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DIY audio preamp B1

May 26th, 2015

First Watt audio preamp B1 DIY
Attention! This post provides the instruction how to build the audio preamp, however it is related to working with the electricity. That can be risky and create hazard to your life! If you do not posses all required knowledge and skills, please do not follow it!

That was pretty long DIY project, but not because it is very complex. One of the reasons was that I had to buy quite many electronic components from eBay for the project. But at the end, I can say it was worth to do it 😉

Vacuum tube sound

You might already heart about vacuum tube sound before. I mean how natural and pleasant it is for the human eye. This phenomena is related to the nature of the human ear, which hears all sounds with even-order harmonics in the nature. You can read more about that e.g. here.

I like the sound of the vacuum tubes, however I do not like that it takes up to 30 minutes for them to warm up in order to reach the full potential. Another limitation is that they have relatively short lifetime.

I came across the audio preamp B1 on this DIY’ers forum, which is based on JFET transistors. I read quite a lot of good feedback of excited DIY’ers about that preamp at the same forum.

JFETs alternative to vacuum tubes

Normally, the pre-amp is connected between the source of audio and the (power) amplifier. In preamp B1 case, it performs also the function of matching the impedance between the audio source and the (power) amplifier.

I got very curious if the Toshiba JFETs based pre-amplifier can perform so well by (a) adding the even-order harmonics to the audio signal and (b) matching the impedance.

You can find the circuit with the description on original First Watt website. Here is a circuit:

First Watt Audio preamp B1 DIY

The circuit looked quite simple to repeat and not very costly. So, I decided to give a try and I repeated everything from the resistors marked as R102 and R202 to the right (skipping what was on the left of those resistors). I did not need additional volume control in my setup.

I thought that one of the biggest challenges will be to find those exotic 2SK170BL JFETs which were already discontinued by Toshiba. But the solution appeared not that difficult 🙂 One of the www.diyaudio.com forum’s guys managed to get a box of those JFETs (2SK170BL), matched them in pairs and started to sell on eBay. Well, that’s cool, the probability was quite low (or no?) that the guy would not cheat 😉

First Watt audio preamp B1 DIY


After waiting for something one week+, I got my JFETs from that guy and started my preamp B1 project 😉

Deciding how to build your audio pre-amp B1

You can follow quite different strategies to build this pre-amp.

In my case, that was a complete DIY project. I bought all resistors separately and chose the best matches between them. It appeared that to find the matches for the resistors is not so hard, however to find very specific values across them can be not so easy 😉 I did not have any 10uF MKP capacitors lying around, so I had also to get them from eBay. For that, I chose the Audiophiler MKP capacitors. Once everything arrived, I started to assemble the pcbs. But that was a bit hard way.

More simple way is just to get this pre-amp kit, assemble it and enjoy 😉

Power supply

I had a toroidal transformer at home, which was producing something 30VA. so I needed only regulated +18V source and the chip LM317 was chosen for that. Keep in mind, that this regulator should be feeded by at least 2V higher voltage compared what are you are expecting to get after.

Last tweak – power filter

There are quite many different home appliances in each house or flat. As a result, the power line gets pretty noisy. It appeared that filtering the power line even before the transformer can be very beneficial for audio gear.

I came across those two posts about the EMI filters here and here. Both links are in Russian, so Google translate might help you to understand. In one of the links, there was shown the EMI filter installation into a CD player.

One of the suggestions for DIY’ers was that one of the easiest ways to find all parts needed for EMI filter – just use the parts from some old PC ATX power supply 🙂

I followed that simple advice. Even more, I de-soldered all filter parts and cut the part of the pcb of the filter. As a result, I got all needed parts and the pcb. The EMI board together with other pcbs nicely fitted into the case of old satellite receiver.

First Watt audio preamp B1 DIY

Fully assembled pre-amplifier B1

Finally, everything was in place. A ‘borrowed’ EMI filter can be seen in the left corner with the toroidal transformer nearby. A regulated power supply found its place on the right. Only the filtering capacitor was too high, so I had to turn by 90 degrees. Two channels with FETs are further on the right.

That’s it, the time is to test all that effort together with these reasonable priced RCA interconnects for my audio setup.

First Watt DIY audio preamp B1 DIY

Final thoughts

I shared my experience of upgrading integrated RPi’s soundcard in one of the previous posts, what brought a huge improvement to my audio setup.

You will experience the difference again with the preamp B1. The sound became a lot more pleasant to the ear and really more natural. After listening one album, you want to just to listen a second 🙂

I fully agree with those guys on the forum who shared their positive experiences about this pre-amp. It is hard to imagine another audio upgrade for something like ~30 Eur in total for such quality improvement.

At end,  I would like just to add that you can get either only PCB (~10USD) or a full B1 kit (~50USD) from eBay. I think it is good value for money 😉

Enjoy an improved sound of your audio system!