February 2002
Tenor Audio 75
Wi 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 at 120V AC line voltage.
 Input signal was applied to the "direct" input.
 Plate current was adjusted to 300mA per tube when warmed up.
 Power output plotted with one channel driven (this is a mono
amplifier).
 Gain: 33.3x, 30.5dB.
 Output noise, 8ohm load, 1000ohm input termination, volume
control set for 1W/8 ohms with
1kHz 500mV input signal to "input 4" in normal (not direct) mode: wideband
0.291mV, 79.8dBW; A weighted 0.078mV, 91.2dBW.
 AC line current draw at idle: 5.0A
 Output impedance (measured by an injection of a constant 1A
of current at 50Hz): 0.76 ohms.
 This amplifier does not invert polarity.
Power output with 1kHz test signal
 8ohm load at 1% THD: 79W
 8ohm load at 10% THD: 120W
 4ohm load at 1% THD: 25W
 4ohm load at 10% THD: 90W
 16ohm load at 1% THD: 89W
 16ohm load at 10% THD: 120W
General
The Tenor 75 Wi
appears to be a robust, welldesigned outputtransformerless power amplifier. Ironically,
many of its characteristics, like distortion and output impedance, are typical of good
tube amplifiers having output transformers. As can be seen in Chart 1, frequency response
is wide, with a 3dB highfrequency point in excess of 200kHz. There is some small rise at
very low frequencies. This verylowfrequency rise has effects on the lowfrequency
damping factor characteristic, as can seen in Chart 4. Output impedance is fairly low,
meaning that the frequency response delivered to a connected speaker will be affected
minimally. The response with the NHT dummy speaker load was within about +/ 0.5dB. Chart
2 shows that output power at clipping is fairly constant at 7080W with loads over a
416ohm range. Harmonic distortion is reasonably low and stays below 1% for an 8ohm load
to better than 80W output. Harmonic distortion increases only moderately with frequency,
as shown in Chart 3. As predicted from Chart 1, output impedance is fairly low, and this
translates to a damping factor of over 10 over most of the audio frequency range. Finally,
in Chart 5, the harmonicdistortion residual spectrum shows the ACline harmonics to be
reasonably low and the signaldistortion components are dominantly second and third
harmonic.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
Magenta line: open circuit
Green line: NHT dummy speaker load
Red line: 8ohm load
Blue line: 4ohm load
Chart 2  Distortion as a Function
of Power Output and Output Loading 
(line alignment at 10W)
Top line: 8ohm SMPTE IM
Second line: 4ohm THD+N
Third line: 8ohm THD+N
Bottom line: 16ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
8ohm output loading
Cyan line: 85W
Blue line: 30W
Magenta line: 10W
Red line: 1W
Chart 4  Damping Factor
as a Function of Frequency 
Damping factor = output impedance (measured by an injection of a constant 1A at all
frequencies) divided into 8
Chart 5  Distortion and
Noise Spectrum 
1kHz signal at 10W into an 8ohm load
