27
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.
