The
Heider FD312 is a special and unique preamp module that
stands out in today’s crowded 500 series market. The
preamp stage has been carefully modeled after the
preamps in the famous Wally Heider Recording Studio 4
console custom built by Frank DeMedio circa 1972. The
original pre’s were stock 312 cards slightly modified by
DeMedio. Frank’s mod was centered around removing the
(1:7) AP2622 input transformer and replacing it with a
custom wound Ed Reichenbach (1:8) RE-0887. The only
differences to the audio path are how DeMedio treated
the RE-0887. He removed the commonly used 1000pF
decoupling capacitors found on the AP2622’s primaries.
He also removed the Zobel network from the secondary.
Instead of a typical Zobel network he used only a low
value load resistor. Besides being just enough to safely
dampen any high frequency ringing, this load resistor
creates an unusually low input impedance and a slightly
higher than typical THD. All of this geek talk equals a
very special tonal quality with some of the most buttery
smooth mids you will ever encounter! CineMag has done a
fantastic job with the current RE-0887’s, working
straight off of Ed Reichenbach’s original notes and
winding data from 1972. For those who don’t know, Ed
Reichenbach’s son Tom went on to form CineMag in 1979
after working with his father for some 25 plus years. I
am so very glad they kept all of the old notes!
To
enhance this special preamp circuit even further, I
chose to employ a stepped “fader” and active booster
stage, very reminiscent to my popular 2-stage VP28
preamp. This is essentially the same booster circuit
that was used in the early to mid ‘70’s vintage API
consoles. In remaining true to the 312 circuit, all AC
coupling capacitors have been removed from the audio
path. I don’t think coupling caps are evil, they are
just not needed in this particular circuit. This fader
booster stage has 12dB of gain “in hand”, exactly like a
real console. This allows for many different gain
combinations. You can crank the fader and reduce the
preamp gain or in contrast, attenuate the 2nd stage and
crank the preamp gain for some sweet, additional
harmonic content.
The
original Heider/DeMedio 312 cards used stock AP2503
output transformers, as found on all 312 cards. With the
desire to have something unique, I was luckily able to
persuade David Geren of CineMag to create his very own
version of the AP2503. Through our conversations
regarding this project, David said that he had been
asked many, many times over the years to wind an AP2503.
Until now, he had always denied these requests. Somehow
I was able to convince him to proceed and the CM-3301
was born. In trying to be loyal to the console signal
path, both gain stages have a CM-3301 output
transformer. While the preamp stage output transformer
is not an exact requirement, it does provide 6dB of
gain, blocks DC from the stepped fader and imparts a
little more character into the overall circuit.
The
operational amplifiers are the pure heart of a preamp
like the Heider FD312. I naturally wanted to stay as
close as possible to the early ‘70’s 2520’s. In a
current commercially available product, Scott Liebers’
SL-2520 Red Dots were a no-brainer choice. As most of
you already know, they perform and sound outstanding all
while having a very long life expectancy due to their
un-potted construction. Each Heider FD312 contains two
of these glorious Red Dots.
In
a goal to stay domestic, I would like to point out some
interesting info. The Heider FD312 is assembled by a
Chicago area electronics contract manufacturing house.
Not the cheapest guys around but their work is
outstanding. Each and every FD312 is fully tested,
calibrated, burned in and packaged by myself, at the
CAPI laboratory. The following items used in the build
are fully manufactured in the USA: All metalwork
including zinc plating, anodizing and silkscreen
printing, PCB’s, all transformers, opamps, aluminum
control knobs, colored knob inserts and Grayhill
switches. That basically leaves board level components
as the only imported items.
Street
Price: $799 available from Rack-N-Roll Audio,
Nashville
Key
Operating Features:
In
MIC mode, the stepped PREAMP GAIN ranges from +22dB to
+57dB, with an input impedance of 167 ohms.
In
LINE mode (MIC pushbutton out or disengaged), the
stepped PREAMP GAIN ranges from -14dB to +21dB, with an
input impedance of 9K ohms.
Engaging
the Hi Z pushbutton in MIC or LINE mode will increase
the gain by 3dB. In MIC mode, the resulting input
impedance will also be raised to 300 ohms. Since the
upper ten PREAMP GAIN steps are also in 3dB increments,
this allows for many creative tonal
combinations.
The
PAD pushbutton equates to 16dB of attenuation in either
MIC or LINE mode. With the PAD pushbutton engaged while
in MIC mode, the input impedance is 765 ohms.
The
stepped CHANNEL FADER switch and 2nd stage booster amp
provide a range of –16dB to +12dB in addition to the
above-mentioned preamp gain ranges. This equates to a
maximum gain of 69dB when in MIC mode and 72dB when in
MIC mode with the Hi Z pushbutton engaged.
Accurate
metering of the final output signal is achieved using
LM339 quad comparators in a “console” style
configuration.
Physically
and electronically fully VPR compliant.
