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.
Chart 1
- Frequency Response of Output Voltage as a Function of Output Loading |
Ultralinear - Maximum
Feedback
![](chart1_a.gif)
Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load
Cyan line: NHT dummy-speaker load
Triode - Minimum Feedback
![](chart1_b.gif)
Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load
Cyan line: NHT dummy-speaker load
Chart 2 - Distortion as a Function
of Power Output and Output Loading |
Ultralinear - Maximum
Feedback
![](chart2_a.gif)
(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
![](chart2_b.gif)
(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
Chart 3 - Distortion
as a Function of Power Output and Frequency |
Ultralinear - Maximum
Feedback
![](chart3_a.gif)
4-ohm output loading
Cyan line: 30W
Blue line: 20W
Magenta line: 5W
Red line: 1W
Triode - Minimum Feedback
![](chart3_b.gif)
4-ohm output loading
Cyan line: 15W
Blue line: 10W
Magenta line: 5W
Red line: 1W
Chart 4 - Damping Factor
as a Function of Frequency |
Ultralinear - Maximum
Feedback
![](chart4_a.gif)
Damping factor = output impedance divided into 8
Triode - Minimum Feedback
![](chart4_b.gif)
Damping factor = output impedance divided into 8
Chart 5 - Distortion and
Noise Spectrum |
Ultralinear - Maximum
Feedback
![](chart5_a.gif)
1kHz signal at 10W into an 8-ohm load
Triode - Minimum Feedback
![](chart5_b.gif)
1kHz signal at 10W into an 8-ohm load
|