Project files: The “MoFo” power follower

I did this version of the “MoFo”-design a while ago and also mentioned it briefly (here) but didn’t manage to complete it or even test the boards. In the mean time the “official” boards have become available from the diyaudio store, but since I now finally got round to testing my boards I still thought I’d share my version as well.

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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 🙂

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 🙂

Project files: The RJM Emerald RIAA

Last week I showed my version of the “Emerald” RIAA design by Richard J. Murdey. The Emerald is a neat little design: It has switchable gain and load for MM/MC, if you use good components it’s got a very accurate RIAA-curve, and of course with just two opamps per channel as the active devices, it’s very easy to build.

Richard has graciously shared the Eagle-files for his version and so it seems only right that I do the same here. Richard is also selling boards from his website, so if you want something that is proven to be working and comes with support then I suggest you buy your boards from him instead.

Project files: The Kuartlotron Buffer

Sometimes projects that have been on hold for a long time can restart with just a tiny nudge. A while ago I built (and showed) a clone of the Kuartlotron buffers. My original prototypes had one obvious mistake (an incorrectly connected Q3) which I fixed, but I still couldn’t properly zero the offset as described. I left the project, did nothing about it and then a few days ago by accident went back into the discussion thread on diyaudio. Here there was a single post discussing exactly that issue and a very short response from Keantoken (the “inventor”) that offset had to be zeroed with the input open. This is not what you normally do so I didn’t think about it after building my boards, but that small clue was enough for me to go back to the prototype boards and confirm they were OK. With the problem fixed I can finally share the project files here 🙂

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 😀

In search of synergy…

Slightly off-topic post, but I have written a few times about how I think that system-matching is much more important than any “absolute” sound quality, at least as far as enjoying the music is concerned. Well, today was another reminder that I still think this is the case 🙂

A couple of months ago I got new speakers, trading my old (and much-loved) standmount Sonus Fabers for some floor standing Scansonics that offered a bit more low-end slam. I was quite happy with the trade from the beginning and I have absolutely no regrets, but after a time the inevitable restlessness sets in and you start thinking about change (at least I do…). I’ve been running the Scansonics with a simple 125ASX amp on my Harman/Kardon preamp, but just to try it I dug out another ICEpower-amp from my collection, this time based on the older 200ASC-modules.

Although I would definitely still class the 125ASX as the better amp overall, the Scansonics (which are just a little bit bright) immediately benefitted from the more “closed-in” presentation style of the 200ASC, so as usual after initially listening to half a track I started to go through my normal playlist of tracks I know well and just enjoyed listening to some music that I would normally say I know back-to-front already.

To be fair I am honestly not surprised at this, because I saw the same change when I switched from the even older Elac speakers that much preferred the warmer sound of a 50ASX amp whereas the Sonus Fabers really came to life with the more lively presentation of the 125ASX. However, I still think that it’s nice to be reminded once again what really matters when putting a well-rounded system together and of course experimentation is always fun (although it can sometimes be very expensive as well…)

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 😀