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 🙂

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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 😀

A parallel amplifier with the LM3886…

Gainclones or chipamps are a popular DIY-topic and I’ve done a couple of designs myself and assembled a few others as well. The only one of the “original” National semi amplifier IC’s that I haven’t really done anything with – and coincidentally the only one that’s still in production – is the LM3886.

But not any more, because I just finished a simple design with two LM3886s in parallel configuration. The circuit is built (mostly) according to the “PA100” design from the original National application note (AN-1192) and not the Jeff Rowland-derived PA150/BPA300 that has different configuration and of course a third IC per board.

The configuration with two parallel ICs gives full current output at +/-35V into 4 ohms where a single IC would otherwise be thermally limited, but of course the power is still modest. As I recently swapped my faithful Sonus Faber speakers for a set of Scansonic MB towers which have a fairly low impedance, that’s exactly what I needed though (not to mention that I had a 35V supply left over from another project 🙂 ). The two-chip configuration also means boards can be kept small (and cheap), and there’s still the option of using two boards per channel in bridge-mode to make a BPA200, although the supply voltage would have to be reduced – only the BPA300 will run at 35V rails in BTL-mode as well.

The boards worked first time on power-up and seem to be well-behaved (quick tests only though). I need to do a bit more testing and make some minor (mechanical) changes to the layout and then I’ll publish the project files 😀

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 😀

Meet the BalBUF…

For a while I have been looking for a simple buffer/preamp circuit that could be used with balanced inputs. In general it would be nice to have, but I have a specific project in mind that would need it (no, I am not going to tell you just yet 😀 ). Also, it would have to be compact and would have to operate on existing supplies. An obvious candidate that I have been interested in for a (long) while is a Fully-differential Amplifier (FDA) in the form of the TI OPA1632.

The OPA1632 includes a Nelson Pass patent called SuperSymmetry (SuSy for short) that gives an inherently balanced topology and therefore allows for all combinations of Bal and SE to be used on both inputs and outputs. To supplement the FDA is what’s called an instrumentation amp front end using a dual opamp. This performs input buffering to keep the FDA happy and can add gain if needed.

The OPA1632 isn’t a new IC by any means, but it is still interesting and something I have been fiddling with for a while (actually for years). However, it had remained on the drawing board and as some prototype boards that I for a long time didn’t really dare assemble and test – I didn’t fully understand the concept of an FDA and so I could not really be sure I had the schematic figured out correctly as I was starting from bits and pieces collated from other designs.

When AMB picked up the OPA1632 again for his Alpha24 (A24) and KappaDCX designs then I finally had a very clear schematic to work from and so I decided to dust off the old board designs and see if I could get it to work this time. In comparison to the A24 I have omitted some of the configurable options of the A24 and deleted the last stage that sums the balanced signal back to SE – that would be rather pointless here 🙂 I was also going to break out and use the enable-pin as well, but as AMB reported (here) that it doesn’t really work well as a mute circuit I decided not to bother.

Technically I haven’t actually used the OPA1632 yet, but instead its “industrial” cousin, the THS413x. There were speculation when the ICs were released that these two are actually the same die but just tested and marked as two different parts depending on achieved specs. Even if that isn’t the case (I don’t think it was ever actually confirmed) they chips are pin compatible and close enough in specs that the differences should not matter.

For the front end I used an OPA1642 which is TIs current highest-spec FET input opamp. It sounds great, but just about any dual SOIC opamp should be fine as a substitute – I just happened to have 3 left over from something else.

As the pics show I’ve just built a single prototype for now which I will keep for testing, but I need to build a new pair as well. Now, as mentioned I am not going to reveal exactly what these are going to be used for because there is a still a piece missing, but if everything works then I think this is actually a very important design (or designs I should say because there is a second PCB on the way as well…)

Building a Kuartlotron… (part 1)

No, I don’t know where the name “Kuartlotron” comes from either, but I can assure you it sounds scarier than it really is 😉

I wouldn’t say simple buffer circuits are a mainstay of this site, but they are definitely both useful and enticing and so when a reader pointed me to the Kuartlotron and its accompanying (and quite massive) diyaudio-thread a while ago, it did pique my interest. A bit of reading later and I was a) still interested and b) deciding to have a go at my own PCB-layout.

The Kuartlotron is the work of diyuser Keantoken and unlike most other simple discrete buffer circuits, it doesn’t employ traditional feedback but instead a type of error correction invented by Professor Malcolm Hawksford. I’ll be honest – I haven’t bothered too much with reading the theory and the technical details, because it’s a simple circuit so it was easier to just have a go at building it.

What I have done in comparison to the original circuit is to lose the thermal coupling between the transistors (which may or may not be a good idea), lose one of the trimpots (because I don’t have the ability to measure THD anyway) and try to minimise the board layout as much as I can.

Now the status of the project so far is that it more or less works. First off I had some major offset-problems and some weird noise issues. Making sure all 4 the transistors were hooked up correctly mostly cured that (…), and so now the noise performance is definitely where I would expected it to be (i.e. there isn’t really any noise 😀 ). The DC-offset is lower than before, but still higher than I would prefer at 25-30mV and the offset adjustment pot doesn’t seem to work as I expected. A few more tests to go then and potentially a rerun of the PCB and then I might publish my final work here, but if I can get it to work then it’s potentially a very interesting circuit either for class D modules in SE-mode or as a simple “no-gain” preamp.

As usual, if you can’t wait for my experiments or if you want something that is tried, tested and supported then I highly recommend buying a PCB from Keantoken instead.

Naim the clone…

Yes, sorry for that terrible headline 😉 One of the ebay-kits I mentioned I my last post is this one.

It’s (supposedly) a clone of the Naim NAC152XS preamp. Now, I’ve not spent a lot of time working out the circuit details (a bit of information available here), but apparently it consists of a simple gain stage and an active filter which also serves as a buffer (and a volume pot in between). It could well be that this is a somewhat bastardised version of the original Naim circuits, but that’s not terrible important for me.

Although part of what made this kit interesting to try was that Naim is one of those hifi-brands that have a distinct sound signature – and a loyal following because of that (some would undoubtedly claim that you definitely don’t buy Naim gear for the looks so it has to sound good… 😀 ). What really caught my eye was that it is a very simple discrete circuit and it is single-supply. Discrete is always fun and while single supply circuits do have some drawbacks (additional capacitors in the signal path etc.), they also have some advantages for DIY’ers. One large advantage is that the single supply rail is normally easier to make and certainly easier to transfer between boxes, so an external PSU suddenly becomes a more viable/desirable option – and that’s where this will go as well when I get that far.

My contribution to this project – apart from soldering all the parts in the right places – consisted of replacing the capacitors in the signal path with Nichicon ES bipolars which are a bit better suited to the job (and nice and green!), and then just matching the supplied transistors as good as I could to make two identical channels. I also supplied the four board-mounted RCA connectors which I had left over – and then immediately after soldering them in place I remembered I had actually put them aside for another project 😉

Now, as usual I don’t post detailed impressions of the sound quality (mainly because I don’t have any right now), but my initial impressions are definitely positive enough that I’ll go forward with finding a suitable enclosure for these boards because I think they deserve that 🙂

Spring cleaning…

Although spring is still some way off weather-wise around here, the Easter break certainly gives time for some (much-needed) spring cleaning. We’ve got quite a few official holidays in Denmark already, so with just three extra holidays from your allowance you can get a full 10-day break – nice!

As the weather most of the week turned out to be well-suited for indoor activities (…) this meant I had time to work on getting my “lab” in order and actually start work on some of the various “backlog” projects that my moving uncovered. Highlights worth mentioning 🙂

I mentioned a while ago that I was building an F4 and that is a backlog design that I would really like to finish and try out. I managed to confirm that the boards are working correctly so more or less all that’s left now is to fix a stupid mistake that I made in the (otherwise nearly completed) chassis work.

Another “no-gain” amplifier design is the “MoFo”. As I alluded to in an earlier post I have made my own board which I would also like to finish and test. I’ll be using the smaller 193T chokes as per the build article, so it’s not much power but it should still be an interesting listen. As you can see from the pictures I have the heat sinks sorted, but a few parts are still missing (incl. the chokes). Of course my plan to get the last remaining board parts here in time for Easter did not work when Mouser shipped late, so they should turn up next week instead. But hey, then there’s something to do for next weekend as well 🙂

I also managed to half-finish a couple of new ebay-kit designs, so another Mouser-order is called for to secure the remaining parts. No ETA on either of these, it’s just stuff I bought because I thought they looked like interesting designs.

Last but not least, since I’m not the only one spring cleaning I’ve been keeping my eyes on the local classified sites and picked up a couple of interesting (for me) products that I have been testing. That experience has (once again) convinced me that I am not looking for absolute sound quality, but more an optimal match between the various parts of the system to get the most pleasing result. Maybe a bit contentious for some, but the difference in musical enjoyment is enough to convince me that I am right 😀