April 2009

JE Audio VS70 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 at 120V AC line voltage with both channels being driven. Measurements made on the 8-ohm outputs of the left channel and balanced inputs unless otherwise noted.
• Input/output polarity: non-inverting
• AC line current draw at idle: 1.67A
• Input sensitivity for 1W output into 8 ohms, volume at maximum, Lch/Rch
• Unbalanced inputs: 82.4mV / 81.9mV
• Balanced inputs: 117.3mV / 116.9mV
• Input impedance @ 1kHz
• Unbalanced inputs: 52.0k ohms
• Balanced inputs: 95.0k ohms
• Output impedance at 50Hz: 2.76 ohms
• Gain, output voltage divided by input voltage, volume at maximum, Lch/Rch
• Unbalanced inputs: 34.3X, 30.7 dB / 34.5X, 30.8 dB
• Balanced inputs: 24.1X, 27.6dB / 24.2X, 27.7dB
• Output noise, unbalanced inputs, 8-ohm load, 1k-ohm input termination, Lch/Rch
  • Volume control at maximum
    • wideband: 2.49mV, -61.1dBW / 1.20mV, -67.5dBW
    • A weighted: 0.46mV, -75.6dBW / 0.33mV, -78.7dBW
  • Volume control at reference setting
    • wideband: 2.06mV, -62.8dBW / 1.07mV, -68.4dBW
    • A weighted: 0.40mV, -77.0dBW / 0.27mV, -80.4dBW
  • Volume control full counterclockwise
    • wideband: 2.32mV, -61.7dBW / 1.19mV, -67.5dBW
    • A weighted: 0.38mV, -77.4dBW / 0.25mV, -81.1dBW
• Output noise, balanced inputs, 8-ohm load, 1k-ohm input termination, Lch/Rch
  • Volume control at maximum
    • wideband: 2.59mV, -60.8dBW / 1.39mV, -66.2dBW
    • A weighted: 0.48mV, -75.4dBW / 0.34mV, -78.4dBW
  • Volume control at reference setting
    • wideband: 2.22mV, -62.1dBW / 1.20mV, -67.5dBW
    • A weighted: 0.42mV, -76.6dBW / 0.30mV, -79.5dBW
  • Volume control full counterclockwise
    • wideband: 2.35mV, -61.6dBW / 1.24mV, -67.2dBW
    • A weighted: 0.40mV, -77.0dBW / 0.26mV, -80.7dBW

Power output with 1kHz test signal

• 8-ohm load at 1% THD: 35.9W
• 8-ohm load at 10% THD: 45.5W
  
  
• 4-ohm load at 1% THD: 24.0W
• 4-ohm load at 10% THD: 38.9W

General

The JE Audio VS70 is a medium-power stereo tube power amplifier. Although not an integrated amplifier per se, its front-panel volume control allows adjustment with a single source.

Chart 1 shows the frequency response of the amp with varying loads. This plot was made with the reference volume-control position set for 0.5V input to produce 5W output into an 8-ohm load. The high-frequency response is moderately wide with an approximate -3dB point of about 40kHz. High-frequency roll-off is nicely controlled for all loading conditions. Output impedance as judged by the closeness of spacing between the curves of open-circuit, 8-ohm, and 4-ohm loading is typical of many tube amplifiers. Variation in output as a function of frequency with the NHT dummy speaker load is a little less than +/- 2dB -- an amount that could be quite audible with some speakers. The frequency response of this unit was little affected by volume-control setting. Tracking between channels was within about 0.5dB from full up to -50dB of attenuation.

Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. This design is optimized for an impedance close to the output terminal value (i.e., 8 ohms on the 8-ohm output and 4 ohms on the 4-ohm output), as half loading reduces the power output at 1% THD+N considerably. Distortion is not particularly low with this design despite the owner’s manual claims about "unique balanced circuitry."

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3 for 4-ohm loading. The amount of rise in distortion at high frequencies is moderate and a good thing, but there is quite a rise at low frequencies at powers much greater than a few watts.

Damping factor vs. frequency is shown in Chart 4 and is of a value and nature typical of many tube power amplifiers, being relatively low and uniform over most of the audio range.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal into 8 ohms is plotted in Chart 5. The magnitudes of the AC-line harmonics are fairly high and complex in nature. Further, intermodulation components of line harmonics with signal harmonics are also moderately prominent. Signal harmonics consist of a complex spectrum of even and odd harmonics, but with the second and third harmonics being dominant.

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: 8-ohm THD+N
Bottom line: 4-ohm THD+N

4-ohm output loading
Green line: 35W
Cyan: 25W
Blue line: 15W
Magenta line: 5W
Red line: 1W

Damping factor = output impedance divided into 8

1kHz signal at 10W into an 8-ohm load

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