"REAL WORLD SLEW EFFECTS:
Let’s assume the source current and sink current capability are the same. [Which, strictly speaking is definitely *not* real world, but keeps the analysis simple]. Then at high frequencies, and low enough levels, the system is linear, and a sine wave in produces a sine wave output, neglecting distortion produced in the tubes, transistors or whatever active devices you care to consider. The frequency response will be limited by the effective R and C of the system. At higher levels, slew rate limiting can occur, if the current required to meet the I = C * dV/dt is not available. This will first be noticed at or near the zero crossings, since this is the highest slope condition. If even more level is asked for, this will distort more and more of the waveform. As the slew limiting distortion approaches the maximum amplitude of the sine wave, further increase in input level (demand) will not cause the output to increase; and the waveform will look sort of triangular, but with a somewhat rounded top.
Note that this differs from a simple clip condition where the top or bottom or both of the output waveform will be “clipped” or flattened. However, as we will see below, this can also occur.
Note that I did not need to invoke any feedback mechanism in order to cause slew limiting to occur. Well, does feedback affect slew limiting? You bet it does. Why? The feedback “error” signal will indicate to the system that “more” signal is needed, and the driver will merrily try to pump more signal into the slew limited node. This will cause the output to look even more triangular, and probably clipped as well, since the error signal will never “correct” the errant condition.
This is *very* similar to what happens in non-slew limiting condition when a feedback amplifier “clips”. This drives a high level signal into the clipping stage. In fact, this is THE one example where clipping AND feedback can actually induce slew limiting that might not otherwise occur in a non-feedback amplifier. [Since the amplifier may try to pump, say, 20dB more signal than normal into the node, 10x the slew rate is also being called for. This may slew limit the driving stage while the driven stage is clipping]."
http://ken-gilbert.com/slew-rate-limiting-bench
"Although bare-bones engineering indicates that only a few milliamps are needed for full power at 20kHz, it’s easy to forget that slew distortion is merely the current equivalent of voltage clipping … and we all know distortion doesn’t just go from zero to full. Instead, distortion gradually falls with decreasing level. The same thing happens with *current* distortion - the less current demanded, the more linear the driver is. Well, we can’t do much about the current demand of the 300B grid, but we can certainly increase the current running through the driver … by several times if transformer, choke, or active loading is employed.
Improving the slew rate by several times doesn’t sound like what you’d expect. People expect shimmery triangles and amazing transients, but that’s not what you notice. Instead, there is a dramatic reduction in the “electronic” coloration of the sound - in fact, most of the “tubey” vintage aspect of the sound disappears.
Since slew distortion is greatest at the zero crossing of the waveform, anything we can do to remove even the slightest trace of slew distortion (which is a different animal than generic THD) will give an astonishing improvement in hearing subtle details in complex passages. After all, we can all live with some distortion on the signal peaks, but we *don’t* want any distortion at all in the zero-crossing region.
Personally, I think it’s impossible to overdesign a driver or power supply. You want the most linearity possible with lots of linear current available, and the power supply needs to deliver lots of peak current while isolating the audio circuits from rectifier switch-noise and AC line noise.
If you’re tired of vintage sound, improve the driver. Go nuts. Aim for 3 to 6dB of headroom or more, at least 15-20 mA of operating current, and transformer, choke, or active loading. (And yes, they all sound different!)"
http://ken-gilbert.com/slew-rate-redoux
