Just behind tube phono preamps and tube headphone amplifiers, the largest number of circuit requests I receive is for tube-based crossovers. Using tubes rather than ICs in a crossover requires no justification for those who already have a tube-based system. But for those not so lucky, or for those solid-state besotted, they will be pleased to learn that the tube does not sacrifice too much measured performance compared to the IC in this application; however, they will be saddened by the fourfold increase in cost. So is a tube crossover worth the cost and effort? One friend tells me that the single biggest improvement in his system resulted from replacing his expensive solid-state active crossover with a tube-based one.
Fortunately, the vacuum tube is better suited to the task of actively buffering filter components than it is to other tasks such as amplifying the frail phono cartridge's output or driving the headphone's punishingly low impedance. However, taking advantage of the vacuum tube's attributes requires a little extra thought when designing a tube-based crossover to maximize the performance and to protect the tube from possible damage.
Bipolar Power Supplies
The easiest tube topology to implement is based on cathode followers and bi-polar power supplies. The cathode followers both isolate the filter sections and provide the means of interjecting positive feedback back into the filter. The positive feedback aspect is seldom considered, but it is vital to creating an active filter. You see in the absence of the reactive pairing of inductor and capacitor, there is only lost gain and gain is necessary to compensate for the otherwise sagging output of a purely capacitive and resistive network. The unity gain output of a cathode follower bootstraps signal going into the filter, which allows for the elimination of the inductor. (And as was covered in the previous article, inductors are a nuisance.)