The STC Tone Circuits require about 3mA of operating current. Nearly all phantom power circuits feed the voltage through 6.8K resistors. 3mA flowing through a 6.8K resistor will produce a 20.4-volt drop across the resistor. If you are working from a 48-volt phantom power supply, this will leave approximately 27 volts to work with in powering the STC. Placing a jumper across the battery terminals will not allow the unit to work from a phantom power source. It will be necessary to connect the red battery wire to the hot terminal of the output jack. It will also be a good idea to decouple the onboard controls from the DC voltage of the phantom power by inserting a fairly large value capacitor between the phantom power connection and the signal wire coming from the master volume. 1uF/50V or larger should be used.
Three other important points to be aware of:
1. The voltage drop mentioned previously will always occur no matter what the phantom power voltage is, so, if you are only starting with a 15V phantom power source, you will basically have inadequate voltage to operate the STC preamp.
2. Many phantom power sources are very poorly filtered because they are intended to be used in conjunction with a balanced system that will tend to cancel noise. The STC preamp does not have a balanced input and is working in single ended voltage mode. It will be very susceptible to power supply noise, so if you suddenly experience much more noise than operation with a battery, this is the reason.
3. The maximum voltage that I would apply to the STC preamp would be around 30 volts. Cross above this voltage and you will be exceeding the voltage ratings of some of the capacitors in the STC circuit exceed 36 volts and you will blow the op-amp up within a minute or so. Check it will a voltmeter at the output jack with everything fully connected.