August 2006
Hyperion Sound
Design HT88 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.
 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 (8ohm load): 14.6X, 12.3dB.
 Output noise, 8ohm load, 1kohm input termination: wideband
0.145mV, 85.8dBW; A weighted 0.049mV, 95.2dBW.
Power output with 1kHz test signal
 8ohm load at 1% THD: 0.76W
 8ohm load at 10% THD: 13.5W
 4ohm load at 1% THD: 0.33W
 4ohm load at 10% THD: 11.8W
General
The Hyperion HT88 is a singleended 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 opencircuit, 8ohm, and 4ohm 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 8ohm loads. The
attainable maximum power, generally rated at 10% distortion for tube amps, is greatest
with an 8ohm load on the 8ohm 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 singleended designs.
Damping factor vs. frequency as measured on the 8ohm
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 ACline harmonics is admirably
low. The signalfrequency 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: 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 10W to determine lines)
Top line: 4ohm SMPTE IM
Second line: 8ohm SMPTE IM
Third line: 4ohm THD+N
Bottom line: 8ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
4ohm 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 8ohm load
