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RE: Tube preamplifier output coupling capacitor install

The coupling cap, whether it is located at the output of the source or the input of the receiver, along with the input impedance of the receiving device, form a 1st order capacitor/resistor high pass filter. A high pass filter circuit only passes signals above the selected cut-off point eliminating the low frequency signals from the waveform. That makes it fairly easy to determine the correct value for the coupling cap once we define the lowest frequency that we want to pass through the device using the formula fc = 1/(2πRC). Fc is the cutoff frequency, R= the input impedance and C= the capacitor value.

So, for example if the receiving device has an input impedance of 10k ohms and we select a 2uF capacitor, the cutoff frequency will be 7.95Hz (use the calculator below). With a 1st order filter the slope of the filter is 6dB and the output at the cutoff frequency will be 70.07% of the non attenuated signal. What this means is that the effects of the high pass filter will affect frequencies above the cutoff frequency. So for instance in this example, the filter will affect frequencies up to about 12Hz.

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