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guitar amp preamp output impedance circuit

What does this calculator do?

This calculator computes the voltage gain and output impedance of a common-cathode amplifier. Output impedance represents the ability of a circuit to drive the next stage. An output impedance of zero ohms signifies that the driving circuit is a pure voltage source. It can supply any amount of current to the next stage. A non-zero output impedance represents the amount of voltage sag that occurs depending on how much current is required. This concept is portrayed by the so-called Thevenin Equivalent Circuit, which represents the driving circuit as a pure voltage source in series with its output impedance.

Thevenin Equivalent

guitar amp preamp output thevenin equivalent

The higher the current, the more voltage drop across the output impedance, and the more the magnitude of the output voltage is reduced. If the source voltage V is positive, the output becomes less positive. If the source is negative the output voltage becomes less negative.

Several of our online calculators require knowledge of the output impedance of the driving circuit. In many situations the exact value is insignificant. For example, if the output impedance is from a typical guitar pickup (usually less than 10k) or a cathode follower (usually less than 1k), the actual value is very low in comparison to traditional guitar amp circuits.

Higher output impedances are important to consider, but often easily computed. The output impedance of a guitar with its controls at less than maximum depends on the guitar circuit and its potentiometer settings, but is relatively easy to determine. Pentodes have a very high plate resistance, so a typical pentode amplifier has an output impedance approximately equal to its plate resistor value RL.

Triode amplifiers, like the one shown here, also have significant output impedance that needs to be quantified to achieve accurate results.

The calculator computes the unloaded voltage gain (the gain without being connected to the next stage) and the output impedance of a triode amplifier. Both of these values depend on the tube, resistor values, and whether or not the cathode resistor is fully bypassed by a capacitor CK. (For Partially Bypassed Cathode Resistors the output impedance is somewhere in between and depends on the specific frequency.)


1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).