EXO Electronic crossover kit
EXO overview
The EXO is a 2 way electronic crossover to use with a stereo power amplifier to directly drive the tweeter (HF) and woofer (LF) drive units in an active capable loudspeaker.
Two EXO modules are required for a stereo system one for left and one for right.
The module has a single line level input and two outputs, one for HF and one for LF.
Level control presets for HF and LF can be used to balance the HF and LF outputs to the speaker to ensure an even frequency response.
A plug in filter board is also supplied which is specific to the chosen loudspeaker or supplied blank for the experimenter who wishes to develop their own filter.
The module requires a single 24V (or greater) supply and the module has provision for higher voltage DC supplies with an extra voltage dropper resistor (not supplied in the kit) on the PCB.
Circuit description
The line level input signal arrives via Cn1 and is immediately split and sent to the two level controls which feed the HF and LF active filters.
HF section
Transistor Tr1 is a simple emitter follower buffer amplifier which supplies the HF filter on pins 3 and 4 of the filter board connector Cn3. The signal is filtered by a 1/2/3rd order CR filter on the filter board. The output of the filter is fed to the base of Tr2 which together with Tr3 form an amplifier to overcome losses in the filter. The gain of this amplifier is set by a resistor and capacitor on the filter PCB which connect via pins 11 and 12.
The output from the amplifier is split two ways.
One output is fed back to the filter via pins 13 and 14 of Cn3 and reduces the filter loss to approximately 0.5dB
The second output is fed to the output buffer transistor Tr4 which has a constant current source (Tr5) which greatly reduces the distortion of the output stage especially at higher output levels.
The output from the HF section feeds terminals 1 and 2 of the output connector Cn2.
LF section
The LF section is similar to the HF section with Tr7 being the input buffer feeding the filter input on pins 3 and 4 of the LF connector Cn4. Then output of the filter returns on pins 9 and 10 of Cn4 and fed to an extra buffer amplifier (Tr8) which feeds back to the filter on pins 5 and 6 to reduce filter losses.
The output of Tr8 feeds the LF amplifier Tr6, Tr7 which is another adjustable gain amplifier with feedback to a resistor and capacitor on the filter board. However the LF amplifier also has the option on the filter board to produce LF boost for baffle step compensation or a bit of bass boost for small loudspeakers. These extra feedback components on the filter board need to be fed from the output of the amplifier which is done on pins 13 and 14 of Cn4.
The output of the variable gain amplifier is fed to an identical output buffer and constant current source to the HF section and delivered to pins 3 and 4 of the output connector Cn2.
Rl1 is a muting relay and shorts the HF and LF outputs to earth to mute the loudspeaker outputs.
The mute relay’s coil is connected to pins 3 and 4 of the Power / Mute connector Cn5.
How to use the mute relay is discussed in more detail in the wiring section of the appendix.
DC power
DC power is delivered to the module using pin1 (+ve) and 2 (-ve).
The power passes through an optional dropper resistor to two DC regulators (Tr6, Tr13) which feed the HF and LF sections. Zener diode D3 provides the reference voltage for the regulators.
Construction