DIY First Watt Nelson Pass B1 Buffer Pre-amplifier

If you into this DIY audio for quite long time, you should already know the audio guru: Nelson Pass with his First Watt products. His idea and concept in making the amplifier is simplicity. All of his amplifier are made by using only few FET transistors. The most important thing about Nelson Pass is he kind enough to shares all his design to this DIY audio community, so you can build one and having the same experience like he did with his design.

Nelson Pass also designing pre-amplifier with the same concept. In this post I will share the B1 buffer pre-amplifier building process. The main function for this buffer pre-amplifier is to match the impedance between input and output. So the next equipment, such as the amplifier can be work optimally.

This B1 buffer pre-amplifier is consist pairs of SK170 FET transistor per channel. It is simple to build without any PCB, or just with a point-to-point connection.

I starting this project by collecting the main components first. B1 buffer need 4 pieces 2SK170 to make a stereo. The recommendation for this JFET grade is BL, but you can use the lower grade such as the GR as well.

In this build, my 2SK170 are not HFE pre-matching. You can do like I did but maybe you could buy more spares transistor and then test the HFE for each transistor one by one. Take the same parameter result for each transistor and put them in this B1 buffer build for more precise result.

The others supporting components such as resistors and capacitors can be pick whatever you comfortable with. I am using the cheap MKP output capacitor combine with high quality MBM paper-in-oil for the input capacitor. The resistor I use here are combine from Takman, Roedenstein and few generic brands resistors from Taiwan.

 


I think the most expensive parts in this build is this big electrolytic capacitor. It is MEPCO capacitor rated 19000uF each. We need 2 as per schematic provided by First Watt.

  

Because this B1 buffer pre-amplifier doesn't need so much current to make it operate properly, I can use almost any transformer size. The transformer rated above 10VA will do very well.

The power supply that I made here is using voltage regulator from LM317. It is better for regulating DC voltage compares to fixed regulator IC such as 7818.

Because this B1 buffer acting as the pre-amplifier as well, so I put the volume control on each input and output. I fully turn the input volume control all the way up and the output volume control is act as the main volume adjustment.

This B1 buffer need a minute or two to warm up before it can operate properly. This is because the big capacitor need to be charged first. The distorted sound will occur if you try to play the music as it turn ON.

I am using the Sony Playstation SCPH-1001 as my CD player. I also using the DIY Gainclone LM1875 amplifier. My speakers in this test is quite special. Previously I have built a single driver full-range in the port box. You can check my review here.

In this test, I playing the songs from my CD collection such as Fourplay, Salena Jones, Nicole Wang, and various instrumental music. The sound quality difference between using and not using this B1 buffer is huge. Although this B1 is only a buffer and not make any gain, but it colored the sound in a good way.

This B1 buffer can be consider as natural sounding signature. It give more treble extension with smooth vocal presentation. I also notice the music separation is definitely improved with this B1 buffer.

I am so enjoying listen the music with this B1 buffer pre-amplifier and I also very impressed on how simple this B1 schematic but at the same time improve my audio system.

Although I am a typical person who doesn't like any active pre-amp so far, but suddenly changes my mind after testing this B1.

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