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 🙂

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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 😀

The “Whammy” headphone amplifier…

Although I did my own version of the Pass/Colburn “Whammy” headphone amplifier before there were boards available for sale in the diyaudio store (and before it was officially called the Whammy), I have still considered getting an original all-in-one board as well.

The cost of shipping from the US originally deterred me enough to do my own version, but a couple of weeks ago a board popped up on diyaudio from a fellow hobbyist in Europe, so I was able to get one at a reasonable cost. Unlike the diyaudio board this one is green (which I massively approve of) and also 2mm thick and plated with gold (ENIG) so it looks and feels really great. Because the board was thicker than usual and I knew I had to mount it in a big case I decided to go “all out”, use tall caps and heatsinks and maybe experiment with turning up the current compared to normal (haven’t do that yet though).

The power supply is running at 20V courtesy of some 7×18-regulators and a pair of green LEDs. This limits my choice of opamps, more or less to either the original OPA2604 or the (now-discontinued) LME49860 which is supposed to be a 22V-tolerant LME49720. Not sure if that is true, but I did chose the latter and I have no complaints about the sound. I might try the OPA2604 at some point instead since I haven’t listened to that since the comparison was an OPA2134 – that’s been a while. The output FETs are the recommended 2SK2013/2SJ313 which I already had matched pairs of, but obviously plenty of other options available that are easier to source.

Just like my clone version this one worked immediately after being powered up, but that is probably more to Wayne’s credit than mine 🙂 I don’t have a case idea just yet, so for the moment it’s going in a box until I come up with a plan for what to do next – still sounds great though 😀

Project files: PassHP headphone amp

What is it?
It’s the project files for the PassHP headphone amplifier from last week’s post and judging by the number of views since then they are eagerly awaited 😀
As mentioned last time, this design is a clone of the one from here and my version consists of a mono amplifier board and a stereo PSU board instead of the original “all-in-one” design.

How big are the boards?
The amplifier boards measure 2.95” x 3.0” (app. 75 x 76 mm.) and the PSU board measures 2.0” x 5.05” (app. 51 x 128 mm.).

What is the status of the boards?
Both boards are in version 1.0 as the prototype seems to work well and I couldn’t be bothered to make any cosmetic changes 😉

Does it use any special/expensive/hard-to-find parts?
Well, the recommended 2SJ313/2SK2013 output transistors are a bit hard to find, but there are plenty of substitutes available. This is a fairly simple design, so otherwise no problems.

Anything else I need to know?

• Resistors: I’ve used RN60-type resistors which are rated 0.5W, but that probably isn’t necessary – at least not for all the positions.
• Heatsinks: The heat sink profile is the one Fischer calls SK104 but there are many substitutes. The power dissipation isn’t great so even the small 25mm high version should suffice, but if you want to use bigger ones for cosmetic reasons that should be just fine 🙂
• Transistors: I’ve used 2SJ313/2SK2013 output devices because I had them, but if you don’t then I recommend using IRF610/9610 or one of the other substitutes mentioned in the diyaudio build thread. The 2SJ/2SK pairs are now either very expensive or very fake (and sometimes even both!), so using parts that are still in production should be safer.
• Optocoupler: In theory this is also substitutable for something else, but in all honesty I don’t know exactly how the optical bias-system works so it’s probably best to stick with the standard 4N35.
• Gain: The default gain is app. 6 but that can be lowered or raised by tweaking the value of R4. In theory you should recalculate the BW-limiting capacitor across the resistor if you change the value, but in practice you’ll probably be fine unless you make major changes. My prototype version has a gain of 3 (R4 = 2k) and I haven’t observed any problems.
• Opamp: My version uses a single-channel opamp which gives a bit more choice. Start out with something like the OPA604, OPA134 or LME49710 and then experiment from there if you want to change the sound.
  Most opamps have a max. supply voltage of +/-15V so as a starting point I’d recommend this as the supply voltage. If you want more voltage swing use the OPA604 which is good up to +/-22V.
• PSU voltage adjustment: Just as in the original you can use LEDs to raise the output voltage of the supply above the regulator voltage (although I’ve ditched the resistor option). Using 7×15-regulators and green/red LEDs should give you around 17V output whereas using 7×18-regulators and LEDs should bump that to app. 20V. If you just want the regulator voltage as the output, remember to jumper across the LED pins and omit the capacitor.

Downloads:
Download design files here

Related information:
You really should chew your way through the diyaudio-thread for information about the amplifier. As mentioned this version was mostly because I did not like the original form factor. If you just want a functioning amplifier then I strongly recommend that you buy one of the “real” boards from Wayne Colburn via DIYaudio (or wait a few weeks for when the boards show up in the diyaudio store).

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

New PassDIY headphone amp…

One of the (few) PCB-projects that I have been able to spend time lately on is a version of a new PassDIY headphone amp developed by Wayne Colburn. I’ve been looking at this from when it was first posted (after all, most diy’ers watch with interest when Nelson and his gang are up to something new 🙂 ) When the design was finally released a few months ago and Wayne started offering boards I didn’t jump on it immediately though.

First off all there was a question of cost – paying $35 in shipping for $25 worth of PCB is a bit annoying although I probably would have survived that. Secondly, the “all-in-one” form factor has some very obvious benefits, but also some significant drawbacks to me. You’re generally tied to one particular chassis, one particular transformer, one specific potentiometer etc. and I wanted more flexibility.

So instead of getting a ready-made board I started thinking about making a more flexible version by splitting the board up into smaller sections – two mono amplifier channels and a separate supply board to which you need to add a separate volume control and a separate transformer. The design is simple and so there were no major issues and my protoype fortunately worked the first time.

As usual I have only done quick bench testing right now, but the design seems to be solid (no doubt more to the designer’s credit than mine 😀 ). There is no DC-offset worth mentioning although it spikes a little when you turn the power off, and even in a “birds nest” test setup with wires all over the place, the amp was completely silent. As far as I can see, the bias is spot on and stable as well.

Project files coming in a few weeks when I’ve had time to compile them 🙂 In the mean time, you can still get the original boards through the thread and the original Gerbers are there as well if you want to get your own boards made instead.

(Yet another) anniversary!

Yes, it’s that time of the year again – and this year it’s the fourth anniversary of the blog 😀

Not a lot to say that I haven’t already said the last couple of years, but I still expect to continue writing as much as time allows. I am also still very excited and greatly appreciative of your questions and comments, so keep it up 🙂

Picture below is of what is (currently) sitting near the top of my project pipeline, namely four 4U diyaudio special-edition pre-drilled heatsinks. These are specifically intended to accelerate (as much as possible) the completion of my Pass VFET project as well as one other Pass project using boards from the diyaudio store that I have wanted to do for some time now 🙂

Pass V-FET kits are here!

Forgot to post this a week ago when they arrived, but I managed to secure a couple of the Nelson Pass V-FET kits which I am quite excited about.

In short, this is a low-power class A amplifier based on some complementary Sony V-FET (SIT) transistors that have been out of production more or less since before I was born. The actual devices were bought as NOS (new old stock) by Nelson Pass himself and offered to the diyaudio community through the diyaudio store as a (more or less) one-off opportunity. I was lucky enough to register my interest early on and so managed to secure a couple of kits to keep me busy on those long Scandinavian winter nights when they come around 😀

There’s a big discussion thread on diyaudio and also an article on the FirstWatt website about the design, in addition to the information in Nelsons previous articles on SITs (also on the FW website). As usual, I don’t really need these and the class A heat is a bit impractical in a small apartment, but a limited-edition amplifier kit with unobtanium transistors that was developed by Nelson Pass himself was an opportunity I simply could not pass up (pardon the stupid pun 🙂 ).

The Firstwatt F5 is still one of the best amplifiers I’ve heard in my system so I have very high expectations for this new design. The lower power of the VFET could be an issue, but I’ll have to build it and try I guess – with my current speakers it should be OK and if not, I can always get a pair of very inefficient planar magnetic headphones instead :D.