Tube DIY Asylum: Re: Confessions of a plate choke convert & a few CCS questions (long) by Lynn Olson

One of the traps people fall into is not realizing just how low DHT distortion really is ... and that DHT's need a not only a lot of voltage, but enough current to overcome a substantial amount of capacitance (this controls slew rate).

How low? Well, the Amity has no trouble putting out 16 PP watts at 0.3% distortion, and 1.6 watts at 0.07%. That's *including* the distortion of a PP 7119 driver. If you really went nuts on the driver and used something like another 300B (!) as a driver, I'd bet the distortion might fall even more, maybe down to 0.2% or less.

300B's need anything from 42 to 60V rms of extremely linear drive to realize their full potential. I doubt that most people have even heard what a 300B really sounds like ... many DHT amps have more driver distortion than the final stage, and the drivers have the additional disadvantage of not delivering enough current to the DHT grid.

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!)