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, 8ohm load, unbalanced input, 1kohm 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
 8ohm load at 1% THD (ultralinear): 30W
 8ohm load at 1% THD (triode): 12W
 8ohm load at 10% THD (ultralinear): 38W
 8ohm load at 10% THD (triode): 23W
 4ohm load at 1% THD (ultralinear): 25W
 4ohm load at 1% THD (triode): 8W
 4ohm load at 10% THD (ultralinear): 40W
 4ohm load at 10% THD (triode): 25W
General
The Manley Mahi is a lowtomediumpower tube amplifier
utilizing two pairs of EL84 output tubes operated in pushpull parallel. The design is
quite flexible as it has two toggle switches that change the outputtube 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 outputstage
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, 8ohm, and 4ohm 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, theabovetheaudiorange
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 8ohm loads. In chart 2B, the attainable
power is slightly higher for the 4ohm 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 ACline harmonics is typical for many amplifiers, and
intermodulation components of line harmonics with signal harmonics are also visible around
the nulledout fundamental signal frequency and the lower harmonics. The signalharmonic
spectrum tails off reasonably rapidly.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
Ultralinear  Maximum
Feedback
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Cyan line: NHT dummyspeaker load
Triode  Minimum Feedback
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Cyan line: NHT dummyspeaker load
Chart 2  Distortion as a Function
of Power Output and Output Loading 
Ultralinear  Maximum
Feedback
(line up at 10W to determine lines)
Top line: 4ohm SMPTE IM
Second line: 8ohm SMPTE IM
Third line: 4ohm THD+N
Bottom line: 8ohm THD+N
Triode  Minimum Feedback
(line up at 1W to determine lines)
Top line: 4ohm SMPTE IM
Second line: 8ohm SMPTE IM
Third line: 4ohm THD+N
Bottom line: 8ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
Ultralinear  Maximum
Feedback
4ohm output loading
Cyan line: 30W
Blue line: 20W
Magenta line: 5W
Red line: 1W
Triode  Minimum Feedback
4ohm 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
Damping factor = output impedance divided into 8
Triode  Minimum Feedback
Damping factor = output impedance divided into 8
Chart 5  Distortion and
Noise Spectrum 
Ultralinear  Maximum
Feedback
1kHz signal at 10W into an 8ohm load
Triode  Minimum Feedback
1kHz signal at 10W into an 8ohm load
