Recording - March 2014 - 38

B Y PA U L V N U K J R .

Earthworks

521 ZDT Preamp

An incredibly clear mic preamp in a handy new format
While best known for its line of ultra-accurate, miniature
capsule omni microphones, Earthworks has also been
making mic preamps since the beginning, first with the
LAB series reviewed back in our April 1998 issue and
then the Zero Distortion Technology (ZDT) line.
We introduced readers to the ZDT line in our January
2013 review of the of the1022 2-channel rack preamp.
Earthworks also makes the 4-channel 1024 and the halfrack single-channel 1021; the latest model is the 521
ZDT, a single-channel unit in the popular API 500 Series
module format.
Zero Distortion Technology in a lunch box
As noted in my last review, this David Blackmerdesigned preamp is a Class A bipolar input circuitry
device that contains almost no nonlinear distortions of any
kind, less than 1 part per million. It uses a balanced input
and a symmetrical, balanced output with no transformers
or electrolytic capacitors in the signal path.
Some sample specs: frequency response of 2 Hz to 100
kHz ±0.1 dB, 1 Hz to 200 kHz ±0.5 dB, an impulse
response of 1.4µs, equivalent input noise of -132 dBV at
20 dB gain and -143 dBV at 60 dB gain, a slew rate of
22V/µs, and of course distortion of less than 1ppm
(0.0001%) at the 1/4" out, 0.001% via the XLR out.
The ZDT preamps belong to the "straight wire with
gain" family also inhabited by the likes of Millennia
Media, John Hardy, Grace Design, and AEA. Put another
way, you hear the sound of the source and the character
of the microphones, not the preamp.
Like the previous ZDT models the 521 comes in a lightly powder-coated matte black finish with white lettering. Its
500 series chassis is fully enclosed and its internal circuit
board is clean and well laid out with high quality pots and
switches. It is possibly also the lightest 500 Series box I
have ever felt weighing only 0.8 lb.
As a fun little extra, it comes in its own little black and
white lunch box-no, not the API kind, but a real lunch box like most of us would have
used in grade school!
Controls and features
The 521 starts with a large input knob, a 12-position stepped gain switch with a
range of 5 dB to 60 dB in 5 dB steps. This controls signal flow from the 500 Series
enclosure's XLR input directly to its output. As with previous models, there is also a second balanced output via a front mounted 1/4" jack, controlled by a small variable knob
that can attenuate the signal -20 dB.
Other controls include 48V phantom power, polarity switching, and a Standby
switch for muting the unit when switching mics or engaging phantom power.
38

RECORDING March 2014

In use
As with my 1022 review, I compared
the 521 to my own 8-channel Millennia
Media HV3D and overall my impressions
were the same. If I listened really hard, I
could discern a 5-10% difference in the
two, with the 521 ZDT being just a touch
more forward in the lower mids.
From drums to voice and instruments in
between, this is a preamp that does as
advertised and adds nothing to the
sound. Where the 521 really excels is in
how gosh darn quiet it is! Not only with
the company's own mics (one of which
we will look at below), but with low-output ribbons and dynamics.
The only time noise starts to become
apparent is when pushing the output up
past 55 dB, but even at 50 dB you will be
able to easily use a Shure SM7 for clean
clear spoken word duties or a low-powered ribbon for quiet sources like nylonstring classical guitars.
On the flip side, if you have really highoutput mics and a loud source, switching to
the variable output allows you to bring the
sound down further should you need it. I
did try using this feature to induce some
intentional clipping and breakup by turning
the input to 60 dB and the variable output
down to -15 dB. All I can say is, it took a
lot of work and a loud source to accomplish, and the result was not "vibey" as on
some other preamps, it was just clipped. In
other words, the ZDT delivers on its sonic
promise! It's not a preamp you'd choose for
harmonic distortion and vibe.
Conclusion
Earthworks' ZDT preamps have always
lived quietly in the shadows of their
famous microphone siblings, but the line
does have a respectable following
among audiophile recording types, and
with good reason.
With the growing popularity of the 500
Series and with clean uncolored preamps
being in vogue again, I suspect that the
521 ZDT could be the breakout model of
the series. If you are after clean, uncolored, and exceptionally quiet performance, and you own a 500 Series box,
you should check it out.
Price: $999 street
More from: Earthworks,
www.earthworksaudio.com

http://www.earthworksaudio.com

Recording - March 2014

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