ROSTEC LMA2 Microphone and Line Preamplifier
5
Clock sync input and output
The LMA2 uses an internal high performance master
clock generator to run all the necessary clocks for the
various digital circuits. It has an input and an output for
clock synchronization on the standard BNC connectors
at the back.
When the LMA2 is connected to a host computer via
USB, the sampling frequency is controlled by the
operating system, or it is selected from within the DAW.
Manual control of the sampling frequency is disabled
when the USB connection is used.
When no USB connection is used, the sampling
frequency can be selected manually by the pushbutton
at the front.
However, when an incoming word clock is present at the
BNC clock input connector, the LMA2 automatically
switches to the corresponding sampling frequency, and
manual control of the sampling frequency is disabled.
The LMA2 automatically detects when an incoming word
clock is present at the BNC connector at the back panel,
and the internal generator locks on to it immediately.
The internal generator softly glides from the internal
crystal reference to the external clock reference without
any jumps or disruptions of the internal clock signals
It achieves lock in typically less than 0.3 seconds, and if
the incoming sync is lost, it softly glides back to the
internal crystal reference again. There are no gaps or
interruption of the internal clocks or any audio data.
Shorter gaps in the incoming sync are efficiently
absorbed that way. The soft gliding back and forth is
sufficiently gentle and well damped in order for the digital
circuits to handle this without any degradation of the
audio signal.
Further, the generator has an extensive ability to clean
op a malformed and distorted incoming clock.
The input uses a high-speed comparator with hysteresis
and a "sweet spot" detector, which performs an accurate
auto-slicing of the input.
This means that the circuit automatically chooses the
most useful part of the input signal, thus being able to
clean-up and reconstruct a ringing and noisy input clock
into a perfectly shaped clock for internal use
An input clock with high level of jitter gets the treatment
too. The internal PLL uses a multi pole filter network to
make it largely immune to incoming jitter.
Put in another way, when the timing of the leading edge
of the incoming clock varies with time (this is what jitter
is!) the crystal oscillator won't follow these fluctuations,
but chooses the average position of the leading edge of
the incoming clock. The result is a cleaned-up reference
clock with typically less than 0.1 nsec RMS jitter.
This solid jitter performance and high stability of the
clock generator provide an ideal environment for the AD
converter, the USB state machine and the AES/SPDIF
transmitter circuit.
This is one of the basic reasons for the exceptional sonic
quality of the LMA2 digital interface.
The meter
The meter is a true peak meter with fast attack and slow
release, with a correct professional ballistics profile (not
like the rubbery and sloppy meters on a typical DAW).
The switch on the front panel selects two measuring
points: SEND measures the insert point send level
(same as analog out), and RTN measures the insert
point return level. Note that the meter is a measuring
device, it does not route any signals. The meter provides
the user with a trustworthy indication of levels and
clipping of the digital converters, microphone amplifiers
and the inserted analog equipment
Power Supply
The power supply is pure analog, with an oversized
toroidal transformer and low noise linear regulators.
This configuration creates an electromagnetic quiet
environment, free from the typical radiation pollution
from switch-mode power supplies
A switch mode power supply generates strong repetitive
electromagnetic pulses that travel through air and sharp
current pulses that travel through the ground system.
When this pollution hit the analog circuitry, it disrupt the
smooth operation of the circuitry by pressing the
amplifiers into slew-rate-mode momentarily
Slew rate mode, is when an amplifier is presented with
a signal that moves faster that the amplifiers maximum
speed capability. It then tries to “slew” as fast as it can,
to cope with the signal. When the amplifier is in this
mode, it cannot process any further information; it is in
fact blocked from reproducing incoming audio signals.
This happens in short durations, when the pulses from a
switch mode power supply hit the circuit.
When this electromagnetic pollution hits the analog
circuit 60.000 times a second, the analog circuit looses,
in small intervals, the ability to reproduce audio.
This is in fact the main reason why audio products with
switch mode power supplies usually sound harsh, flat
and lifeless, with a degraded ability to process details
and depth in the audio.
LMA2 maintains a clean electromagnetic environment in
the box, and the reward is a natural, musical, relaxed
and open sound.
