THE CD PLAYER PLUS - Ultra High Fidelity Magazine

Mov ing coil cartridges
Listening Feedback Room
Allnic Step-up
are often referred to,
incorrectly, as having low
output. In fact what they
have is low impedance. And perhaps that
calls for a little explanation.
A cartridge’s output power can be
defined as its voltage multiplied by its
current (assuming voltage and current
are in phase, but let’s not complicate
things). A typical moving magnet cartridge
will have an output at relatively
high voltage (perhaps 2 to 3 mV) at
low current. A low impedance (MC)
cartridge will have perhaps a tenth of
that output voltage, but it will be able
to supply higher current. The output
is therefore not really low, merely
different.
That causes a problem, however.
A phono preamplifier expects to “see”
voltage, and so low voltage might as well
be low output. The usual practice is to
add extra amplification to increase the
voltage to what you would get from a
high impedance cartridge.
But amplification stages add noise
and distortion, and there is another solution
that used to be common. A step-up
transformer can trade off current against
voltage, and allow a low impedance
cartridge to drive a phono preamp with
no extra amplification.
So why doesn’t everyone use transformers?
It’s because making one good
enough for a tiny signal requires extraordinary
precision work, and it’s not the
sort of craftsmanship you can easily
automate. Because a step-up transformer
44 ULTRA HIGH FIDELITY Magazine
handles such a small signal, it requires
extraordinary shielding against the
electromagnetic interference generated
by our increasingly connected world.
The result is that, over the past decade
or more, we have heard step-up transformers
whose sound can be charitably
described as disastrous.
We ourselves have long owned a
Bryston TF-1 transformer, which was
once an everyday element of our reference
system, and which two decades
ago was surprisingly affordable, perhaps
around $450 (we’re quoting this from
memory). Though Bryston still uses that
transformer in some of its preamplifiers,
it no longer offers it as a standalone
product. Certainly it would cost several
times more today, and it was a bargain
even when we acquired it. Hammertone
Audio shipped us an earlier model of the
Allnic, and then asked us to send it back
and try this newer one instead.
The AUT-2000 is in a massivelyshielded
box, with holes that look as
if they are made for heat dissipation,
though of course the tiny signal produces
no significant heat. The twin switches,
which can be set with a coin, can vary
the gain from the multitap transformer
to provide the best signal that won’t overload
the MM (conventional) input of the
phono preamp. We ran them all the way
up and noted that they did not overload
our phono preamp even on the loudest
passages we could find (obviously, your
mileage may vary).
We did note a glitch: we thought we
had lost the right channel, and began
looking for improperly-inserted interconnect
cables, only to find that one of
the Allnic’s switches was not set just so.
The switch detents are not as positive as
they could be.
We did our listening in our Alpha
room, connecting our Bryston to our
Audiomeca J-1 turntable and Goldring
Excel moving-coil cartridge. We listened
to four selected LP’s, and then
repeated the session using the Allnic
transformer. We used one of our Atlas
Navigator All-Cu interconnect cables
between transformer and preamp.
The first thing we noted is how very
quiet the Allnic was. Of course you would
expect a transformer to be quieter than a
low-impedance amplification stage, but
that will be true only if the shielding is
adequate and if internal grounding has
been done properly. Phono preamplifiers
are often noisy in our Alpha room,
whose ungrounded lead-filled side wall
pulls in plenty of less-than-delightful
radio frequency interference. Not in this
case.
The rest of this article can be found in
the complete print or electronic version
of UHF No. 84. Order the print issue
from www.uhfmag.com/IndividualIssue.html
(it’s case sensitive). Or subscribe
at www.uhfmag.com/Subscription.html.
The electronic issue is available from
www.magzee.com.
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