BOZ Bias Current

[gema]

In original Mr. Pass’s article we can learn, that the correct bias current should be 40mA or so.
But more then one DIYer pointed out that with such bias, BOZ clips very asymmetrically. And to correct this the bias current round 30mA is needed.

And here is my dilemma.

It is possible that I somewhere missed Mr. Pass’s explanation why 40mA? Is 40mA nevertheless the right figure? Is 40mA a mistake? Mr. Pass?

On the other hand I realize that ZEN clips way before BOZ does. Is that the reason that we can afford higher bias current?

Regards.

[GRollins]

If you're clipping a preamp circuit, you're running way too hot a signal.
Besides, the Zen and Bride of Zen aren't necessarily the ultimate circuits. Nelson's intent in designing them was to show how much can be done with just a few parts and a simple circuit. There are scads of ways to improve on them, but nearly all of them will make them more complicated and more expensive.

Grey

[Nelson Pass]

The 40 mA figure is indeed correct, and was
chosen as a balance between better performance
at low levels vs high level clipping.

You can regard the output of the preamp as being
high quality up to about 3 volts or so, and as you
can see from the curves, the distortion continues to
climb until it clips somewhere above 10 volts rms
or about 15 volts peak.

I continue to recommend the addition of an input
pot as an adjustment, say 10 to 25 K ohm. This is
set by placing the output pot at max and adjusting
the input pot until the output level is as high as you
are likely to want in real use. After that, use the
output pot as a volume control.