February 2004
ConradJohnson
Premier 140 Stereo 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 with 120V AC line voltage.
 Measurements were made on the left channel.
 Gain: 24.6x, 27.8dB.
 Output noise, 8ohm load, unbalanced input, 1kohm input
termination: wideband 0.255mV, 80.9dBW; A weighted 0.103mV, 88.8dBW.
 AC line current draw at idle: 3.6A.
 Output impedance at 50Hz: 0.98 ohms.
 This amplifier does not invert polarity.
Power output with 1kHz test signal
 8ohm load at 1% THD: 98W
 8ohm load at 10% THD: 110W
 4ohm load at 1% THD: 140W
 4ohm load at 10% THD: 160W
General
The ConradJohnson Premier 140 tube amp is an
interestinglooking piece. Its physical layout is unusual in that the front and rear
panels are on the sides relative to the enclosed transformer and capacitor cover. This amp
is a stereo pushpull design rated at a nominal 140W. A potentiometer and LED indicator
set outputtube idling current adjacent to each output tube.
Highfrequency bandwith, as seen in Chart 1, is unusually
wide for a tube design having a 3dB down point of about 140kHz. The lowfrequency response
holds up down to 10Hz nicely at the 1W level of the test. utput impedance is typical of
many tube amplifiers, giving less than a 2dB frequencyresponse variation with the NHT
dummy speaker load. Total harmonic distortion plus noise and SMPTE IM distortion as a
function of power output and load for a test frequency of 1kHz is plotted in Chart 2. More
power is delivered with the 4ohm load as that is the default way the amp is delivered.
(The amp can be configured to be optimum for 8 or 16ohm loads.) Amount of distortion is
admirably low for powers up to perhaps 1020W, where the kernel of most music resides.
Total harmonic distortion plus noise as a function of frequency at several power levels is
plotted in Chart 3 for a 4ohm load. Admirable is the relatively low amount of distortion
increase at the higher frequencies. However, distortion does rise considerably below 20Hz
at higher powers. The rise in distortion at low frequencies is a strong function of how
closely the bias is adjusted for equal current in all four output tubes. When I first
measured the left channel of this amp, the rise in distortion at low frequencies was
considerably more pronounced. When I went to more carefully adjust the bias, I found that
two of the four output tubes were down in current compared to the other two. When I really
tweaked the bias so that the red indicator LEDs just went out at 120V AC line input for
all four tubes, the performance of the left channel more closely matched that of the right
channel. Damping factor vs. frequency referred to an 8ohm load is plotted in Chart 4, and
it is quite consistent over most of the audio range. In the spectral plot of distortion
and noise for a 10W 1kHz signal into a 4ohm load plotted in Chart 5, the signal
distortion components are dominated by the second and third harmonics with higherorder
harmonics at reduced and decreasing amplitude with frequency. As I have seen in quite a
number of other amplifiers measured, there is quite a bit of 120Hz powersupply hum
modulation around the suppressed fundamental 1kHz test frequency.
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: 8ohm SMPTE IM
Second line: 4ohm 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
Green line: 140W
Blue line: 30W
Magenta line: 10W
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 an 4ohm load
