Pass Labs B1 Buffer

I decided to make a B1 buffer for my Nelson Pass F5 power amp after listening to a friends version. The B1 is a no feedback JFet buffer that offers low distortion and noise with a very wide bandwidth. Being a buffer it has no gain and just acts as an impedance matching device with a volume control. It can be used between source and power amplifier and even used with another active stage. For example I have used it between my tube pre-amp and the power amp with the volume on the B1 set at maximum so it just acts as the impedance matcher. It is fairly easy to make and if you wish can be run off 2 x 9v batteries in series or a simple brick wall power supply that outputs 18 to 24V. If you do not need volume control you can make it as a pure buffer / impedance matching device.

Build

Most parts in my build are nothing too exotic. Input caps are low voltage film so suitable for coupling duty here. Output caps are MKP 10uF Metallized Film 100V off eBay. Transformer is a 50VA potted one, from my spares box, normally used for 12V Halogen lighting so vastly over-rated here. Rectified and smoothed this gives just under 18V, close enough for me as 18V is needed. All audio signal wire is from Diy HiFi Supply, solid copper routed in some clear Teflon tubing with thicker copper for the two audio earth runs to the PCB from RCA sockets and Pot. The Volume Pot is a miniature Green Alps. The case and volume knob are solid aluminium types, also from Diy HiFi Supply.

The earth is lifted by a 24R resistor. The B1 does not strictly need this when used on its own but sometimes I have tested it feeding a Vacuum Tube active stage in a separate case; such as the 12B4 pre for example. In other words I have CD Player --> B1 --> Active Tube Stage --> Power Amp. This then gives some final gain (about 5db) to drive the F5 power amp. As a result the audio earth needs to be separated from the B1 by some small R to avoid an earth loop.

This is perhaps an unorthodox combination, essentially to most people it looks like I am using 2 pre-amps when viewed on the Hi-Fi rack. Well strictly it is 1 unity gain buffer (impedance matcher) and 1 5dB tube pre-amp stage but it is only the same if that extra final gain stage was housed inside the power amp. Doing this combo depends on the power amp in use I guess. If the power amp is a little 'deaf' and insensitive on its input then an active stage with some current output such as the 12B4A tube gives that extra drive. This really gives the dynamics a major boost in my system. The battle for detail and dynamics occurs at the pre-amp stage.

Sound

Sound of the B1 on its own is very good if not the best I have heard. It sounds neutral, clean, tidy and fast but after a while I find I miss that little bit of tube harmonics, colouration and warmth of a tube pre-amp. If you want a pre-amp or buffer that has little colouration then the B1 is ideal.

Tweaks

Thanks to Val for the 0.01uF Russian Silver Mica which I used to by-pass the input audio coupling caps. I also by-passed the output caps with some 0.22uF Obbligato I had spare. I also managed to replace some of the copper in Teflon with silver in Teflon. I also lowered the value of R102, R202, R104 and R204 from 1K to 220R.