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 🙂

Advertisements

Encore?

A quiet last few weeks here – at least on the surface. Two reasons for that really: 1) With an Xmas-break looming on the horizon the pace at work is picking up a bit and 2) for quite a lot of projects I am in the annoying phase where lots of important work is done, but it doesn’t really look like you are getting closer to a finished product and so it’s not really worth showing here. If nothing else though, it’s nice to have a good pipeline for next year 😉

However, one thing there is always time for is to buy new projects for the shelves 😀 As regular readers will know I have made lots of ICEpower-based projects, but practically nothing with the various Hypex-modules. However, recently one of the new Ncore NC502MP modules came up on ebay and so I pounced on that. The module looks very nice but I need to test it for a while to check the sound (waiting for proper cables at the moment) and then I’ll decide on a suitable enclosure for it. The original goal was to built a custom high-power integrated amp, but I may end up going in a different direction and do a pure power amp instead. One deciding factor will definitely be whether mounting the module on a simple aluminium bottom plate proves to be enough heat sinking, because if the module has to be on a “real” heat sink, then all my current enclosure ideas are definitely out the window!

Soundwise, I still expect that the benchmark for the Ncore to beat (at least in class D) is going to be my trusty 125ASX-based stereo amp and the 700ASC-monos (which incidentally are also among the designs that are I am currently inching closer to completion…)

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 🙂

Improving a Mean Well IRM PSU…

If you’ve been here before, you might have noticed that I have been using Mean Wells IRM-xx series PSU modules quite a bit. They are small, cheap, easy to use and available from many of the parts sources I normally buy from. Being cheap switching supplies they have quite a bit of ripple and noise which on the face of it is a problem. However, in practice many supporting applications aren’t too fussy about the quality of power and for those that are, alternative PSU arrangements can usually be found.

But what if the noise from the IRMs could be removed? One application where that would be useful (and where several people other than me have tried it already), is for Kevin Gilmores various discrete amplifiers (Dynalo, CFP etc.). They are normally powered by 16-24V DC and require a few hundred milliamps per board, so it’s pretty ideal for an 24V IRM with some additional filtering and regulation. Here I’ve made a single channel PSU intended for a 15/20W IRM-module, so output currents are in the region of 0.8-1.5A or so.

Most integrated voltage regulators have low rejection of HF-noise, but as the IRMs have a high switching frequency a passive filter seems an ideal way of damping the noise before it gets to the regulator. I can’t find any spec to state how much capacitance the IRMs will tolerate so I’ve stayed on the conservative side, but even so a small passive CRC/CLC filter is very effective at 100kHz so it should be fine.

Instead of the “usual” LT108x LDO voltage regulator I’ve gone for an LM2941 instead. This has an even lower drop-out voltage which will help if the starting point is a 15V module. The downside is a max. output voltage of 20V – 22V would have been better (and while we are at it, can we get 9V and 18 versions of the IRM20 please Mean Well :D). Actually, I’m going to try to do an LT10xx-based version as well, but for now the LM2941 works fine. For this test example I’ve set the output voltage to 19.5V and I get 19.4V even under load so that is perfect. Next step: Build two more boards for my first “real” use-case for these 🙂

A real gem…

I’ve already had my hands on a couple of designs by Richard J. Murdey, mainly here and here. However, Richard has quite a few other interesting designs on his site and one of them I’ve had my eyes on for a while, namely the “Sapphire” which is a line/headphone amplifier. Unfortunately my first order of Sapphire boards never showed up and while I was working on the Sapphire I also had a look at the Emerald MM/MC RIAA. Those boards did show up and so while I wait for the reordered Sapphire boards to (hopefully) show up soon, let’s look at the Emerald instead 🙂

The Emerald is very simple RIAA-design with two opamps per channel and selectable gain for both MM and MC cartridges. The RIAA correction is a bit special as described in Richards write-up and the design also includes an on-board discrete voltage regulator.

Richard has graciously shared the Eagle files for his designs so that was what I started from, but what was originally meant to be minor touch-ups ended up taking me a bit further away from the original than I expected. Although the schematic is still (more or less) the same, I’ve made quite a few changes to the board so that it now looks more like one of mine and uses the parts footprints that I normally use – the latter is absolutely intentional, the former a bit less so. I’ve also done quite a bit of rerouting – which I think of as optimizing the layout, but others might disagree – and managed to shrink the overall board dimensions a little in the process as well.

I haven’t really sacrificed anything from the original, except maybe by downsizing the footprint for the output cap a little bit, but that’s hopefully an acceptable compromise for most people. Project files (probably) coming shortly 😀

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 🙂

Improving small DC-DC converters…

I’ve written a few posts about DC-DC converters and I’ve found them very useful for many circuits where a “complex” (= multi-rail with fixed voltages) power supply needs to be replaced with something “simpler” and more flexible (= single-rail with large variations allowed). However, I have previously largely ignored small SIP-type converters because I didn’t believe they were powerful enough to be of much use. As I (recently) realised you can actually get 3W from several manufacturers and even 5-6W from certain others. That makes it’s possible to get 100mA or more at +/-12-15V which is more than plenty for most small opamp circuits such as buffers, preamps and RIAAs, and of course single-rail 5-12V currents more than suitable for small auxiliary circuits in power amps etc.

Now, there are a few drawbacks to these converters: Even the small converters normally still have quite large ripple voltages and I expect there is quite a bit of HF-noise as well, but I’ve tried to use a passive filter to compensate for that. The basic idea is that because the switching frequency of the converter is very high (typically 50-100 kHz) which is nearly 1000 times higher than a linear supply, a simple passive filter is also 1000 times better at removing ripple and noise and so even small capacitor/resistor values for the filter gets you very far. A second drawback is that the converters have limited tolerance for capacitive loading, so it’s normally a good idea to think the power source into the design/build of the consumer circuit. That’s normally also doable though.

The basic SIP-8 form factor is used by several manufacturers so there are quite a few different converters to choose from, both cheap and not-so-cheap. One thing that differs between manufacturers seems to be the allowed capacitance load that the converters will tolerate. Here, the more expensive Recoms and Tracos list considerably better specs than the cheaper converters, so that’s worth looking into before you choose. Given how the converter works, this restriction mainly applies to higher voltages of 12-15V or higher – at 5V the load margin is likely to be fine even for the cheaper converters.

The boards I’ve made are both a single and a dual version with the same form factor and they both work as expected. However, after I received the boards and assembled the prototypes I had a couple of ideas to improve the filtering a bit so I’m going to hold off making the board files public until I’ve tried those ideas 🙂

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 😀

Waiting for parts…

The summer weather still doesn’t show any signs of slowing down here – at least not significantly – and so building is a little on the backburner. However, I have been keeping up a steady flow of PCB-orders over the last weeks (partly my own designs, partly not) so that when I go on holiday in a couple of weeks the finished boards should be waiting for me. Assuming the weather is more suitable for indoor activities at that point, there should be a few interesting things coming up in the not-too-distant future then 😀

Already now though, I have started putting together a few things including another line-level buffer, an ebay tube-kit and a couple of headphone amplifiers but it’s stop-start traffic most of the way. A constant interruption to these builds are a lack of parts – not massively so, but a resistor here and a capacitor there is enough to slow everything down. Case in point is a buffer by Kevin Gilmore where I have the boards (and have had them for a while) and most of the assembly is done, except that I am missing four ceramic caps (odd value and specific form factor) and four RN60 resistors (a standard value that I simply ran out of).

For some odd reason this actually tends to delay overall progress by quite a lot because by the time I’ve accumulated enough volume for an order from a specific vendor and the missing parts show up, usually something else has caught my eye…  😀

Anyway, Mouser order just completed so the last parts for the buffer and a few other half-finished projects should be here by the end of the week. Maybe I should spend my holidays working out a queuing system for new builds of some sort instead? 😀

Inching forward…

Another long(ish) break from posting – this time mostly courtesy of some extremely nice late-spring weather and a couple of house-related DIY-projects. Just about the only thing that has moved forward (at least enough to notice) are my ICEpower 700ASC-based mono blocks (which I discussed here). A couple of weeks ago I got the mounting plates I designed for the modules + supporting circuitry which meant I could drill the chassis and start putting some mechanicals together at last.

Some of you may have guessed that this is where my BalBUF design is supposed to end up, but there was a piece missing. A matching power supply to drop the 700ASC’s 15V aux power supply to something more manageable for the OPA1632 (which gets very hot in operation). Because I was running out of space in the enclosure I wanted to use, a key design criteria was that the PSU should be “stackable” with the BalBUF board.

I quickly found what looks like the perfect device for this use – the TPS7A39 from TI – which is a dual pos/neg low-noise regulator with the right specs. Unfortunately, it is also only available in a 3×3 mm leadless package and as my odds of hand-soldering that are pretty much = 0 I dropped that pretty quickly. Instead I went for a bog-standard LM3x7-based design, but managed to squeeze it down to size because of the modest heat sinking requirements.

In a nod to “reusability”, which is something I always aim for where possible, the PSU board includes SMD resistors on the bottom in front of the caps, which means it can also be used with the unregulated supplies on the other ASX-boards such as the 50ASX and 125ASX. This means that you can use the BalBUF with any ASX-module without a separate offboard supply for the low-voltage circuitry, and because the BalBUF and the PSU stack on top of each other it should be very compact. Assuming everything works as expected with the 700ASC when I test it, I’m pretty sure that means I’ve just figured out what to do with my last remaining pair of 50ASX’es 😀

The sketch for the rear panels is also pretty much done, but given that Schaeffer/FPX panel work is getting more and more expensive I have decided not to order the rear panels “blind”, i.e. before I have tested that the monos work electrically. If this weather continues, that might be a while though 😀