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Equipment Measurements

August 2007

Convergent Audio Technology JL2 Signature Mk 2 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.

Additional Data
  • Measurements were made at 120 AC line voltage, with both channels driven, and on the left channel unless otherwise noted.
  • This amplifier does not invert polarity.
  • AC-line current draw at idle: 5.8A.
  • Input impedance @ 1kHz: 55k ohms.
  • Output impedance at 50Hz: 0.7 ohms.
  • Gain (8-ohm load), Lch | Rch: 28.3X, 29.0dB | 27.9X / 28.9dB
  • Output noise, 8-ohm load, 1k-ohm input termination, Lch/Rch:
    • Wideband: 0.076mV, -91.5dBW / 0.142mV, -86.0dBW
    • A weighted: 0.033mV, -98.3dBW / 0.098mV, -89.3dBW
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 105W
  • 8-ohm load at 10% THD: 133W

  • 4-ohm load at 1% THD: 103W
  • 4-ohm load at 10% THD: 163W

General

The Convergent Audio Technology JL2 Signature Mk 2 is a medium-power stereo push-pull tube amplifier utilizing four triode-connected pairs of 6550 output tubes per channel. Having the output tubes connected as triodes means that the resulting output stage will be more tolerant of load-impedance changes than if a smaller number of output tubes were connected in ultralinear or pentode modes. Furthermore, the larger number of output tubes means that the primary impedance of the output transformer can be lower, resulting in potentially better and more controlled high-frequency response.

Chart 1 shows the frequency response of the amp with varying loads. The output impedance, as judged by the closeness of spacing between the curves of open-circuit, 8-ohm, and 4-ohm loading, is reasonably and acceptably low for a tube power amplifier. With the NHT dummy speaker load, the variation is better than +/-0.6dB. Of particular interest is the beautifully controlled and consistent way the high-frequency response retains its shape with the different load impedances -- not a usual quality of most power amplifiers, tube or solid state.

Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. Of interest, this amplifier’s general distortion characteristic is typical of many tube amplifiers and is one of continuously increasing distortion with increasing power. As is the case with most power amplifiers, this one puts out more power into the 4-ohm load.

Total harmonic distortion plus noise as a function of frequency at four different power levels is plotted in Chart 3. The amount of rise in distortion at high frequencies is reasonably low, and there is the typical rise in distortion at low frequencies due to the characteristics of output transformers.

Damping factor vs. frequency is shown in Chart 4. Notable here is the reasonably flat characteristic with frequency over most 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 principal signal harmonics are the second and third, with all higher harmonics some 34dB or so lower. AC-line harmonics are low, although there are some line-harmonic-related sidebands around the nulled-out 1kHz test signal fundamental -– something I have seen from quite a few of the power amplifiers that I have tested, but not all of them.

Chart 1 - Frequency Response of Output Voltage as a Function of Output Loading


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

Chart 2 - Distortion as a Function of Power Output and Output Loading


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

Chart 3 - Distortion as a Function of Power Output and Frequency


8-ohm output loading
Green line: 100W
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 8-ohm load

 

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