Aphex
Thermionics
- Division of Aphex Systems Ltd.
Model 1100 MkII Mic Preamp - Owner’s Manual
Page 9-2
White Paper
IV. Designing the Aphex Model 1100 MkII Tube Microphone Preamplifier
1. Noise
One of the primary design goals of the Model 1100 MkII was to have as wide a
dynamic range as possible. Several key inventions combined with a no-compromise
selection of components (see below) create a microphone preamplifier with unprec-
edented performance. The EIN with 65dB of gain is an incredible -135dBu. That
means that the Model 1100 MkII adds less that 1dB of noise to the natural self noise
of a 150 ohm microphone. The worst case dynamic range is 97dB and is a high as
101dB. But low noise is only part of the story.
2. 20dB extra headroom
As described above, a conventional preamplifier must be set for sufficient headroom
in order to avoid overload. The Model 1100 MkII has two inventions that actually
provide up to 20dB of extra input headroom so that it is virtually impossible to over-
load the preamplifier. A third invention does not directly increase headroom, but
maximizes available headroom in the digital domain.
a. MicLim
The first invention is the Microphone Limiter (MicLim), first used on the Model 1788.
It comprises a custom designed optical attenuator directly on the microphone input
line. It smoothly limits the microphone output signal prior to the preamplification by
up to 20dB. The peak limit detector is located after the preamplifier input stage and
feeds a control current back to the attenuator so that the input signal remains below
clipping. MicLim has no effect whatsoever on the input signal until the preamplifier's
output approaches clipping.
b. Low Frequency Cancellation Filter (LoCaf)
The second invention is a tunable low frequency cancellation filter (LoCaf). It is a
second order (12dB/octave) modified Butterworth filter meshed into the nodal inter-
sections of the first and second amplifying stages in a servo configuration. The servo
affects only frequencies below the corner frequency, thus it contributes nothing to
the audible signal. Imposing the servo filter in such a manner gives the preamplifier
about 20dB more overload headroom in the low cut range as compared to conven-
tional techniques. Additionally, the added low frequency headroom eliminates the
need for the MicLim to trigger earlier than necessary from excessive low frequency
energy.
c. Drift Stabilized A/D Converter
Conventional analog to digital converters utilize high pass filters in the digital
domain to block any DC generated in the conversion process or already in input
signal. While this is effective in eliminating the DC, it requires extra headroom in the
converter to allow for the DC offset. The patented drift stabilized A/D eliminates the
DC offset in the analog domain so that the input peaks of both polarities can reach
the true maximum level. Since there is no high pass filter in the digital domain, all
ringing and time delays from that filter are also eliminated.
3. Other Features
a. Full Featured AES/EBU Digital Audio Output
AES/EBU XLR output is standard. Clock synchronization options allow for locking to
standard "word clock" received at the standard BNC clock input jack. Internal clock
options provide low jitter 44.1, 48, 88.2, 96, and 192kHz sample rates. When a unit
is set for internal clock, its internal word clock reference is sent to the rear panel
word clock BNC output jack to serve as clock reference to other units. When the unit
is set for an external clock reference, the clock input BNC jack is directly tied to the
output clock BNC jack for easy daisy-chaining of model 1100 MkII units from the
master clock source. All digital audio settings are controlled and displayed on the
front panel.
The A/D converter receives signal from the soft mute stage just prior to the analog
output level control and triode output stage. This means you can use both the digital
and analog outputs independently, with full and proper calibration of both regard-
less of the analog output level settings. The analog and digital outputs respond
equally to the input gain, low-cut filter, and all front-end conditioning effects.
d. Bifurcated 20dB Pad and Phase Reverse
Many preamplifiers, even the more expensive models, switch microphone level
signals directly through switch contacts. It is well known that even the best quality
switches will eventually suffer from dry contact diode effects causing noise and dis-
