Let’s start with why tubes have high output impedance.

This post is not about the burger chain, though I must give them a hat tip for their excellent (off menu) grilled cheese with all the trimmings – a delight for vegetarians like me.

A number of readers have asked why tube equipment has high output impedance. High enough, in fact, that problems with low input impedance can be an issue.

Let’s start with why tubes have high output impedance.

Tubes run on relatively high voltage and that means commensurately higher impedance values. A vacuum tube preamplifier, for example, might operate with 100 to 300 volts while a solid state version runs at 1/10th of that. If the current is to remain the same between the two designs, the resistor values of the tube circuit will be ten times higher than those of the solid state version. Thus the tube preamp will almost always have higher output impedance.

Tube designers have choices when it comes to controlling impedance. Adding an extra tube at the output can significantly lower this figure. So, for example, a simple vacuum tube preamplifier might have an output impedance of 10KΩ. Add a second tube configured in what is known as a cathode follower, and that impedance drops to 600Ω.

It’s always helpful to check the output impedance specs of your tube preamplifier before matching it up to another product. We’d like to see an approximate difference of 100X between the two, though anything greater than 20X is probably adequate.

Thus, if your preamp has an output impedance of 600Ω, an amplifier with an input impedance of 50KΩ or higher would be perfect.