4
Where amplifiers with unbalanced inputs are used, the ‘hot’ or +ve pin of the
amplifier input should be connected to either pin 2 or pin 3 of the TX3 output XLR,
according to which pin is the ‘hot’ pin on the device connected at the input. The
unused signal pin (3 or 2 respectively) should then be linked to pin 1 for grounding.
The use of amplifiers with unbalanced inputs - rather unusual for professional
products - is not recommended, among other drawbacks it will cause the level to
drop by 6dB on the outputs of the controller.
Operation
The unit should be powered on (green LED illuminated on the front panel) before the
amplifiers are switched on, this will avoid any switch noise or power surges which
could damage the loudspeakers. Similar precautions should be taken when power is
switched off (i.e. in reverse order).
Figure 3. TX3 front panel
The simplified block diagram (Fig. 4.) shows TX3 operation with and without the sub-
bass system.
When the rear panel ‘SUB ON’ push-button is in the ‘out’ position, 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 button is pushed in, with the red SUB-BASS LED on the front panel
illuminated, 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 (T300 running full range without additional sub-bass).
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 risk of phase cancellation
ocurring.
The SUB-BASS level control on the front panel can attenuate the sub-bass level by
12dB (-6 to +6). The potentiometer on the front panel is flush mounted in order to
discourage any unauthorised adjustment. A small, flat blade screwdriver is required
to adjust the Sub level.
The EQ MODE MONITOR push button is intended to tailor the bottom end for floor
standing/monitor applications. When the EQ MODE MONITOR button is pressed ‘in’,
the excess bottom end generated due to the close proximity of the floor (when used
