Vacuum Tube Mixers

Vacuum Tube mixers have been around for many years. In fact, many of the old mixer designs are still in use today. The pro audio industry and hard core audiophiles swear by their "perfect Sound" quality. Some say you can "restore" that "Tube Sound" utilizing sound shaping processors - but the Audiophiles who are die hard will tell you "No Way". The argument that a "Tube" Amp or Circuit will "Require a Warm Up Period" doesn't fly either. After all - who turns their amp on and off between CDs or Records? After a 30-45 second warm up - the tube circuit performs just like any other audio circuit - it does what it is designed to do.
This section is therefore dedicated to the Analog Vacumm Tube Mixer, it's design, its characteristics, and it's applications. We start off with a simple mixer circuit utilizing the 6922/6DJ8 Triode Vacuum Tube - the European Equivalent is the ECC88 Tube. The 6922/ECC88 has gold plated pins (which reduces the tendancy of Pin Oxidation while in-use) and is therefore a higher quality tube, compared with the 6DJ8 Tube - which has tin plated pins. This vacuum tube has a plate voltage requirement of only 90 Volts D.C. The filament requires 6.3 Volts.


The Simple tube mixer
Using the 6922/ECC88 Tube

       
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.



Feedback and the Simple Tube Mixer

        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.


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