November 2002

Manley Laboratories Snapper Mono Amplifiers: 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.
• Output tube plate current adjusted to 30mA per tube when warmed up.
• Power output and distortion plotted with both channels driven.
• Gain, unbalanced input/balanced input: 36.4x, 31.2dB/19.2x, 25.7dB.
• Output noise, 8-ohm load, unbalanced input, 1-kohm input termination: wideband 0.6mV, -73.5dBW; A weighted 0.13mV, -86.7dBW.
• AC line current draw at idle: 1.4A.
• Output impedance at 50Hz: 1.8 ohms.
• This amplifier does not invert polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD: 60W
• 8-ohm load at 10% THD: 120W
  
  
• 4-ohm load at 1% THD: 10W
• 4-ohm load at 10% THD: 150W
  
  
• 16-ohm load at 1% THD: 53W
• 16-ohm load at 10% THD: 82W

General

This amp is a push-pull design rated at a nominal 100W. Bias is set by convenient test points and adjustable controls on the top part of the chassis. The idling current as received when warmed up was quite uniform between the four output tubes at about 30mA (0.3V across assumed 10-ohm resistors) and was not adjusted for the measurements.

Measurements were made using the unbalanced input. It was found that results with the balanced inputs were virtually the same. Frequency response, as seen in Chart 1, is beautifully controlled in the high-frequency end as a function of load. The low-frequency response holds up down to 10Hz nicely at the 1W level of the test. Output impedance is typical of many tube amplifiers giving an approximate +/-1dB frequency-response variation with the NHT dummy-speaker load. Total Harmonic distortion plus noise as a function of power output and load for a test frequency of 1kHz is plotted in Chart 2. Also shown in this chart is the SMPTE IM distortion for an 8-ohm load. Not having separate 4- and 8-ohm outputs available, this amp is clearly designed for an optimum load lower than 8 ohm, most likely around 6 ohms. This can be seen as the attainable output power is more like 140-150W into 4 ohms. As a result of this, considerably less power is available into 16-ohm loads. Total harmonic distortion plus noise as a function of frequency at several power levels is plotted in Chart 3 for an 8-ohm load. Amount of distortion over the main midrange energy band is less than 1% for power outputs of 30W or less. Admirable is the relatively low amount of distortion increase at the higher frequencies. However, distortion does rise considerably below 20Hz at higher power levels. Still, this is very good performance indicating a good output transformer design (as was the nicely controlled high-frequency response). Damping factor vs. frequency referred to an 8-ohm load is plotted in Chart 4 and is approximately 4.5 over most of the audio range. In the spectral plot of distortion and noise for a 10W 1kHz signal into an 8-ohm load on the 8-ohm output, the signal distortion components are dominated by the second and third harmonics with higher-order harmonics at reduced and decreasing amplitude with frequency. There is quite a bit of hum modulation around the suppressed fundamental 1kHz test frequency.

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

(line up at 5W to determine lines)
Top line: 8-ohm SMPTE IM
Second line: 4-ohm THD+N
Third line: 8-ohm THD+N
Bottom line: 16-ohm THD+N

8-ohm output loading
Cyan line: 100W
Blue line: 30W
Magenta line: 10W
Red line: 1W

Damping factor = output impedance divided into 8

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

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