Tuesday, September 19, 2017

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All amplifier measurements are performed independently by BHK Labs. All measurement data and graphical information displayed below are the property of the SoundStage! Network and Schneider Publishing Inc. Reproduction in any format is not permitted.

Note: Measurements were made at 120V AC line voltage with both channels being driven. Measurements made on left channel through the balanced inputs and 8-ohm outputs unless otherwise noted.

Power output

  • Output power at 1% THD+N: 26.0W @ 8 ohms, 15.0W @ 4 ohms
  • Output power at 10% THD+N: 40.0W @ 8 ohms, 32.0W @ 4 ohms

Additional data

  • This amplifier does not invert polarity.
  • AC-line current draw at idle: 1.42A, 0.89PF, 151W
  • Gain: output voltage divided by input voltage, volume at maximum, Lch/Rch: 18.61X, 25.4dB:
    •      Balanced inputs: 39.75X, 32.0dB / 40.31X, 32.1dB
    •      Unbalanced inputs: 50.32X, 34.0dB / 52.18X, 34.4dB
  • Input sensitivity for 1W output into 8 ohms, volume at maximum, Lch/Rch:
    •      Balanced inputs: 0.071mV / 0.70mV
    •      Unbalanced inputs: 0.56mV / 0.54mV
  • Output impedance @ 50Hz: 2.15 ohms
  • Input impedance @ 1kHz: 50k ohms
  • Output noise, 8-ohm load, balanced inputs, termination 600 ohms, Lch/Rch
    •      Volume control at reference position
                Wideband: 0.89 mV, -70.0dBW / 4.88mV, -55.3dBW
                A weighted: 0.21 mV, -82.6dBW / 0.70 mV, -72.1dBW
    •      Volume control full clockwise
                Wideband: 1.11mV, -68.2dBW / 5.08mV, -54.9dBW
                A weighted: 0.28mV, -80.1dBW / 0.71mV, -72.0dBW
    •      Volume control full counterclockwise
                Wideband: 0.89mV, -70.0dBW / 4.89 mV, -55.2 dBW
                A weighted: 0.18mV, -83.9dBW / 0.68 mV, -72.4 dBW
  • Output noise, 8-ohm load, unbalanced inputs, termination 1k ohm, Lch/Rch
    •      Volume control at reference position
                Wideband: 1.12mV, -68.1dBW / 5.09mV, -54.9dBW
                A weighted: 0.20mV, -83.0dBW / 0.74mV, -71.6dBW
    •      Volume control full clockwise
                Wideband: 1.59mV, -65.0dBW / 4.97mV, -55.1dBW
                A weighted: 0.25mV, -81.1dBW / 0.73mV, -71.8dBW
    •      Volume control full counterclockwise
                Wideband: 0.95mV, -69.5dBW / 5.32mV, -54.5dBW
                A weighted: 0.19 mV, -83.5 dBW / 0.72 mV, -71.9 dBW

Measurements summary

The JE Audio VS70.1 is a medium-power tubed power amplifier with a front-panel volume control. Its overall gain is 6-7dB over the "nominal" power gain of 26dB. However, the extra gain would be useful for low-output CDs, such as Reference Recordings’ HRx discs.

Chart 1 shows the VS70.1’s frequency response with varying loads. This was made at the usual 1W/8-ohm level, with the volume control turned fully clockwise. The frequency-response variation with the NHT dummy load is of the order of ±1.5dB. The high-frequency rolloff is nicely controlled in that its shape is quite independent of load.

With the reference condition set up for measuring integrated amplifiers -- i.e., volume control set for 5W output into 8 ohms for 500mV input -- another response was run, and is shown in Chart 1A. Here we see that the HF response is a little more extended, and the very low-frequency level is starting to roll off because of output-transformer limitations. Channel tracking as a function of volume-control position down from this reference condition was generally within ±1dB down to 50dB of attenuation, below which hum, noise, and capacitive coupling altered the response and the channel separation.

Chart 2 illustrates how total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. It is apparent that the power is maximized for the 8-ohm load, whereas the power for a 4-ohm load is lower at higher distortion. Further, the amounts of intermodulation distortion are rather excessive at the higher power levels.

Chart 3 plots THD+N as a function of frequency for 8-ohm loading at several different power levels. The rise in distortion at higher frequencies is reasonably low. Because of output-transformer characteristics, low-frequency distortion gets progressively worse as the bass frequency rises and the power level increases.

The VS70.1’s damping factor vs. frequency (Chart 4) is fairly low, but not unusually so for a tubed power amplifier. What is good is that it is quite flat with frequency.

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 quite high and complex, as are the signal harmonics. Intermodulation components of line harmonics with signal harmonics are numerous, and can be seen at the baseline of the signal harmonics.

Chart 1 - Frequency response of output voltage as a function of output loading

Chart 1

(Volume control fully clockwise)
Red line = open circuit
Magenta line = 8-ohm load
Blue line = 4-ohm load
Cyan line = NHT dummy load

Chart 1A (Additional Measurement) - Frequency response with volume control at reference position (8-ohm load)

Chart 1A

Red line = left channel
Blue line = right channel

Chart 2 - Distortion as a function of power output and output loading

Chart 2 

(Line up at 20W to determine lines)
Top line = 4-ohm SMPTE IM distortion
Second line = 8-ohm SMPTE IM distortion
Third line = 4-ohm THD+N
Bottom line = 8-ohm THD+N

Chart 3 - Distortion as a function of power output and frequency

Chart 3

(8-ohm loading)
Red line = 1W
Magenta line = 5W
Blue line = 15W
Cyan line = 30W
Green line = 35W

Chart 4 - Damping factor as a function of frequency

Chart 4

Damping factor = output impedance divided into 8

Chart 5 - Distortion and noise spectrum

Chart 5

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

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