The Input Section
About the Input Transformer
The Vipre features an exclusive input transformer
custom-made in California to the highest industry
standards. A key feature is that it's a multitap
transformer, allowing a selection from four different
load impedances for a microphone. (Some other
"variable impedance" products on the market simply
switch in additional resistors, which actually makes
them "variable resistance"??a subtle but potentially
audible difference because of the insertion losses that
resistors cause.)
Benefits of transformers, generally
While modern solid-state input circuitry can deliver
excellent results at very low cost, input transformers
deliver several significant advantages:
? Greater CMRR (common mode rejection ratio):
this means that noise from the cable that appears
on both the + and ? leads simultaneously is more
likely to be rejected. Solid-state differentiallybalanced
circuits may have a good CMRR spec at
1 kHz, but often aren't as good at other
frequencies.
? More resistance to RF (radio frequency
interference): the windings of a transformer,
being naturally inductive, reject ultra-high radio
frequencies without requiring a capacitor in the
audio path as electronically-balanced circuits do.
? Lower impedance: it is easier to build a lowimpedance
input transformer than a lowimpedance
transistor input (which is one of the
reasons the industry went to bridging impedances
in the last few decades).
? A transformer is fully floating, isolating the circuit
from DC offsets, and to a certain extent from
surges and stray signals. You could think of it as
a magnetic link that keeps the circuits free from
mutual contamination.
For these and other reasons, the highest-level audio
consoles, especially those used by touring sound
companies, have always used input transformers. But
good quality input transformers are expensive, and
cheap input transformers used in affordable consoles
several decades ago gave transformers a bad name
with some engineers due to their irregular frequency
response and distortion.
The input transformer used in the Vipre is a totally
different story. Special care is taken to shield the
transformer windings from noise: a high-quality
double Mu-metal can surrounds the transformer. The
transformer itself is a "humbucking" type, with
specially-wound oppositional coils that cancel out
induced noise from the environment or nearby
circuitry.
Impedance: Matching vs. bridging
In early audio devices, circuits were designed to
maximize power transfer between units. This meant
that input and output impedances should be matched,
e.g. a 150-ohm source would be terminated by a 150-
ohm load. This impedance matching maximized the
current in the load, at a time when every bit of current
was necessary due to primitive amplification methods.
One obvious down side to impedance matching was
that a single source couldn't easily feed several inputs
at once.
After the advent of high-impedance solid-state
circuitry, the era of maximum power transfer was over
and the focus shifted to conveying signals through
voltage, not current. The microphone or other source
generates a varying voltage, but very little current is
drawn in the circuit because the load it "sees" is so
high that it may as well be connected to nothing at all.
Most modern audio equipment is designed to present
a very high impedance load to the source. The typical
input impedance of a load is designed to be 10 to 20
times the output impedance of a typical source. This
is called a bridging input. (For a more in-depth
explanation of the importance of impedance, see page
31.)
But what is the actual impedance of the microphone
you're plugging in? And what kind of circuit was that
microphone designed to "see" for optimum
performance? Particularly in the case of vintage
condenser and ribbon microphones, the original
design usually were set up for impedance m atching ,
not bridging. When these microphones are plugged
into the MIC IN jacks of today's solid-state mic
preamps or consoles, they're seeing a very different
load than they were designed for. When they're
plugged into the input of the Vipre and the impedance
is set to lower values, they exhibit the more open
sound heard on classic recordings. Yet, other
microphones may need to see a higher, bridging load
for best performance. In any case, altering the load
against which the microphone has to push
fundamentally alters the tone and character of the
output signal. The Vipre gives you the best of both
worlds. A key to this flexibility is the input transformer,
which is the first component the microphone "sees"
when the INPUT SELECT switch is in the XFMR
positions.
