55
High
Frequency
Response
Probably, from the sonic quality point of view, the most important
improvement is the power MOSFETs vastly superior high frequency
response. A large proportion of the power transistors used in modern hi-fi
amplifiers start to show a decline in efficiency from 10kHz upwards. The
efficiency of the power MOSFET does not start to decline until about
2MHz and is only down 3dB at 30MHz. This is due to the energy transfer
being accomplished with minority charged carriers in the power MOSFET
as opposed to majority charged carriers within the transistor, and results in
hole storage at high frequencies causing the transistor to dissipate
increasing amounts of energy within itself as the frequency increases.
Further sonic degradation of the transistor power amplifier occurs due to
hole storage of the output transistors. As the output distortion increases
with increasing signal frequency, it is obvious that the distortion products
in the negative feedback path also increase.
Because the negative feedback system is employed to reduce distortion by
cancellation, at high frequencies it causes even more power to be
consumed within the output transistor just to cancel out the distortion.
Transient intermodulation (TIM) is also more prevalent in transistor power
amplifiers because the signal transition in time is relatively slow. This
means the distortion products in the signal of, say, a fast transient will not
travel through the negative feedback system into the output stage fast
enough to cancel at exactly 180 degrees out of phase – resulting in the
amplifier being overloaded. This is not possible in power MOSFET
amplifiers.
Other
Advantages
Further sonic improvement is achieved in power MOSFET amplifiers due
to reduced crossover distortion, as power MOSFETs have a sharper “knee”
than transistors at cut-off and provide a greater linearity when crossing
over from one device to the other. Because crossover distortion is a major
cause of odd order harmonic distortion in transistor amplifiers (be it small,
i.e. 0.05% total) they are usually considered to sound more harsh than
valve amplifiers which generally have large amounts of even order
harmonic distortion up to 5% and are thought to sound more pleasant and
musical.
However, which is more accurate? The valve amplifier at 5% THD with a
pleasant sound and even order harmonics; the transistor amplifier with
0.05% THD with relatively unpleasant sound with even and odd harmonic
output, or a power MOSFET amplifier with 0.02% THD and relatively
pleasant even order harmonic distortion? In our opinion, the power
MOSFET amplifier because the THD generated is virtually all second or
even order harmonic distortion total 0.02% or less at 20kHz and down to
0.004% or less at 1kHz.
It can be seen that power MOSFETs are here to stay and that there are major
sonic and electrical improvements to be had over other output devices.
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