April 2008

JAS Audio Bravo 2.3 Integrated 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.
• This integrated amplifier does not invert polarity.
• AC line current draw: 2.5A.
• Input impedance @ 1kHz: 35.0k ohms
• Output impedance at 50Hz (8-ohm output)
• Minimum feedback: 5 ohms
• Maximum feedback: 1.7 ohms
• Gain, output voltage divided by input voltage, volume at maximum, Lch/Rch
• Minimum feedback: 14.5mV, 23.2dBW / 17.3mV, 24.8 dBW
• Maximum feedback: 6.64mV, 16.4dBW / 7.03mV, 16.9 dB
• Output noise, 8-ohm load, 1k-ohm input termination, Lch/Rch

  Minimum feedback

  • Volume control at reference position
    • wideband: 3.36mV, -58.5dBW / 4.29mV, -56.4dBW
    • A weighted: 0.858mV, -70.4dBW / 1.17mV, -67.7dBW
  • Volume control full clockwise
    • wideband: 1.72mV, -64.3dBW / 2.29mV, -61.8dBW
    • A weighted: 0.479mV, -75.4dBW / 0.606mV, -73.4dBW
  • Volume control full counterclockwise
    • wideband: 1.06mV, -68.5dBW / 1.37mV, -66.3dBW
    • A weighted: 0.264mV, -80.9dBW / 0.320mV, -78.9dBW

  Maximum feedback

  • Volume control at reference position
    • wideband: 1.51mV, -65.5dBW / 1.81mV, -63.9dBW
    • A weighted: 0.383mV, -77.4dBW / 0.473mV, -75.5dBW
  • Volume control full clockwise
    • wideband: 0.549mV, -74.2dBW / 0.612mV, -73.3dBW
    • A weighted: 0.185mV, -83.7dBW / 0.204mV, -82.8dBW
  • Volume control full counterclockwise
    • wideband: 0.515mV, -74.8dBW / 0.573mV, -73.9dBW
    • A weighted: 0.123mV, -87.2dBW / 0.130mV, -86.8dBW

Power output with 1kHz test signal - minimum feedback

• 8-ohm load at 1% THD: 0.94W
• 8-ohm load at 10% THD: 17.3W
  
  
• 4-ohm load at 1% THD: 120mW
• 4-ohm load at 10% THD: 5.3W

Power output with 1kHz test signal - maximum feedback

• 8-ohm load at 1% THD: 5.0W
• 8-ohm load at 10% THD: 19.3W
  
  
• 4-ohm load at 1% THD: 0.21W
• 4-ohm load at 10% THD: 6.9W

General

The Bravo 2.3 is a single-ended, tube-based design utilizing one of the large Russian 6C33 carbon-plate triodes for the output tube in each channel. The first-stage tube looks to be an octal-based pentode type that I haven’t seen before. A GZ34 rectifier is used for each channel. Since the design has a front-panel negative-feedback control, measurements were made for the extreme positions of this control -- minimum and maximum feedback.

Charts 1A and 1B show the frequency response of the amp with varying loads. As can be seen in chart 1A, with minimum feedback the output impedance, as judged by the distance of spacing between the curves of open-circuit, 8-ohm, and 4-ohm loading, is fairly high, with the result that there is likely to be significant coloration with speakers with widely varying impedance curves. The variation with the NHT dummy speaker load is in the range of +2dB to -3dB over the audio range. With the feedback control set to counterclockwise (maximum feedback), the measurements in chart 1B are more reasonable and more in line with the measurements of many tube amplifiers. Frequency response as a function of volume-control setting was essentially unchanged from full up to 50dB of attenuation. Volume-control tracking between channels was within 1dB down to 60dB of attenuation.

Charts 2A and 2B illustrate how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals at 4- and 8-ohm loads. The attainable maximum power, generally rated at 10% distortion, is greatest with an 8-ohm load on the 8-ohm output connection. The reduced power and higher distortion with 4-ohm loading on the 8-ohm output would not be the case when a 4-ohm load is driven from the 4-ohm output. Then the performance would be essentially like the 8-ohm loading on the 8-ohm output. Not tested, 16-ohm loading on the 8-ohm output would likely yield less power but with lower distortion than either 8- or 4-ohm loading.

Total harmonic distortion plus noise as a function of frequency at several different power levels for 8-ohm loading on the 8-ohm output is plotted in Charts 3A and 3B. As is typical, distortion rises at both ends of the audio range and noticeably more so with maximum negative feedback, as in Chart 3B.

Damping factor vs. frequency as measured on the 8-ohm output is shown in Chart 4. Here, with maximum feedback, the damping factor is highest but with more variation over the audio range.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Charts 5A and 5B. The amount of AC-line harmonics, especially the 120Hz component, are rather high. As shown in the output-noise measurements, hum would likely be audible with high-sensitivity speakers. The decay of the signal harmonics with increasing order is more uniform in Chart 5B with the maximum-feedback condition.

A - Minimum Feedback

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

B - Maximum Feedback

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

A - Minimum Feedback

(line up at 1W 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

B - Maximum Feedback

(line up at 1W 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

A - Minimum Feedback

8-ohm output loading
Green line: 15W
Cyan line: 10W
Blue line: 3W
Magenta line: 1W
Red line: 0.3W

B - Maximum Feedback

8-ohm output loading
Green line: 15W
Cyan line: 10W
Blue line: 3W
Magenta line: 1W
Red line: 0.3W

Damping factor = output impedance divided into 8
Red line: maximum feedback
Magenta line: minimum feedback

A - Minimum Feedback

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

B - Maximum Feedback

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

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