March 2006
Vincent SPT100
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 with one
channel being driven (this is a mono amplifier).
 Gain: 26.3x, 28.4dB.
 Output noise, 8ohm load, unbalanced input, 1kohm input
termination: wideband 0.158mV, 85.1dBW; A weighted 0.051mV, 95.0dBW.
 AC line current draw at idle: 1.2A.
 Output impedance at 50Hz: 0.11 ohms.
 This amplifier does not invert polarity.
Power output with 1kHz test signal
 8ohm load at 1% THD: 107W
 8ohm load at 10% THD: 129W
 4ohm load at 1% THD: 189W
 4ohm load at 10% THD: 225W
General
The Vincent SPT100 is a mediumpower solidstate design
with typically wide bandwidth, low output impedance typical of solidstate power
amplifiers. It is of a hybrid design with vacuum tubes used for the frontend circuitry
and solidstate devices used for the output stage. A vacuum tube rectifier is used for the
tube circuitry supply.
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, 8ohm, and 4ohm loading, is quite low. The variation
with the NHT dummy load in the audio range is of the order of +/0.1dB.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for 1 kHz and SMPTE IM test signals and amplifier output load. As
can be seen, attainable power is greater for the 4ohm load, as is usual for most power
amplifiers. Furthermore, the way that the distortion increases as power nears maximum is a
much softer curve than is typical for a solidstate amplifier. This indicates the
possibility of low amounts of overall feedback in the design. In fact, the manual says the
output stage doesn’t have any feedback taken around it.
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 nonexistent in this design, although there is some
distortion increase at low frequencies at the 180W level.
Damping factor vs. frequency is shown in Chart 4 and is
reasonably constant with frequency.
A spectrum of the harmonic distortion and noise residue of
a 10W 1kHz test signal is plotted in Chart 5. The magnitude of the ACline harmonics are
quite low as are intermodulation components of line harmonics with signal harmonics. The
signal harmonics above the second and third are admirably low in number and magnitude for
the 8ohm loading shown. For the 4ohm loading case, the higher harmonics do increase but
are still all below 0.005%.
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 20W 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: 180W
Blue line: 60W
Magenta line: 20W
Red line: 2W
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
