September 2005

Manley Laboratories Mahi 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 and one channel driven (this is a mono amplifier).
• Gain
  • Ultralinear: 28.9x, 29.5dB.
  • Triode: 52.3x, 34.4dB.
• Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination
  • Ultralinear: wideband 1.68mV, -64.5dBW; A weighted 0.345mV, -78.30dBW.
  • Triode: wideband 3.02mV, -59.4dBW; A weighted 0.626mV, -73.1dBW.
• AC line current draw at idle: 0.8A.
• Output impedance at 50Hz
  • Ultralinear: 1.4 ohms.
  • Triode: 3.0 ohms.
• This amplifier does not invert polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD (ultralinear): 30W
• 8-ohm load at 1% THD (triode): 12W
• 8-ohm load at 10% THD (ultralinear): 38W
• 8-ohm load at 10% THD (triode): 23W
  
  
• 4-ohm load at 1% THD (ultralinear): 25W
• 4-ohm load at 1% THD (triode): 8W
• 4-ohm load at 10% THD (ultralinear): 40W
• 4-ohm load at 10% THD (triode): 25W

General

The Manley Mahi is a low-to-medium-power tube amplifier utilizing two pairs of EL84 output tubes operated in push-pull parallel. The design is quite flexible as it has two toggle switches that change the output-tube operating mode from ultralinear to triode, and change the amount of negative feedback in three steps. Rated power is 40W in ultralinear mode and 20W in triode mode.

Because there are really six combinations of output-stage mode and amount of feedback, I am going to give characteristics for two extremes: ultralinear with maximum feedback and triode with minimum feedback.

Chart 1A shows the frequency response of the amp in ultralinear mode with varying loads. As can be seen, the output impedance, as judged by the closeness of spacing between the curves of open circuit, 8-ohm, and 4-ohm loading, is of a relatively low value for a tube amplifier. The variation with the NHT dummy load in the audio range is of the order of +/-1dB. Chart 1B illustrates the frequency response in triode mode with minimum feedback. Here it can be seen that the spacing of the curves is much greater with consequent lower damping factor. Further, the-above-the-audio-range peaking of the ultralinear output is absent in the triode output. Not shown is the fact that the relative curve spacing is about the same for ultralinear and triode for the three different amounts of feedback.

Chart 2A shows how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. In ultralinear, the attainable power is about the same for 4- and 8-ohm loads. In chart 2B, the attainable power is slightly higher for the 4-ohm load.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in chart 3A for ultralinear mode. Amount of rise in distortion at low and high frequencies is reasonable and typical for a tube power amplifier. In chart 3B for triode, the amount of distortion is generally similar to that of the ultralinear output except the amount of rise at high frequencies is a bit greater.

Damping factor vs. frequency is shown in chart 4A and 4B for ultralinear and triode, respectively.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in chart 5A and chart 5B for ultralinear and triode modes. The magnitude of the AC-line harmonics is typical for many amplifiers, and intermodulation components of line harmonics with signal harmonics are also visible around the nulled-out fundamental signal frequency and the lower harmonics. The signal-harmonic spectrum tails off reasonably rapidly.

Ultralinear - Maximum Feedback

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

Triode - Minimum Feedback

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

Ultralinear - Maximum Feedback

(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

Triode - 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

Ultralinear - Maximum Feedback

4-ohm output loading
Cyan line: 30W
Blue line: 20W
Magenta line: 5W
Red line: 1W

Triode - Minimum Feedback

4-ohm output loading
Cyan line: 15W
Blue line: 10W
Magenta line: 5W
Red line: 1W

Ultralinear - Maximum Feedback

Damping factor = output impedance divided into 8

Triode - Minimum Feedback

Damping factor = output impedance divided into 8

Ultralinear - Maximum Feedback

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

Triode - Minimum Feedback

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

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