![]() ![]() In fact, a negative power supply rail can be added or, what might be better still, a FET-based constant-current source could be used, which would not require the bipolar power supply. Of course, the 5687 and 12AX7 are not the only tubes that can be used two 5687s or two 6SN7s can be used instead, or a 6BX7 and 6SL7 might make a good pairing as might a 5751 and two 12B4s. Still, this is an interesting circuit and I wish I had time to throw a working example together. The power supply must be well-filtered or regulated, as this topology does not enjoy any Aikido techniques to sidestep the power-supply noise. Much like the Aikido circuit, this line stage amplifier will require a heater power supply that is referenced to ¼ of the B+ voltage, in this example, +75V. The total gain is roughly +20dB (1:10) the open-loop gain is closer to +34dB (1:50). ![]() The output triode's cathode resistor is made up of three sub-resistors, which works as a voltage divider to present the feedback to the bottom 12AX7’s grid. The rest of the topology falls into place, with the second 5687 delivering a low-distortion and low-output-impedance in the cathode-follower-based output stage. Then this bottom 12AX7 is loaded at its plate by another 12AX7, which helps lower its distortion, as the symmetrical loading offers a countervailing twist to an already twisted active circuit element. Note that 5687 triodes are not used throughout instead, a 12AX7 receives its input signal at its cathode from the input 5687’s cathode. (1,360 ohms is shown in the schematic, because the 12AX7’s current conduction must be factored in the resistor’s value.) So, while a negative power supply and a much larger cathode resistor will produce less distortion, the 1,500-ohm cathode resistor isn’t that much worse. A 1Vpk input signal is only 1/15th as large, which means that the triode will only have to undergo a small change in current conduction to accommodate the input signal (10mA/15). The 5687’s low mu (about 17) and the B+ voltage of 300V requires a -15V grid-to-cathode voltage to maintain an idle current of 10mA. Well, the same trick can be applied to a common-cathode amplifier, as shown below: Instead, rely on a low-mu triode and a high B+ voltage to allow a higher cathode voltage and, in turn, a larger-valued cathode resistor, largely bypassing the need for a negative power supply rail. ![]() Long story, short: don't use a negative power supply. ![]() The last blog entries mentioned the trick of making cathode followers on the cheap. So it was when I reread blog 42: “not bad, not bad at all” was my fond observation.) But after a year or so passes, I look back on the once-hated piece of writing with a small smile. After I have finished writing, I always feel that I have done no more than just sketched out the roughest first draft and that a lot more work is needed. (I suffer from a quirk: I cannot stand anything I have written-well, I can't stand it right after I have written it, to be exact. I do not want to recap the basics of the circuit any further here, as I have covered that aspect of the topology before. The down side to the circuit is the usually used high-voltage negative power supply rail and the issue of dissimilar grid or B+ voltages, as the input triode’s plate connects to the B+ directly, whereas the output triode’s plate sees a plate resistor between its plate and B+. Additionally, a negative feedback loop can readily be applied, because of the inverting, high-impedance input presented by the rightmost triode. This last characteristic is due to a very low input capacitance for the input, as no Miller-effect capacitance is created. This topology offers low distortion, no phase inversion at the output, and a wide high-frequency bandwidth. I am a big fan of the common-cathode amplifier (also known as the “cathode-coupled” or “grounded-grid” amplifier-no thanks to Bruce Rozenblit, whose taxonomic howler has made already murky waters murkier still). ![]()
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