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?

Project files: The ACP+ clone…

Well, both my ACP+ clone boards are now fully populated with relays and working as expected so I guess it is appropriate to share the design files 🙂

Project files: The BBA3FE

Haven’t really had time to fully complete my BBA3FE project yet, but as I am otherwise happy with the design I might as well release it in case anyone wants to have a play with it 🙂

The last F5?

I’ve built a couple variations of the F5 already and I have a couple more that haven’t been finished yet, but the last one of them is here. It’s a pair of mono blocks with fan cooling and a large power supply 🙂

This project was of course driven by the fact that I really liked my original F5 build, but also by the fact that I had some suitable heatsinks and that I got a good offer on a couple of transformers that were really meant for a Pass/FirstWatt mono block build (2x18V/250VA each). The heatsinks actually turned out to be a mixed blessing, because they are just a millimeter or two too tall for most of the readily available enclosures, so I had to go up to a 3U size. I also decided to choose a slightly larger footprint instead of trying to shoehorn everything into a “real” half-width box. That annoyed me in the beginning, but to be honest I am not regretting that now.

Design-wise, you should be able to see the inspiration of some of my other projects in this one and that is no coincidence – there were some concepts I wanted to “recycle”. That last piece to arrive of what you see now was the rear panels which showed up a few days ago. Fitting them was a quick job, but they are quite expensive and so I can’t help but breathe a sigh of relief when they fit as I intended the first time. No matter how many checks and paper mock-ups I have done in the creation process it’s always a huge relief when everything works…

Still a bit of a way to go, but getting this far was very satisfying 🙂

The BoSoZ…

When I started to look around for balanced preamp designs some time ago, the BBA3FE wasn’t the only design that turned up. Another candidate was a sort of “predecessor” for it, namely the Pass Balanced Zen Line Stage (aka BoSoZ). I was a little slower getting started on this one so the BBA3FE came first, but a few weeks later inspiration struck and I managed to finish the layouts for both the BoSoZ and the matching PSU as well.

I chose a mono configuration for the amp board to maximize flexibility and minimize board cost. I’ve only made some minor changes to the schematic, but you should be able to see the resemblance to the BBA3FE layout easily. My prototype version uses 27.5mm output caps because I had some I wanted to use, but the “real” version of the board has space for 37.5mm caps as well and is only slightly larger (app. 5mm deeper).

As you can see I’ve actually also made good headway on the mechanicals of the design so what you can see now is really a semi-completed amplifier. Other than the new amp/psu boards I’ve picked components “off-the-shelf”, i.e. input selectors, an output relay board and an aux PSU that I have previously done, so putting it all together wasn’t that hard to do.

I need to pull myself together a little and get the wiring done before it will play music for real, but other than that it’s looking very promising – and the initial sound quality tests definitely match that as well 🙂

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 🙂

Testing the F3 amplifier…

Earlier this year Nelson Pass graciously started distributing batches of the Lovoltech LU1014 Power-JFETs for free to diy’ers. You could get 4 pcs. and only pay for shipping and of course that offer was hard to turn down – all the more so since I had been looking at trying an F3 amplifier at some point. There are no “official” group buy boards for the F3 at the moment and the few redesign/group-buy initiatives I have seen have been false starts, so I picked up a set of amplifier boards from ebay instead.

The F3 amplifier is one of Nelson’s “unusual” First Watt amplifiers, in that it uses a Power JFET as the gain device. Power JFETs are rare, and as a result for those of us who were a bit slow on the uptake the LU1014 came and went without me buying any. Otherwise the F3 isn’t a very complicated design, but it’s got normal First Watt class A heat levels and even-lesss-than-First Watt levels of output and gain. As a result i am not sure whether I actually have any practical use for the finished amplifier, but as a listening experiment I am still going to give it a try 🙂

The F3 is also a single-rail amplifier, which quickly led me to the realisation that I didn’t really have a PSU board suitable for a single-rail power amp. When you have a good “back catalogue” of designs then that’s quite helpful and so taking one of my existing class A power supply boards and chopping it up to create a single-rail version wasn’t that hard. The end result should hopefully – at some point – end up as a (nearly) dual-mono F3 amplifier (meaning a single transformer is used).

Another complication is that for reasons I can’t really remember I decided to try using 3U heatsinks for this build which may end up being a mistake – they are going to get very hot I guess. Anyway, for now it is an experiment and hopefully I will have it (electrically) completed by the Christmas break so I can hear what it sounds like before I put time and money into finishing the mechanical design 🙂

Experimenting with the BBA3FE…

Not the most obvious acronym to decode, but it stands for “Balanced Burning Amp 3 Front-End” and it’s the first half of a power amp design that Nelson Pass launched for the ”Burning Amp” festival several years ago. It’s very closely related to the F5 circuit and without the accompanying power stages it’s also well-suited for pre-amp duty. The diyaudio store has been selling PCBs for years, but I’ve never been sufficiently interested to grab one (and the shipping cost and import duty for buying PCBs from the US makes it really expensive to get anyway).

Now, fast forward to a few months ago: For a while I’ve looked for a “real” balanced preamplifier circuit. I have several balanced designs already, but some of them are “cheating” by not being developed for balanced use and that obviously won’t do 😉 I’ve looked at the BA-3 before, but some months ago I did a double-take when I noticed a schematic for a balanced version in this thread (and yes, I know the thread is seven years old…). That circuit was more or less what I was looking for, namely a simple discrete circuit optimised for balanced use, and so I decided to try to make one.

My version is more or less the same as the original, but I decided to save some board space (and create some flexibility) by moving the output capacitors off-board and so they are not shown (but still very much required). The major downside of the circuit is that the input is based on 2SK170/2SJ74 JFETs that are obsolete and near-impossible to get. To add insult to injury, they should ideally be matched to around 8mA Idss which is more or less the most commonly required value – and therefore even harder to find!. However, the article also states that while matching is preferred, it is not essential, and so I managed to find some suitable pairs in my parts drawers.

I’ve only briefly tested the boards and they power up fine but the DC-offset is unstable so the output capacitors are definitely needed. Thermal stability and equilibrium with the bias-current is also something I need to work on (it’s going to require leaving the boards on for a while as far as I can see), but so far it is looking promising. As regular blog-followers will know I am a big fan of the B1 design and I don’t normally need gain, but as there are several reports of the BBA3FE sounding significantly better than the B1 I am obviously quite excited to make progress on this build.

PCB files will be coming eventually, but I made a couple of stupid mistakes in this layout that need to be corrected, and since I was so focused on this being a balanced pre I forgot to make it easy to do the SE-version as well. Translation: I really need to get a v1.1 ready and ordered first 🙂

Building an(other) F5…

Although I recently built a new type of F5 amplifier, I haven’t completely abandoned the original F5 design 🙂 Hiding in one of my many boxes were a pair of half-finished F5 boards and some matching matching fan heatsinks that only needed the last bits of assembly and calibration. That honestly didn’t take long to do once the right parts showed up and I then managed to confirm the boards were indeed working.

The boards were originally bought from ebay and are more or less the same as my original F5 build – nothing special there. I have some matching PSU boards as well, only missing the last few parts which are now in the queue for my next order and that’s going to be a standard C-R-C type thing as well.

The mechanical design is from the same time as my JLH mono blocks, so the idea is also more or less the same. This heat sink profile is too large to fit in most enclosures though, so cracking what to do took some time but I think I have it figured out now. It’s also going to be monoblocks, but much larger ones than the JLHs. From my first tests during calibration of the boards I think a slow-speed fan should be enough to keep the heat under control, so hopefully they will be living-room friendly when they are done 🙂

Building a different F5…

As I have mentioned a few times, the First Watt F5 is one of my favourite amplifier designs (and of course I am not the only one who likes it). It’s very simple to build, it’s reasonably priced and it sounds exceptionally good. The only drawbacks are the heat and the relatively low power (which is why I sold my original build), but with both new speakers and a new room comes new opportunities so I wanted to try the design again.

I actually have a few F5 clone boards more or less done, but that’s a story for another time because the original F5 design has spawned a few variations. One of them by diyaudio-user Juma is based on using several smaller output devices in the form of Toshiba 2SK2013/2SJ313 (which of course are obsolete…). For reasons I don’t really pretend to understand these devices are very linear and so the sound of this F5-version should be even more special – we’ll see about that I guess.

I’ve looked at this particular F5-design before and it’s not exactly new, but sometime you have to wait a (long) while for inspiration to strike and in this case it only did a few weeks ago, so the finished boards turned up only this week.

My version has four device pairs in the output to allow a bit more idle current for low-impedance loads. Also included is some additional rail capacitance close to the outputs (mostly because it seemed wasteful not to use the board space for anything), but otherwise it is that same as Jumas original circuit. I’ve only bench-tested it for now and I can’t do proper trimming of idle current and offset until I’ve drilled some heatsinks to mount the board on, but it powers up like an F5 and it responds to the trimpots, so hopefully it should adjust properly when the time comes. For now I’m just excited to have gotten it this far 😀