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thematt, on Mar 25 2007, 08:33 AM, said:
Would buying a solid state, and attaching a tube preamp (such as the Milbert or the TruTech) produce a similar sort of sound to a full tube, but without the downsides??
I'll try to answer this question as briefly as possible.
The characteristic "valve sound" has a rich and very clear midrange. This is produced by more 2nd order harmonic distortion, and by rolling off the trebles and bass somewhat. Also, because valve circuits are relatively simple, there is less electronics in the way which produces a cleaner sound. A well designed valve amp will roll off the frequency extremes somewhat less, but still produce a harmonic distortion spectrum that favours low order harmonics, especially even order harmonics. The exact harmonic spectra will depend on circuit topology, whether negative feedback (NFB) is used, etc.
OTOH, the "transistor sound" is linear from top to bottom. However, because transistors are easily saturated and because they are such nonlinear devices, more complex circuitry with higher amounts of NFB are required. This has the effect of cancelling out low order harmonics but pushing up more high order harmonics (5th, 7th, etc). Because most THD ratings only count the first 5-7 harmonics (and ignore the higher order harmonics), transistor amps quote lower THD ratings. Also, the presence of large amounts of NFB cause the amp to tolerate clipping poorly. Because of the complex circuitry involved, poorly designed transistor amps often sound less transparent. Presence of high order harmonics can make them sound harsh, lack of 2nd order harmonics make them sound sterile, and the poor clipping performance means that transistor amps require a LOT of power headroom to ensure that they never clip.
Note that all my comments so far are only restricted to Class A amps. Class B amps are subject to crossover distortion, which can never be completely eliminated. This again creates a lot of high order harmonics which contributes to the harsh sound. Furthermore, crossover distortion is not related to gain - the same amount of crossover distortion is present whether played at low gain or high gain. THD measurements are typically taken with the amp at full power - meaning that as a proportion to its total output, crossover distortion is relatively low - which is another trick the designers use to pass their amps off as having low THD. But in normal listening, you are sure to hear the crossover distortion.
For a full discussion as to the effect of harmonic distortion and sound quality, read this article I posted on our sister website, stereo.net.au.
To answer your question, I am suspicious of those little tube preamps. A tube circuit needs a good rectifier stage (preferably valve). It needs high voltages to make those electrons jump the vaccuum gap. I have my doubts if it is possible to miniaturize the bulky components to fit into a single DIN slot and run at pure Class A with minimal NFB. If my suspicion is correct (that these designs have too many engineering compromises), then it would colour the sound like a bad valve preamp - presenting you with a sound rich in 2nd order harmonics, with the impression of a fat midrange produced by rolling off the highs and lows.
