Tube-Based Crossovers

   The bipolar power supply allows us to keep the heater power supply referenced to ground and it allows for DC coupling at the input, two real benefits. Yet many tube aficionados won't touch a bipolar power supply. True it is harder to make a bipolar power supply with tube rectifiers and off-the-shelf power transformers. But making a bipolar power supply with solid-state rectifiers and center-tapped off-the-shelf power transformers is a breeze.
    Still, the advantages that derive from using the bipolar power supply are worth looking into. For example, the goal of not using the signal  ground for any purpose other than signal processing is much more easily done and often power supply noises can be made to cancel with a bipolar power supply. The circuit below shows a tube filter that uses a bipolar power supply.

    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.)

Bipolar power supply based crossover

    This is a 2nd order lowpass filter that can be made to realize a Bessel, Butterworth, Gaussian, or Linkwitz-Riley alignment depending on the relative values of resistors R1, R2 and capacitors C1, C2. The 6N1P is a Russian tube that has earned the respect of many tube fans. It has a mu of 34 and an rp of 9750 ohms at 200 volts and 5 mA. It is similar to the 6DJ8, but not at all directly interchangeable with it. This circuit protects its tubes by including three diodes. The first two, the zener-diode pair, only conduct when the cathode falls below about 10.7 volts. The last diode is normally reverse biased and only conducts when the grid is over .7 volts more positive than the cathode (something that should never happen). The only time any of these diodes should conduct is when power is first applied and the tube is still cold.

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