John Broskie's Guide to Tube Circuit Analysis & Design

25 November 2016


Quasi-Single-Ended Amplifier
I have lots more to say on the topic of single-ended amplifiers, but I only have so much time today, so here is a quick post. I have enjoyed an email exchange with TCJer, Sheldon, who is designing interesting hybrid power amplifier topologies. He pointed out that the new hot thing is to load a chip power amplifier's output, such as the LM3886, with a constant-current source, which will cause a displacement of the amplifier's crossover points within the output stage transistors. Cambridge Audio calls the technique Class-XD, which I covered in post number 88, in 2006.

In 2002, in the Tube CAD Journal, you will find the following schematic.

This idea is old-hat to those working with OpAmps, so I wasn't claiming any new topology, let alone creating a new class of operation, rather I was pointing out that such a setup could be implemented externally to an existing power amplifier; in other words, as an add-on enhancement in its own enclosure.

Of course, the huge problem is heat, as the existing power amplifier's output stage will now greatly increase its dissipation. With chip amplifiers, this problem is particularly acute, as they are relatively small and, thus, quite heat sensitive. Think about it: a power amplifier made of discrete parts might hold six output transistors per channel, each transistor being rated for 150W and each offering a thermal resistance of only 0.7C/W. in contrast, the LM3886's thermal resistance is 1C/W for the entire amplifier.

The workaround I came up with is to feed the chip amplifier's positive rail voltage through a quasi-class-G setup. It's not real class-G, as only one positive power-supply rail is used, not two. (If you want, you can think of the following amplifier as being a 4W single-ended design, which can put out 36W in push-pull class-AB.)

Okay, I agree that the schematic looks super complicated, but the idea behind it is simple. As the amplifier's output swings positively, its positive power-supply rail voltage rises. As the amplifier's output swings negatively, its positive power-supply rail voltage falls, but only down to about 10Vdc, where it remains until output voltage swings back up.

Why bother? Why not just let the positive rail voltage follow the output signal?

It looks good, but looks can be deceiving, as we all know all too well. The big problem here is that although the amplifier's output can swing -25V, the input stage only sees a -1Vpk input signal, but the power-supply rail positive voltage is at -12.5V. Not good. In fact, the LM3886's positive rail voltage should never fall below 9V, according to its data-sheet; thus the need for a lower voltage limit of +10V.

The more that I thought about it, the more I began to worry about the zener being able to engage quickly enough, say at 20kHz. It might pass the SPICE test, but not the reality test. The workaround is to keep the zener constantly conducting, as show below.

Now, when the output swings sufficiently negative to forward bias the 1N4148 signal diode, the MOSFET stops following the output voltage.

Okay, John, this is all good stuff, but where are the dang tubes?

Tubes, you want tubes? No problem.

The tube-base input buffer runs off the LM3886's power-supply rails. The resistor labeled with a "?" serves two purposes. The first is to allow a path to ground for the 1µcoupling capacitor at startup, when the triodes are cold and still not conducting. The second is that allows us to force some 2nd harmonic distortion on the buffer's output.

I like to run my 6DJ8 triodes, however, with a bit more than 30V of cathode-to-plate voltage. The following design uses 80V of cathode-to-plate voltage.

Before anyone panic, let me assure that getting the four power-supply rail voltages from a single center-tapped secondary is no big deal.

Note the +/-90 volt rails, not +/-80V rails. I assume that two RC filters would be used. A 44Vac CT power transformer would be used.

Happy Black Friday.



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