M2Tech
www.m2tech.biz
MITCHELL
STEREO ANALOG ELECTRONIC CROSSOVER
REVISION P
RB
– DECEMBER 2020
Copyright © 2020, M2Tech Srl
17
7. Theory of Operation
The
MITCHELL
is an all-analog stereo, 3-way electronic crossover. Its purpose is to separate
the signal from a preamplifier into up to three signals with different frequency contents.
This way it is possible to eliminate the passive crossover in a speakers system and drive
each driver with a dedicated power amplifier. This technique is called “active multi-
amplification”.
The advantages of active multi-amplification are various and more than subtle:
more clarity and transparency in the sound due to elimination of the side-effects of
non-linearities in passive power components like inductors and capacitors;
more accurate pulse response with increased quality of soundstage presentation;
higher dynamic;
less distortion as each power amplifier sees an easier load to drive.
Of course, this comes at a cost, as more power amplifiers are needed than in an usual
system: at least a stereo power amplifier each way, plus the crossover. E.g., in a two way,
bi-amplified system the following units are required: a
MITCHELL
and two stereo power
amplifiers. A more expensive, more refined system with 5-way speakers will need two
MITCHELL
set in mono mode and at least 5 stereo power amplifiers (or up to 10 monoblock
power amplifiers).
Digital electronic crossovers are usually extremely versatile as their configuration software
allows for shaping cut-off curves with great precision. On the other hand, they need to
convert the sound from analog to digital before processing it and convert the processed
signals back from digital to analog. This is often unwanted as conversions apply a strong
signature to the sound and usually systems sound like the crossover's converters rather
then like their high quality analog components.
To partially avoid this problem, somebody drives the digital crossover by its digital input
when available. This allows to skip the input A-to-D converter and reduce the conversion
artefacts. However, the crossover digital input limitations apply: quite often the highest
sample rate that the input can accommodate is 96kHz or 192kHz, and DSD is never
accepted as it can't be processed natively by digital crossovers. This reduces the choice of
music that can be played on the system or requires real-time format conversion in the
player which introduces artefacts similar to those produced by the A-to-D converter.
For the reasons above, digital crossovers should be avoided in any high-end system,
particularly if both digital and analog sources are used. The solution is to use an analog
electronic crossover. This may be a problem as most of the analog electronic crossovers
on the market are generally too low-end or not enough versatile to properly fit a high-end
system needs. Particularly, the kind of filter which each way can implement is fixed (low-
pass or band-pass or high-pass) and crossover frequency are always same for adjacent
filters (e.g. the cut-off frequency of the low-pass filter is usually same as the low-side cut-
off frequency of the band-pass filter). This makes using electronic crossovers difficult when
a unusual speakers configuration is chosen. Example: in a 2-way and half speakers
system in which one driver is for low frequencies only, another driver works up to midrange
