Solid State Amps
Most solid state amps have a very clean and fast response time in the amplification but also a
sharp edge of going into distortion when reaching its maximum performance level. The distortion
created when the amp reaches its clipping point is very rarely desirable and most users prefer to
not ever experience that in their amp. Furthermore quite a few transistor designs have the
unfortunate feature of ‘output stage lockup’ which in essence means that when the amp goes into
distortion, the output circuit itself gets stuck in the distorting mode for several hundred
milliseconds longer than the original audio impulse i.e. the transistor circuit adds more distortion
than the audio signal itself created in the first place.
With the dynamics of bass signals in mind, this means that a transistor amp has to be able to
cleanly amplify very high peaks in order to reach a decent perceived loudness or power level
without distorting.
In the example below (fig.3), the 800w amp plays back the transients or peaks cleanly at its max.,
but because of the high crest factor of bass signals, the average level and power is more likely to
be perceived as 300w by the listener.
Figure 3: Transistor: Actual vs. Perceived power
Tube Amps
The nature of tube amps is very different from that of transistor amps. Rather than suddenly
hitting the distortion point, tubes gradually go into saturation and tend to ‘round off’ signals in a
very soft way when getting to its maximum performance. The overdrive created by tube amps is
quite pleasant to our ears and is known by many as a central part of the highly complex ‘tube
sound’ that have been pursued in many bass amp designs.
Again, looking at bass signals, the gradual saturation of tubes essentially ‘compresses’ or limits
the high peaks of the bass with only little resulting overdrive thereby altering the original crest
factor to become quite a bit lower than the original signal with a consequently higher perceived
loudness at a lower power rating – this is a quite known effect by many users, usually referred to
as the difference between ‘transistor watts’ and ‘tube watts’ (fig. 4)
In the example below the 400w tube amp has reduced the peaks, and hereby the crest factor, of
the original bass signal, achieving a perceived level and power of 300w.
This means that the 400w tube amp and 800w transistor amp are perceived to have the same
power even though they are vastly different in the actual power measurable.
