Project files: B1 R2 buffer/preamp

It’s been a while since my last project files-post, so it’s a good feeling to get this published – and on Christmas eve to boot! 🙂

Improving the B1…

If you’ve been following the blog for some years, you’ll likely know that I am a big fan of the Pass B1 buffer. I’ve built several versions and my own board layout has been one of the most popular designs shown here. It’s also almost 15 years old now (!) – oh how time flies!

The reasons for my fascination with the B1 are many: It’s a simple and elegant design; it sounds very good; and the lack of extra gain is very often a big plus because many modern signal sources have a high enough output level as-is. However, the single-supply original version of the B1 requires bulky/expensive/poor-sounding capacitors in the signal path, and so a DC-coupled version with a dual-rail supply that doesn’t require capacitor coupling seems an obvious next step.

There have been a few of those (for example the “DCB1”) but they’ve been more complex and so in my view don’t have the same appeal as the original. However, it turns out Nelson Pass also did a ”B1 R2” version with a complementary JFET pair (SK170/SJ74). I think I saw this some time ago, but I was a bit shocked to learn that it is actually much older. Since it’s possibly even more simple and elegant than the original B1, it was the perfect project to restart doing PCB layouts for the winter season (as described here).

The result is one of the cleanest PCBs I can remember doing, and fortunately it also works as expected. The version shown here has an onboard volume pot to make it a complete preamp, but there is also a version without the pot that should be even more versatile (and usable e.g. as a class D input buffer).

I don’t need this right now, but as it is a very versatile design it’s a good one to have in the arsenal for future use. Project files coming soon, probably just after X-mas 🙂

Harmonic experiments…

Not much to say about this one really – I finished casing my build of the Korg Nutube B1 preamp (as shown originally here). By using a built-in power supply based on a filtered IRM power module I managed to keep the unit compact and still avoid the external PSU which I quite like.

I’ve only done very quick listening tests with the amp, but considering how varied opinions I have read on the sound quality I am positively surprised. The post title is obviously in reference to the fact that it is possible to “tune” the profile of harmonic distortion that the amp produces which I haven’t really tried, but I actually would like to try it and get a sense of where my personal preferences lie.

I am also positively surprised that the amp does not seem to have a big turn-on/turn-off thump (as I have seen some people mention). Not sure why (maybe it’s the PSU?), but it definitely makes it more likely that I will actually use it in my system where “livability” is a major factor (even if it’s not an actual word…). Maybe I should do a “second generation” of my original B1-based office system?

A Christmas present in March…

Yes, it’s this season’s last (or the year’s first) Christmas present which finally turned up 🙂 It’s a partial kit version of the “B1 Korg” preamp circuit, which is to say B1 that Nelson Pass modified to use a Korg “Nutube”.

The “Santa” who was kind enough to send the package is none other than Nelson himself who a couple of months ago offered a limited batch of boards and JFETs for just the cost of postage. Naturally I jumped on this and somewhere along the line a Nutube was thrown in the package as well. The only downside was a longer wait, but I would be an idiot if I complained about that – so I don’t! 🙂

The Nutube is basically a solid-state version of a triode tube, meaning it doesn’t require high voltage, doesn’t use (and waste) a lot of power and and doesn’t have a relatively short lifespan. What it retains is the tube “character”, meaning a relatively high and almost completely 2nd harmonic distortion level. I have actually been looking at the Nutube during my trips to Japan, but it seemed a bit expensive on its own and there were no designs available that seemed to scream “build me” as a looked at them. However, now there really was no excuse for not trying it and as regular readers will know I have been a big fan of the original B1 design and its derivatives.

I don’t have all the parts in stock, but I’ll be picking up my list again this weekend and trying to get an order out the door as quickly as possible. The board and the Nutubes are now for sale in the diyaudio store (also courtesy of Nelson) so even if you didn’t get a free kit you can still get in on the action 🙂

The Great B1-binge…

I’m obviously a fan of the Pass B1 design but the last couple of weeks have seen the arrival of no less than three more B1s which is a bit much, even by my standards… 😀

Not sure how this really came about, but it must be something like this: A while ago a saw an ad on a forum for a B1 clone board (the original type which is pretty much the same as the original pasty board). It was quite cheap and so I bought it. Since I now use both analog and digital I thought I could build one with two inputs (because my previous one only has a single input).

Shortly after I received this board and had started populating it, I realised I already had a partially-assembled board of another clone design that was basically only missing the input and output caps in order to be ready (yeah I know, I should keep a list or something… 😀 ). Because of the size of the onboard electrolytic caps, the board I had would fit nicely in a 40mm high enclosure, whereas the new board would require a bit more internal height, i.e. an 80mm chassis.

More or less the only thing I don’t like about these B1 boards is the fact that you have to “air-wire” the input switch, which means criss-crossing the inside of the chassis with long wires carrying the input signals. The obvious solution is to use a relay, so I went ahead and made a couple of small adapter boards to accomplish this. With a 24V relay all that is needed to switch the relay is the supply voltage to the B1 so it makes for very simple connections.

The last design was prompted by a reader email asking if I knew of a B1 with more than 2 inputs. The answer was “no”, but I then decided to build a source selector to match my own B1-board. The selector is a simple relay-based type with four inputs but it should work just fine. As I had some PCB mounted RCAs I made a board version for those, but also a more universal one without onboard connectors (not shown).

I’ve tried to put all of these in fairly nice cases, but it’ll still be a bit of a challenge to decide which one to keep as my “personal reference” 🙂

Project files: B1 buffer/preamp

What is it?
Project files for my miniature version of Nelson Pass’ B1 buffer/preamp (shown here). I was looking through the “back catalogue” my of designs and decided that this has been sitting around for long enough to release 🙂

How big are the boards?
The board measures 3.55″ x 2.825″ (app. 90 x 72 mm.)

What is the status of the boards?
This board is in version 1.1 – tested, working and with a few minor touch-ups afterwards 🙂

Does it use any special/expensive/hard-to-find parts?
Well…

•  The 2SK170 JFETS are obsolete and can be difficult to get (the real ones at least – getting something that is marked “SK170” isn’t hard at all…). There are a few “close enough” substitutes (such as 2SK369, 2SK117, 2SK246 and possibly a few more) which aren’t completely unobtanium yet. Do a google search if you are not sure, make sure to get BL-grades and beware that not all of these have the same pin connections.
• The input and output caps should be MKP-types (polypropylene). The “square” footprints correspond to normal types from Wima, Evox/Rifa and many others.
• The 1 ohm resistor in the power supply is meant to be a 5W MPC-71 type. These can be a bit hard to find – ebay and Aliexpress seem to be the best options (some risk of fakes as usual) – but it is also possible to fit a normal 3-5W type instead.

Anything else I need to know?

• The JFETs should be matched for Idss for best performance. Plenty of guides available for that if you are unable to buy pre-matched pairs, and my JFETmatcher can be used as well.
• I have used smaller PSU capacitors on the board than Nelson did in the original article, but you should still be able to get away with both regulated and unregulated power supplies. Plenty of advice out there for that as well – as long as you stay between 18 and 24V more or less everything should work.

Downloads:
Download design files here

Related information:
Tons on information out there on the B1 and derivates, but the most obvious place to start is (obviously…) Nelson’s original article.

Note: Always read the “intro post” for additional important information about my designs.

The B1 Buffer – Redux

I have built a couple of versions of the Pass B1 Single JFET buffer (one is shown here), but I really wanted a more compact and “elegant” version of the board layout. I like the B1 for a number of reasons, mainly 1) simplicity, 2) great sound and 3) the fact that it has no gain and it is a versatile design that has many uses.

My PCB layout tries to be compact as possible, mainly by tailoring the footprints to the components I had available. Most of the board space is consumed by a pair of large 10uF Wima MKPs for the output caps. Input caps are fairly compact Wima MKPs as well and the space for the power supply capacitors have been shrunk a little. There is no space for an onboard volume control, but instead a couple of three-pin headers that can be used to connect on (or jumpered on the board if a volume control isn’t needed).

Silkscreen rendering made with http://www.circuitpeople.com.

Eagle layout screenshot

The first prototype board shown above is playing music but it isn’t cased yet. However, I have the case more or less ready and only have to cable it up, so that should not be too long until that is done (by my standards of course…)

Oh and again, plans for a balanced version are being drawn up as I got 10 boards to play with 😉

Past projects: An office system with Pass and ICEpower

One of the projects I actually use the most on a daily basis is my amp/preamp office system. This consists of a power amp made with an ICEpower125ASX2 class D module and a matching preamp made from a Pass Labs B1 JFET buffer. The setup normally feeds a pair of Elac 310CE speakers from a Pure i20 iPod dock (with a 160GB iPod classic and ALAC-files).

The preamp is battery powered from a pair of 9V batteries which is a great idea in theory, but not so great in practice. I use the amp quite a lot and I have found out that either you spend a lot of money buying new batteries or you spend a lot of time recharging NiMH-batteries – not sure what is worse here actually. At some point this will probably annoy me enough that I will rig something up so I can use an external power supply, but so far I am just enjoying the sound from this pair. They also showcase that even in my less than ideal settings, the Elacs are absolutely superb little speakers 🙂

In the last picture you can see the system “in situ”. The owl figure (from Züny) is partly filled with metal and provides an important audiophile function – vibration damping :D.

/U.