[quote author="ruairioflaherty"][quote author="mr coffee"]Could this be re-done to use a Siliconix VCRN2 n-chan JFET
?
Aren't they supposed to be better than p-chan?[/quote]
From the PDF page 4 - "The circuit is designed for a p-channel FET. A n-channel device cannot be used because the polarity of the control voltage is not correct."
I'm only quoting, I don't know that you couldn't use an n-channel if the circuit was modified (perhaps by using A4 in non-inverting mode, I'm sure there are other issues too)
cheers,
Ruairi
Thanks for the link JCM, really enjoyed studying this tonight.[/quote]
It would be sufficient I think to reverse diode and transistor polarities, and the rail for the Q's and R8 would become the negative 15 rather than +15. But there isn't much to be gained for the voltage-controlled resistor applications between P-ch and N-ch IMO. The situation is much different with amplifiers, where the higher mobility of electrons versus holes confers advantages on silicon N-FETs in terms of gain-bandwidth and noise, all other things equal.
Now, if you extended the design to stereo and used a dual FET then you might well want to go with the sex change for parts availability. But as John pointed out you might well reconsider FETs as the active (i.e., ~gain-variable) elements anyway.
BTW JCMaudio: is the complete document only those 5 pages? I should have thought Leach would have provided some performance data.