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Controlling the power
As we established the most appropriate method of control is by varying the supply voltage of the mains power supply. Most
mains power supply units have a trimmer that you can use to set the output voltage. You can remove that trimmer and
connect two leads to the control board header J11 (F).
Fig. 36: Block diagram of the Amplifier Control Board v4.0
There are mains power supply units out there that have dedicated control voltage input, typically this control voltage needs to
be in the 0-5V range. You can connect header J11 (F) directly to this control voltage input. In order for this to work you
have to install the two jumpers J11. This mode of operation is called mode B.
Another thing to consider is that with some mains power supplies the output voltage rises proportionally with the control
voltage and with other it is inversely proportionally to the control voltage. You can compensate for this with J20 (J). It lets
you select falling or rising mode.
You can also use control voltage (mode B) to control adjustable attenuator. This is usefull in linear mode applications, such
as TV amplifiers. Low power adjustable attenuators are relatively easy to construct.
For more advanced users here is a diagram of the control circuitry.
Fig. 37: Diagram of the control circuitry of the Amplifier Control Board v4.0
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