UBx Universal JFET Buffer
A while back, one of the diyAudio forum members who was building the nx-Amp (here’s the thread on DIYaudio.com) enquired about either reducing the amplifier’s overall gain, or providing a volume control facility. Since both the 15 W class A sx-Amp, and its bigger 100W sibling the nx-Amp, are Current Feedback Amplifiers, performance is quite carefully optimized for a specific gain – they are not as tolerant as voltage feedback amplifiers in this regard. Although not specifically raised in the thread, a consensus seem to develop that what was actually needed was a general purpose buffer that would accommodate the input from a pot, and be able to comfortably drive a power amplifier with typical input impedance of 10 k Ω.
No originality is claimed – Nelson Pass’s B1 buffer has been around for years and is well regarded – the UBx ‘Universal Buffer’, as this will I hope come to be known, also uses this classic configuration (but without the cascode), which I believe was first published by either Siliconix or National Semiconductor way back in the late 1960’s.
You can download the PDF file here UBx