February 2004

Conrad-Johnson 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, 8-ohm load, unbalanced input, 1k-ohm 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

• 8-ohm load at 1% THD: 98W
• 8-ohm load at 10% THD: 110W
  
  
• 4-ohm load at 1% THD: 140W
• 4-ohm load at 10% THD: 160W

General

The Conrad-Johnson Premier 140 tube amp is an interesting-looking 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 push-pull design rated at a nominal 140W. A potentiometer and LED indicator set output-tube idling current adjacent to each output tube.

High-frequency bandwith, as seen in Chart 1, is unusually wide for a tube design having a 3dB down point of about 140kHz. The low-frequency 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 frequency-response 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 4-ohm load as that is the default way the amp is delivered. (The amp can be configured to be optimum for 8- or 16-ohm loads.) Amount of distortion is admirably low for powers up to perhaps 10-20W, 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 4-ohm 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 8-ohm 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 4-ohm load plotted in Chart 5, the signal distortion components are dominated by the second and third harmonics with higher-order 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 power-supply hum modulation around the suppressed fundamental 1-kHz test frequency.

Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load
Cyan line: NHT dummy-speaker load

(line up at 10W to determine lines)
Top line: 8-ohm SMPTE IM
Second line: 4-ohm SMPTE IM
Third line: 4-ohm THD+N
Bottom line: 8-ohm THD+N

4-ohm output loading
Green line: 140W
Blue line: 30W
Magenta line: 10W
Red line: 1W

Damping factor = output impedance divided into 8

1kHz signal at 10W into an 4-ohm load

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