Aphex
Thermionics
- Division of Aphex Systems Ltd.
Model 1100 MkII Mic Preamp - Owner’s Manual
tortion. The Model 1100 MkII uses high-grade, bifurcated (milti-finger) gold contact
relays which do not develop these problems. The Gain and Low Cut controls are
sealed gold contact rotary witches.
e. Precision Three Turn Output Level Attenuator
In order to match the analog output of the model 1100 MkII to the user's system
level, the output gain is adjustable from zero dB (max gain) to -14dB. The user will
appreciate the smoothness and precision afforded by the front panel 3-turn high-
grade potentiometer adjustment.
f. 48-volt Phantom Power Circuit
Very slow rise and fall of the phantom voltage is used to eliminate turn-on and turn-
off thumps. Industry standard resistances of 6.81 K-Ohms supply the highly filtered
48-volt source to pin 2 and pin 3 through a voltage ramping active buffer. The phan-
tom powering system can withstand a short circuit to ground on both microphone
jacks indefinitely.
g. Series-Shunt, Optical Soft Mute Attenuator
The second-stage output signal passes through a specialized series-shunt optocou-
pler circuit to provide a soft mute while introducing no distortion or noise.
h. Front Panel Peak Headroom Meter and Function Controls
Each channel contains a 20-segment LED headroom meter, making it easy to opti-
mize the performance of the preamplifier. The headroom meter is calibrated in deci-
bels below clipping, where 0dB is the analog clipping point. This coincides with
the A/D converter's maximum input level, so the headroom meter also indicates the
digital audio level accurately. Each channel contains its own independent controls
over every function.
i. Rear Panel Mute Jack
The mute function may be activated by the front panel push-button, or by a remote
switch plugged into the mute jack. In the absence of a phone plug, the mute jack
serves as a closed circuit, and only the front panel push-button has control. In the
presence of a phone plug, an open circuit mutes the preamplifier while a closed
circuit un-mutes. This facilitates the convenient use of mute controls such as floor
mat switches (step on to un-mute) and musician's footswitches.
j. Internal Switchmode Low Noise Power Supply
One of the technology improvements comprising the model 1100 MkII is a highly
efficient switchmode/linear hybrid power supply. Switchmode design is used to
very efficiently convert the a.c. primary power into secondary d.c. voltages. Great
effort was made to eliminate switching noise and we phenomenally succeeded. All
preamp supply voltages are further stabilized through linear regulators to maintain
the highest audio performance. The tube filaments are run from regulated d.c. for
extremely long life. The whole power supply is magnetically shielded. With this new
power supply design, the 1100 MkII is lighter, runs cooler, and performs better.
V. Sound Quality by Design
While specifications and functions are important, the most important characteristic
of a piece of audio gear, particularly a microphone preamplifier, is how it sounds.
The sound of the Model 1100 MkII is clean, clear, present, open and solid. It is
extraordinarily detailed and spacious. The low end stands up without any muddiness
and the high end is very extended without any harshness.
This sound is achieved through the use of proprietary designs, careful engineering
and the highest quality components. It is a combination of Class A discrete compo-
nents and patented tube circuitry as briefly described below.
1. Ultra Low Noise, Transformerless, Discrete Class A, Bipolar PNP, Variable Gain
Differential Input Stage
No outer feedback is used, thus eliminating the possibility of any dynamic interac-
tion with the microphone's self-impedance. The input impedance remains passive,
providing an optimal load for any microphone. The solid state, class-A PNP bipolar
design achieves high common-mode rejection with extremely low noise, wide band-
width, and low distortion at all gain settings. Robust input overload protection
assures that all performance features will be retained indefinitely. The gain is adjust-
able in 4dB precision steps from 21 to 65dB.
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