4
Figure 3. TX2 front panel
The way the TX2 operates with and without the sub-bass system can be seen on the
simplified block diagram of Fig. 4.
When the front panel push-button (left side of the SUB-BASS window) is depressed,
the main - left and right - channels operate full range with the signal passing through
the LF EQ section, resulting in an extension of the bottom end response of the
system. This section consists of a ‘High Q’ 2nd order high pass filter which provides
electronic boost where required and also cuts very low frequencies.
When the switch is pushed in, with the adjacent red LED coming on, the LF EQ
section is bypassed and the signal on each main channel is re-routed via a high-pass
filter at 100Hz designed to provide an optimised acoustic summation for the sub-
bass system.
On the sub-bass channel, signals from both left and right inputs are summed
together, and the resulting mono signal is low-pass filtered before being sent through
the level control section.
To sum up: the push-button should be ‘in’ when a sub-bass system is used, ‘out’
when not.
However it should be noted that the sub-bass channel itself is not affected by the
position of the switch, which means that it is possible to have the main system
operating full range while the sub-bass is also in operation. In that configuration both
systems would overlap over a wide bandwidth, with a large phase cancellation.
Reversing the polarity on the sub overlapping the main system would on the other
hand give an excess of energy in the 60-100 Hz range, but to the detriment of sound
accuracy, headroom and sensitivity to spatial alignment of the loudspeakers.
Figure 4. TX2 simplified block diagram.
