Measurements

March 2005

Thor Audio 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.

Additional Data
  • 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.
Measurements Summary

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 plate current.

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.

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 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 Noise Spectrum

1kHz signal at 10W into a 8-ohm load


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