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RackPack: Preference Mic Pre
SSM 2019
The Preference Mic Pre is suitable for all common dynamic and con-
denser microphones. It works along the principles of an instrumenta-
tion amplifier which is a technology that is also used in measurement
and medical equipment due to its high common mode rejection of stray
pick-ups. It is based on the semiconductor SSM 2019 which sounds
more balanced than available alternatives. Low noise and distortion
values as well as a broad bandwidth and a fast slew rate are its forte.
Servo Drive Design
The main focus during the development of the Preference Mic Pre was
its acoustic transparency and its natural representation of the source
signal. Reducing DC offsets in the audio signal paths is a decisive part
of this design job.
DC offsets are produced in relatively large amounts especially when
amplifying a microphone signal because the signal is amplified by
extreme factors from the pico volt range to 0dB nominal level. DC off-
sets impair the signal quality as they induce noise and distortion that
lead to a diffuse sound.
The Preference Mic Pre‘s servo drive design minimizes these problems
much more effectively than the common solution based on capacitors
by setting the DC offset to almost 0 mV. In addition to that, the active
servo drive circuitry tends to produce less distortions than passive
capacitors.
The servo drive design incorporates three operational amplifiers. The
SSM 2019 is the first op-amp. The second op-amp acts as a sensor to
detect voltage differences. These differences are then compensated
for in a third op-amp working as summing stage.
Of course an elaborate active servo drive circuitry is a more expen-
sive solution than simply using passive capacitors, but especially in
a microphone preamplifier this effort pays off with improved sound
quality.
Technology
SSM 2019, Servo Drive
