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Equipment Measurements

August 2006

Hyperion Sound Design HT-88 Mono 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.

Additional Data
  • Measurements were made at 120V AC line voltage with one channel driven (this is a mono amplifier).
  • This amplifier does not invert polarity.
  • AC line current draw at idle: 1.25A.
  • Input impedance @ 1kHz: 237k ohms.
  • Output impedance at 50Hz: 2.5 ohms.
  • Gain (8-ohm load): 14.6X, 12.3dB.
  • Output noise, 8-ohm load, 1k-ohm input termination: wideband 0.145mV, -85.8dBW; A weighted 0.049mV, -95.2dBW.
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 0.76W
  • 8-ohm load at 10% THD: 13.5W

  • 4-ohm load at 1% THD: 0.33W
  • 4-ohm load at 10% THD: 11.8W


The Hyperion HT-88 is a single-ended mono tube amplifier utilizing two KT88 tubes triode connected in parallel. A pair of tube rectifiers gives one plenty of tube glow to contemplate when playing music.

Chart 1 shows the frequency response of the amp with varying loads. As can be seen, the output impedance, as judged by the closeness of spacing between the curves of open-circuit, 8-ohm, and 4-ohm loading, is fairly high and typical of many tube amplifiers. The variation with the NHT dummy speaker load is of the order of +1.4/-1.8dB over the audio range.

Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals and 4- and 8-ohm loads. The attainable maximum power, generally rated at 10% distortion for tube amps, is greatest with an 8-ohm load on the 8-ohm output connection.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3. Laudable is the absence of a rise in distortion at high frequencies. There is, however, the rise in distortion at low frequencies, which is typical of most single-ended designs.

Damping factor vs. frequency as measured on the 8-ohm output is shown in Chart 4 and is reasonably constant over the majority of the audio range.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Chart 5. The amount of AC-line harmonics is admirably low. The signal-frequency harmonics are quite high and numerous, as the usual 10W level that I do this test with is near this amplifier's power limits.

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

4-ohm output loading
Cyan line: 10W
Magenta line: 5W
Red line: 1W
Green line: 0.3W

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


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