The Tube CAD screen capture on the previous page shows the first grounded-cathode amplifier configuration that is common to both circuits. The screen capture below shows the first circuit’s second stage. Note the asymmetrical maximum voltage swing (-44V and +98V).

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      The next screen capture shows the second circuit’s second stage’s configuration. Note the larger symmetrical voltage swing, but a lower gain and a higher output impedance.

      Which is better? “Better for what?” is the question. Which is more important: gain, output impedance, output voltage swing, or PSRR? And then, which is worse: having the signal flow through an electrolytic capacitor or a film/oil capacitor?

      Fortunately, testing these two circuits is easy enough: simply build the second circuit and tack solder a coupling capacitor, a 1Meg resistor and a 20µF electrolytic capacitor to ground only. Next test, listen, evaluate. Then tack solder the electrolytic capacitor’s other end to the second triode’s cathode and unsolder the 1Meg resistor’s connection to the 0.1µF capacitor and short this capacitor out with a piece of wire. (Of course, the amplifier has been unplugged from the wall and the power supply capacitors have been discharged first... audio constructors are already going extinct without any extra help.) Once again, test, listen, evaluate.

      (My guess is that if the output tube [or tubes] is an EL34 or 6550, then the first circuit just might be the winner, but if the output tube is a 300B or 211, then the second circuit would clearly be the winner, as these  tubes require a much larger grid voltage swing.)

     So you see, Pawel, going capacitor-less and going without feedback isn’t as easy or as obvious as we might imagine. Furthermore, I haven’t even touched on the topic of safety. Do you really want to risk $600 Grado headphones on a capacitor-free output stage? Hook up a voltmeter to the outputs and blow on side of one tube and see if the reading doesn’t change by a few volts. Speaking of safety, I would never put my son’s father’s ears between two thinly isolated wires that connected to a 100V power supply! (My ears sweat after half an hours worth of headphone listening.) A final quibble is on the topic of potentiometers, like the one used to set the DC balance in your design. Put plainly, most potentiometers are crap. They lose scraper contact, they burn out when overburdened, and they can sound much worse than an electrolytic capacitor. Yet audio neophytes love them for some odd reason (maybe they can understand their inner workings, unlike a pentode or MOSFET). One good rule to follow is never to pass a DC current through the potentiometer’s scraper, unless the potentiometer was specifically designed for the task. Bear in mind that a 1W potentiometer is only capable of withstanding 1 watt of dissipation across its outer leads. Thus the practice of using a potentiometer as a variable resistor is unwise, unless the currents and/or voltages are truly trivial. 

     But enough quibbling, let’s look into your circuit. First of all, just about everyone who dabbles with tube electronics comes up with this topology at some time. It seems to offer everything we want: DC coupling throughout, gain, and a low output impedance. Well, it is DC coupled. When burdened with a 32-ohm load this circuit’s high output impedance (3k from cathode to cathode) realizes very little gain, 0.156 to be exact. In other words, it takes 2 volts of input signal to equal 0.312 volts of output signal. The distortion is fairly good at about 1%, with odd order harmonics leading over even order, which is typical of a push-pull amplifier.

Pawel’s circuit: a DC coupled headphone amplifier    Download B2 Spice A/D circuit