Tube DIY Asylum: Partial slew rate distortion by Lynn Olson

Hi Ken, just read your discourse on your Web site, but I must respectfully disagree about the all-or-nothing nature of slew rate distortion. I've been down this path back at Audionics in the late Seventies with solid-state amplification.

What we saw on the scope and spectrum analyzer accorded with the final voltage-gain stage gradually losing its linearity as more and more current was demanded by the dominant pole of the amplifier, then reaching a "hard" slew limit when all the current was finally used-up by the capacitive load. And yes, feedback certainly worsened the slew limiting, with the worst-case situation being reactive (speaker) loads that ate into the available phase margin. The aspect of the CC-2 amplifier that the late Bob Sickler was most proud of was the combination of generous phase margin, high slew rate, and low intrinsic distortion in the forward path. This resulted in fast settling times even with highly reactive loads, and unexpectedly, much improved Class AB crossover transitions due to adequate phase margin with any load.

Moving on to vacuum-tubes, it's obvious from inspection of the published plate curves that the slope of the load-line affects distortion, and the opening-up of a flat (resistive) load-line into an ellipse (partly reactive) will make distortion significantly worse. When the elliptical load-line swings through the low-current region, abundant upper harmonics are generated.

This is a progressive effect, with distortion increasing with signal level, slope of the load-line, *and* percentage of reactance in the load. What's unfortunate about capacitive (as opposed to inductive) loading is that it falls in a region where the ear is particular sensitive to distortion - distortion audibility roughly parallels the Fletcher-Munson curve, so IM distortion at 5kHz is 20-30dB more audible than distortion at 50 Hz.

A few pF here and there is hardly a concern for transistor circuits that cheerfully operate at 100mA or more, but is a big deal for vacuum-tube circuits that operate at a tenth of that. The "stray pF's" are very hard to avoid: there's the Miller capacitance of the power-tube grid, the stray C of the choke, transformer, or dynamic load, and the inevitable capacitance of the socket and wiring itself. So our driver circuit is always going to see a net load of 50 to 100pF or more, and has the additional awkward requirement of swinging 40 to 60V rms per grid - at distortion figures below the already low DHT figures! This is not a trivial requirement for any driver section - trivial perhaps at 1 kHz, but not so easy at 20 or 50 kHz.

Interestingly enough, conventional RC-coupled PP amplifiers drive each power tube with what amounts to individual SE drive - one driver per output tube, of course. This means the elliptical load-line of the (reactive) power tube is presented to a SE driver circuit.

This is the subtle reason I chose to experiment with the little-explored and archaic path of IT-coupled PP drivers and PP outputs. I wanted to sum the PP drivers in the interstage transformer, so the grid-lines would be straight and parallel (PP Class A operation) instead of the highly curved grid-lines of SE operation. When grid-lines are straight and parallel, a change in load from resistive to reactive has very little effect on distortion. In SE operation, distortion typically increases several-fold when a reactive load is encountered.

The net effect of this choice is the PP 300B grids are presented with a drive that's low-distortion even at the highest frequencies, something that would not be true if conventional RC-coupling was employed. With pentode output tubes, a reduction in driver distortion is much less important, partly due to more moderate driver requirements (in both voltage and capacitance), but more importantly, the substantially higher distortion of pentodes (even triode-connected pentodes) compared to DHT's.

I admit this concentration on maximizing linearity at high frequencies is well off the mainstream - it's partly a result of my time at Audionics and Tektronix, but also partly the bias of a speaker designer that's forced to pick among very imperfect drivers with no recourse to feedback. You can use feedback with moderate success at low frequencies in loudspeakers, but things really fall apart at high frequencies, making feedback impossible to stabilize above 200 to 300 Hz thanks to cone break-up and the difficulty of finding an appropriate sensing point.

In speaker design, the entire speaker is made or broken in the critical 1 to 5 kHz region, the region of peak sensitivity for the ear. I carry this preference forward into electronics, and focus on HF performance into complex loads (which both speakers and triodes certainly are).

This is a broad definition of slewing - current distortion into a capacitive load - but I feel it's a useful distinction from midband distortion, where by definition the circuit is operating well away from edge-of-band inductive and capacitive loading. Just as voltage distortion is not an all-or-nothing phenomenon, neither is current (or load-dependent) distortion. As more and more of the quiescent current in the tube is consumed (or reflected) by the load, the amount of current flowing through the tube becomes a larger and larger percentage of the quiescent value. The plate curves of most triodes are well-behaved at high currents, but get a lot less linear when the current flow drops to 10-20% of the nominal quiescent value.

Another undesirable side-effect of the capacitive load is the phase-shifting of energy ... the capacitance reflects essentially all of the current back to the plate. This 90-degree phase-shift results in the region of maximum current demand falling not the top of the waveform, where you'd expect it, but the zero-crossing region instead. Since audio is a logarithmic perception, tiny degradations 20 to 60dB down from the loudest signal are important and perceptible - and the most likely place to find these small-but-significant signals is in the zero-crossing region. This is why a tiny amount of corrosion in a dry circuit can have devastating impacts on perception of space, dimension, and emotional subtleties - the damage is all occuring in the worst possible portion of the signal swing.

Hard-to-measure bursts of distortion that occur around the zero-crossing region have a subjective impact entirely out of proportion to the averaged-THD numbers. Some of the causes are slew distortion, Class AB crossover distortion, PCM errors around the LSB, or minor corrosion effects in a dry circuit. The reason is simple: although the burst of distortion may be brief as an overall percentage, it is selectively masking or actually removing low-level information from the signal.

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