D.W. Fearn VT-1 Operating instructions manual - page 12

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

parts still available?  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 state — and easily regulated. Electrolytic filter capacitors were smaller.

The only parts that 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 the ‘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

component values 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

response, 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

circuit 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

performance.

The next prototype was built and its performance was even better, largely because of bet-

ter shielding and a better layout. 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. Modern test equipment can quantify and graph parameters that had not even been

discovered back in the heyday of “hollow state.”

This prototype became my 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 for 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.

Why do tubes sound better? All properly designed audio amplifier circuits exhibit low distor-

tion throughout 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

from low distortion to extreme distortion (clipping). This is a good trait, since when operat-

ed right up to their maximum level solid state amplifiers can maintain excellent performance.

Digital audio circuits have similar characteristics.

Vacuum tube circuits, on the other hand, show a gradual increase in distortion throughout

their operating range. 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

transformer 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

headroom when the output voltage exceeds the power supply voltage. The result at this point