A HIGH-PRECISION PHONO PREAMPLIFIER
by Fred Nachbaur, Dogstar Music ©1998, 2001
1: INTRODUCTION
There has been much press lately about the merits (and drawbacks) of the venerable vacuum
tube. How much of this retro movement is based in demonstrable principles, and how much is
rooted in nostalgia or subjectivity is a debate that could fill volumes.
What is clear is that there is considerable renewed interest in vacuum tubes, a technology
that even two decades ago was considered as obsolete as spats and top hats. Now the trend is
reversing, and a number of manufacturers are again supplying tube gear for audiophiles,
musicians and hobbyists. In many cases, these are simply vintage circuits in new packaging.
Other products offer new approaches to vacuum tube technology, adding what we've learned in
the meantime to come up with some truly noteworthy designs. The project described here fits into
the latter category.
Why use tubes anyway?
The central argument for the pro-tube movement is that specs can be almost meaningless, and
that what counts is how it sounds to the individual listener. Highly subjective descriptions are
therefore used, instead of the techno-babble we've more-or-less gotten used to in recent times.
The opposite camp claims that numbers don't lie, and that you can't improve something by
adding distortion of any kind.
Both camps have valid points. Most of us have heard expensive gear with spectacular
specifications, but were left cold by the "too good to be true," almost clinical sound of such
equipment. Similarly, most people will agree that just because something has tubes in it doesn't
make it worth listening to. Turn on one of those old "All-American Five" table-top AM radios if you
want a striking demonstration of just how bad vacuum tube equipment can sound.
Perhaps a way of reconciling the two viewpoints is to consider the distinction between musical
equipment, and reproduction equipment. For musical gear, the individual frequency, waveform
and phase distortions are part of what defines the sound of, say, a Fender tube amp. Just as no-
one would try to define a fine Stradivarius violin with specs and distortion figures, so also it would
be specious to argue that a certain tube guitar amp has over 12% THD (total harmonic distortion)
at 35 watts.
Reproduction equipment, on the other hand, has always been expected to give a perfect rendition
of the signal applied to it. Sounds good in principle; if such a thing existed, we should be able to
exactly and perfectly reproduce everything from a grundge band to the New York Philharmonic,
making the reproductions indistinguishable from the original performances.
But therein lies the rub. The overall sound of a stereo system depends so heavily on the room it's
in, the speakers, volume level, personal preference, and a host of other fuzzy variables that a
perfect reproduction system cannot be said to exist even in this day and age, and probably never
will. Add the fact that most if not all recordings are electronically sweetened to some degree to
make them "sound good" (as opposed to being an exact copy of the performance), and arguments
for the clinical reproduction approach lose credibility.
Carried to extremes, an "ideal" reproduction system wouldn't even have any controls, except
perhaps for selecting inputs. If you think about it, personal preference is the only reason why
stereos have volume controls, equalizers, and other adjustments to let us customize the sound to
suit our very individual ears and brains.
It might be best to view the reproduction gear as a continuation of the same process that started
with the construction of the instruments used in the performance. What we ultimately hear is the
sum total effect of everything from that original instrument design, to the way it is played by the
artist, through the entire recording, mixing and distribution process, to the gear we use to play it,
and how we've set its controls.
There is, therefore, a considerable difference in design approach between the "instrument" and
"reproduction" categories. An earlier design ("The Real McTube") documented a vacuum tube
pre-amplifier for use as an adjunct to electric instruments. The design approach was largely
empirical, and the emphasis was on highlighting the unique distortion characteristics of the
vacuum tube. This project explores the reproduction aspect; the design approach was quite
mathematical and precise, and the emphasis is on controlling the characteristics of the vacuum
tube. The maths used bear a striking resemblance to approaches used in designing solid-state
gear; we're changing the active devices from transistors back to tubes, while retaining the same
careful design techniques.
Vinyl vs. CD
Similar arguments are ongoing regarding vinyl records vs. compact discs. The CD camp points at
the CD's accuracy, definition, and clarity, while vinyl lovers bemoan the CD's lack of warmth and
claim that conventional records sound more "natural."
The waters are muddied by the existence of albums reissued on CD that simply sound bad. Part of
the reason is that master tapes were often "tweaked" (especially by un-naturally boosting the
highs) to compensate for the limitations of the vinyl medium; when such tapes are used to make
CDs, the result is often too bright and brassy. The better re-issues will usually have a notice to
the effect of "re-mastered for CD from the original recordings" as an indication that the engineers
took the trouble to adjust the message to the new medium.
In any event, this project gives you the opportunity to explore these subjective and controversial
debates, letting your own ears be the judge. You may find that there are some recordings that
sounds better through the tube pre-amp, and others that benefit from the improved definition
(whatever that is, technically speaking) that good solid-state gear can offer.
