207d
instruction Manual
Aphex Systems Ltd. Model 207D
Page 6
207d
2-ch tube mic preamp with adc
Page 7
Aphex Systems Ltd. Model 207D
tortion is so natural. The very slight tube distortion of
the Model 207D comprises typically less than 0.02%
of second order harmonics at average levels. What
is even more interesting is that this curve follows
the tube’s law as regards level, and furthermore, it
responds to certain very subtle dynamic space-charge
effects known only in tube amplifiers. Thus, the
sound with an RPA can have a large and open feeling
that is very natural and transparent.
2.3 WHAT ELSE IS SO SPECIAL?
It may sound trite, but everything else about the
Model 207D is special. When we designed its circuitry
and decided upon its features, we took into consid-
eration a very wide range of factors. We “blocked in”
the things that our marketing experts wanted, but
then made sure that every circuit and feature could
work in harmony. Some things were discarded by
engineering because they would have compromised
the sound. Once we had the Model 207D very well
defined, we went to work at embodying the circuits
cleanly and cleverly, using some great ideas from the
Model 107, but redesigning most of it from scratch
through computer modeling and simulation and
finally through actual physical prototypes and listen-
ing sessions. There are no databook or textbook pre-
amplifier circuits in the 207D. It’s all Aphex.
That being said, let us point out just a few of the
things that are special about the Model 207D. Then,
as you read more of this manual, you will gain appre-
ciation for many other special aspects of this product.
First, let’s look at the front end. Other preamps, even
those costing many times more, pass the mic signal
through switch contacts to enact the pad and polar-
ity features. This eventually leads to contact distortion
creeping into the sound because switch contacts will
eventually oxidize and, in the very least, need to be
operated frequently to keep themselves clean. That is
a big problem, especially for fixed installations where
it is impossible to exercise all the switches regularly
(daily?).
We used expensive d.c. operated, sealed, bifurcated
gold contact relays to switch these functions. These
relays never oxidize and can remain stationary for
many years without contact distortion.
Next, let’s look at the low cut filter. It is a second
order active filter that is designed for musicality,
Introduction
not just rumble cutting. We very slightly peaked the
response at 120Hz by about 1dB so there will be a
natural phase compensation effect in the important
low-mid bass region helping regain a greater sense
of bass presence. That benefit is immediately appar-
ent to experienced mic preamp users. We received so
much praise for this type of filter in the Model 107,
that we kept it for the 207D.
Just one more mention. Let’s look at the RPA tube
output stage. Here, we merge a real operating
vacuum tube into the world of semiconductors. The
tube serves as the main gain element and brings
us all of its warmth and clarity while remaining
directly linked to its solid-state support unit. There
are no transformers or high voltage supplies, yet the
strength of the output signal is more than powerful
enough to drive any practical load to full professional
levels with ease. Because of the unique capabilities
of the RPA circuit, we were able to mix the mic and
instrument signals directly to the tube without any
interaction. The result is exceptionally high perfor-
mance.
2.4 WHAT ABOUT THE DIGITAL AUDIO?
With the superb audio quality of the 207D, it is obvi-
ous that only a high grade ADC will be able to capture
that quality and bring it into the digital domain. We
incorporated a 24 bit ADC that has a digital dynamic
range greater than the analog circuitry. In this way,
perfect dithering will be provided by the analog noise
floor. That is the most ideal form of dither since it
avoids the harshness of digitally synthesized dither
noise.
The AES3 and S/PDIF digital audio outputs are of 24-
Bit resolution. The analog noise floor, even though
exceptionally low in level for a mic preamp, will
typically dither the 24 bits into the 16-bit level which
makes it suitable for all projects, including anything
that will eventually end up on CD. Even if your digital
studio truncates the audio word length to 16-Bits,
the 207D’s natural dither will still make it sound
great.