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Stereo processing with two Filtrex-II modules – Application Hints
Author's note: I haven't tried this yet with the Filtrex-II, however, the theory is sound so it
should work without issue. I would be interested in hearing from people who have made a
dual unit. Feel free to contact me either directly by e-mail or via the Forum.
Two Filtrex PCBs must be populated. One should be designated Master and one Slave. The
slave one is identical to the master except that it has no onboard power supply. The slave
receives its power from the master using the MOTM headers.
The two 2-pin headers, STV and ST-A, must also be fitted to both boards. STV is an
abbreviation of 'stereo voltage' and ST-A is for 'stereo-audio'. STV will carry the filter cut-off
CV from the master to the slave. ST-A will carry an audio signal from the slave to the master.
You will also need two SPST (or SPDT) switches, or one DPST (or DPDT) switch,
somewhere on your front panel. This will give you the option of switching between two
completely independent Filtrex channels and one stereo dual Filtrex.
I would expect any dual Filtrex to be fitted to a 2U high rack case. The master would be the
upper one and the slave below it. Use the MOTM header to carry power from the master to
the slave. I would recommend using Amphenol 0.156" MTA connections since the wires from
these come out at right angles to the connector and are therefore lower profile. The PCB has
been designed so when using MTA connectors the wires move away from the front of the
board so can be looped from one board to the other.
The heatsinks fitted to U11 and U15 must be closely monitored after first building up a stereo
unit. When being run from a 15V AC wallwart the heatsinks get only slightly warm when
driving a single Filtrex-II. But doubling the power output of the power supply will double the
power dissipation in those regulators. Therefore you may wish to increase the size of the
heatsinks. The Filtrex-II PCB has been designed to accommodate larger heatsinks than the
standard ones.
To link the two Filtrexes together you need to allow the CV signal from the master Filtrex to
affect the cut-off frequency of the slave. To do this connect pin 1 of STV on the master to pin
2 of STV on the slave. Wire your switch so that this signal can broken by the action of
opening the switch.
To allow the master's envelope follower and threshold detector circuit to work from the
slave's audio input or side channel, that audio signal must be passed up into the master's side
channel amplifier based around U14a. To do this connect pin 1 of ST-A on the slave to pin 2
of ST-A on the master. Again, wire your switch so that the connection can be broken when
the switch is opened.
You can switch STV and ST-A together with a DPST (or DPDT) switch, or you can switch
each independently with a SPST (or SPDT) switch. One switch lends itself to a simpler front
panel design but there may be times when you want the master to not listen to what the slave
is processing.
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