5) On first powering up the amplifier, and during adjustments, use a current-limited supply. If
anything does go wrong, you’ll have more time to notice it before letting the smoke out.
7.2 Power supply
For full 50W power output the PA requires a supply of 20V at about 5 or 6A. Ensure you are using
a power supply which is up to the job! The power supply needs to be capable of supplying full
current suddenly without any voltage drop on key-down. Use THICK cables from the power supply
to the amplifier, to avoid voltage drop.
Note that the QCX should not be powered by the same 20V supply, even if the power supply is
capable of supplying the additional current required by the QCX. A supply voltage of 20V is too
much for a QCX.
If you wish to operate the whole project from the same 20V supply, then use a voltage regulator
such as 7812 (with a suitable heatsink) to drop the voltage to 12V for the QCX.
Alternatively, you could run both the QCX and the PA at a lower voltage, which will give a final
power output of less than 50W (please refer to the measurements in the later section).
7.3 Connection to QCX
The diagram shows an appropriate connection between the QCX and the 50W PA kit.
A simple coaxial cable is used from the QCX RF Out to the 50W PA RF In (note, Out/In
nomenclature here assumes the system is in the transmit state; naturally the direction of signal
flow is reversed during receive).
The QCX should be operated at 12V supply. A full 5W of power output is not necessary to drive
the power amplifier to full 50W output and will over-drive the amplifier, causing potential distortion
and glitches. With 12V supply the slightly lower QCX Power output will be perfect. A 7812 voltage
regulator (TO220 package) with heatsink is a perfect way to power the QCX, if you wish to operate
both the QCX and the Power Amplifier from the same supply.
The “PTT” (Push To Talk) or Transmit-enable signal requires a 3.5mm stereo plug to connect to
the 50W PA board. Connect the “Sleeve” to ground, and the PTT signal to the “ring”. The tip can
be left unconnected as the socket in the 50W PA does not have a connection to this pin. At the
other end of the cable, connect the ground signal to QCX ground, and the “ring” PTT to IC3 pin 11
of the QCX. This signal goes high (+5V) when the QCX keys its own 5W power amplifier.
50W QCX PA kit assembly
1.00q
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