Nelson Pass B1 Buffer Pre-Amplifier DIY



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 is really a source follower and outputs less than its input at less than full volume. It can be used between any source component 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 not silly exotic but not junk parts. 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 so vastly over-rated here. Rectified and smoothed this gives just under 18V, close enough as 18V is needed. All audio signal wire is from Diy HiFi Supply, solid silver and 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 Alps. The case and volume knob are solid single cast aluminium types, also from Diy HiFi Supply. The JFets are 2SK170-BL and a matched quad in this build.

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 a 5dB tube pre-amp stage but it is only the same if that extra final gain stage was housed inside the power amp or inside a single pre-amp box. Doing this combo depends on the power amp and speakers in use I guess. If the system is a little 'deaf' and insensitive then an active stage with some current output such as the 12B4A tube gives that extra punch and drive. This really gives the dynamics a major boost with electrostatics. The battle for detail and dynamics occurs at the pre-amp stage I once read.

 

Sound

Sound of the B1 on its own is very neutral, clean, tidy and fast. It really imparts little sound character of its own; so if you want some colouration or emphasis of certain frequencies then you will be disappointed... get a coloured tube amp. If you want a pre that does not get in the way or create some artificial sound stage or colour then it is perfect.

On its own it is a great match for the F5 and F3 power amps but maybe not the last word in scale. On some set-ups it may be even a little underwhelming where more gain is needed for some extra 'punch' but I think it is the lack of colouration what makes it appear this way initially. The more you listen you start to realise that other pre-amps are quite artificial and corrupt the signal to some degree... the B1 is the polar oposite.

 

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. I later removed the Silver Mica by-pass caps and used some Tin Foil.

** Main circuit inside zip file download below **

 

Nelson Pass B1 Buffer Pre-Amplifier
Nelson Pass B1 Buffer Pre-Amplifier
Nelson Pass B1 Buffer Pre-Amplifier
Nelson Pass B1 Buffer Pre-Amplifier
Nelson Pass B1 Buffer Pre-Amplifier
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