D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
12
Over the years much has changed in the world of electronic components. Were the necessary
par
ts still a
v
ailable? I found out that they were (though not necessarily cheap) and, in many
cases, they were vastly better than the components available back in the age of vacuum tubes.
Carbon resistors could be replaced with quieter metal film types. Sonically superior poly-
styrene and polypropylene capacitors were preferable to the old paper types. The power sup-
plies could be solid st
ate — and easily regulated. Electrolytic filter capacitors were smaller.
The only par
ts t
hat remained to be found were top quality audio transformers that matched
the tube input and output impedances.
A call to the great folks at legendary Jensen Transformers revealed that not only were the
necessary transformers still available but that they were orders of magnitude superior to the
technology of t
he ‘50s and ‘60s.
A couple of months research into the classic tube mic preamp designs gave me a good idea
of how to proceed. A breadboard prototype was constructed and tested, and it worked great!
(Although the open construction resulted in some RFI; while experimenting with different
com
ponent v
alues one night with a pair of headphones on the output, I heard a half hour of
Radio Havana coming through weakly but clearly.) Professional quality specs on frequency
r
esponse, distortion, noise, phase shift, and so on were definitely attainable.
Now it was necessary to squeeze the last dB of performance out of the circuit. Computer
cir
cuit analysis was one tool not available to the designers of the original equipment, and it
was amazing how careful manipulation of values could make a significant improvement in
per
formance.
The next prototype was built and its performance was even better, largely because of bet-
ter shielding and a better la
yout. This one became the testing ground for additional experi-
mentation. There is not a single component in that prototype that hasn’t been changed in an
attempt to improve performance. Some parts of the circuit have been through dozens of iter-
ations. Moder
n test equipment can quantify and graph parameters t
hat had not even been
discovered back in the heyday of “hollow state.”
This pr
ototype became m
y preamp of choice for all my recording. I used it (and another
one I built soon after) for a year of location recording, mostly of classical and choral music,
but also f
or studio sessions. Although I have some very fine commercial and homebuilt mix-
ers, after using the tube preamp, I just couldn’t bring myself to use the solid state preamps
anymore.
Wh
y do tubes sound better? All properly designed audio amplifier circuits exhibit low distor-
tion t
hroughout their operating amplitude range. The difference in sound is particularly evi-
dent when the circuit runs out of headroom. Solid state devices tend to abruptly transition
fr
om lo
w distortion to extreme distortion (clipping). This is a good trait, since when operat-
ed right up to t
heir maximum level solid state amplifiers can maintain excellent performance.
Digit
al audio circuits have similar characteristics.
Vacuum tube circuits, on the other hand, show a gradual increase in distortion throughout
t
heir operating rang
e. But instead of an abrupt break, the distortion increases incremental-
ly
. Until a level is reached where something in the circuit just completely falls apart (e.g. a
transf
ormer saturates), the sound retains most of its original quality.
It’s the nature of the distortion that makes a difference, too. Solid state circuits run out of
headr
oom when t
he output voltage exceeds the power supply voltage. The result at this point
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