The simplest tube mixer is made up of several series input resistors feeding one common grid. The resistors further the isolation between inputs, i.e. serve to limit the back-feeding of input signals. The input signals are summed at the grid and amplifier at the plate. Unfortunately, this circuit is too simple. It fails to truly isolate the inputs; it provides unwanted gain; and it adds excessive distortion. 

       The mixer is an essential tool in professional audio endeavors. Bands, concert halls, recording studios, and radio stations all use mixers to blend audio signals from different sources into one signal. Ideally, the mixer only mixes at its output and not at all at its input. In other words, the input signals should not back-feed into each other. While this is small a concern when low output impedance solid-state devices are the sources, it does become a large concern when the input sources are high output impedance tube circuits. Furthermore, a mixer should not diminish or augment the signal; its job is only to mix input signals.

      Adding one resistor to the circuit makes the difference. This resistor completes a feedback path that will provide much greater isolation, unity gain, and both a low distortion figure and output impedance. This circuit is the function equivalent to the previously shown Op-Amp circuit. Adding a cathode follower to the circuit will substantially lower the circuit's output impedance, as the cathode follower output is included in the feedback loop and its already low output impedance will be decreased by the feedback ratio. For many readers, we need not go any further, as they have all that they want in a tube mixer. For other readers, the feedback aspect annoys. "Couldn't a feedback-less design that functions as well be created?" they wonder. It can.

     With the advent of the solid-state Op-Amplifier, mixer design became trivial. One Op-Amp and a handful of resistor is all that is needed. The circuit below illustrates the simplicity of the modern mixer. Making an effective tube mixer requires a little more work.

< PREVIOUS

NEXT >