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 😀

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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 😀

 

Fame and fortune here I come!

Well OK maybe not, but it looks like I’ve made it to audioXpress magazine and surely that counts for something? 😀

I didn’t subscribe to their newsletter (I do now! 😉 ) so I found out a bit late, but they seem to have open-sourced the Borbely Hybrid Headphone-amp design (kudos for that) and referenced my post as one of the examples of the usage of the circuit. That’s really great and very much appreciated, but next time guys let me know in advance and I’ll provide you with a better board picture, ok? 😀

Since that discovery gave me time to reread the original post, I started googling for turn-on/turn-off protection and delay circuits and maybe it’s time to see if something can be done here? That the amp had dangerous levels of DC offset at turn-on and turn-off was more or less the only real flaw with an otherwise great and very interesting design. Google did provide some inspiration that I am going to look at and see if I can come up with something good. I was tempted for a moment to use a uC, but putting software in a Borbely-design seems inappropriate in some way 😉

Slow-turning wheels….

I’ve been complaining about my pile of unfinished projects for the last few posts, but now I am at least at a stage where I can start to do something about it. The first step was of course to add cupboards where I can hide the mess ( 😃), but obviously the only really viable solution is start finishing up some of these projects, so I will try to get started on that during the Easter break – fingers crossed!.

Meanwhile, there are also other stuff to be worked on. One of the projects I did start warming up to has been my ICEpower700ASC-based amplifiers where I have managed to decide on an overall architecture. It’s going to be monoblock chassis with two switched inputs, provision for some form of buffering and an external trigger option. The ASC-board has a whole host of useful features and connections which it would be a shame not to exploit, but in order to avoid too many air wires I decided that a small breakout-board was in order. This will put the control signals and the aux power on more easily-accessible headers. Prototypes have been ordered! 🙂

Buffering isn’t 100% decided yet but since the 700ASC-module has a balanced input, the requirement was really for a fully-balanced buffer – ideally with Bal/SE conversion built-in. The obvious choice for that (and one which I haven’t really used before) is a fully-differential amplifier (FDA) such as the OPA1632. I’ve previously looked at this IC and done some sample board layouts, but nothing ever really came of it. This time, I’ve started from the schematic of AMBs excellent “Alpha24” design and started hacking it to suit my usage. The starting point for the board layout is one of my old ones, but significantly cleaned up compared to those previous experiments so hopefully everything works as it should (the OPA1632 is fairly high-speed and so board layout is a bit critical to get good performance and low noise).

I’ve also made rough mechanical sketches which are only really waiting for the boards etc. to become available so that the dimensions and placement of the various holes can be 100% finalised – paper mockups are a great way to do the initial prototyping though. The turnaround time for board deliveries from China seem to have slowed down a bit lately, but since all the Danish public holidays are basically in April and May there should be a chance to make some more progress when the boards do show 😀

Signs of life?

This turned out to be a longer break than I had expected – sorry. Turns out that buying a house and moving is a bit of a slow process (at least for me) and throwing in a job change, a holiday and a few other things as well probably didn’t help…

So, what has been been going on since last time? Well, not as much as I would have liked, but a few things nonetheless:

Back in October diyaudio member Michael Rothacher posted this article on the “MoFo” class A MOS-FET follower and kicked off a very long thread on diyaudio. The MoFo is a very simple design that is immediately appealing to me and so rather than wait for “official” boards (which are now for sale at the diyaudio store) I decided to do my own. To be honest, even if the design hadn’t appealed to me I would still be interested because of the article and its obvious nod to Corey Greenberg’s original Stereophile article on a buffered passive preamp which I really like (if you are lost now, read that here 🙂 ) I have most of the parts for the MoFo on hand except the transformers, so hopefully I can put the boards together and check whether they work in the not too distant future. This is just an experiment for now anyway, so I just have to get it to produce sound and then we’ll see about a chassis later on 😀

Also, for whatever I eventually end up doing with my ICEpower 700ASC modules, I will probably at least experiment with adding an input buffer. A very nice design suggestion arrived some time ago from a blog-reader, namely the “Kuartlotron” (which sounds like a device used by a mad scientist in a sci-fi movie by the way). I’ve made some boards, but haven’t tried them yet. They should sound marvellous, so I am looking forward to that – loads of impressions from others here by the way 🙂

Another diyaudio-thread that I managed to keep up with was on a small and very cheap DAC-board based on the new ESS ES9038Q2M dac chip (the “cheap” version for e.g. mobile devices). I mainly bought a board because there is a simple onboard volume control, but the sound quality I think warrants further investigation. Unlike most other of these boards I buy, I actually have something approaching a real need for this one 😀

Last but not least I am – slowly but surely – working my way through the piles of half-finished projects I moved out of my apartment and I am finding plenty of things where I don’t really have many good excuses for not just finishing them. so fingers crossed I will be able to start making some progress here as well. I don’t plan on spending all of my easter holidays doing gardening, but let’s see if that holds 😀

New toys…

As per my last post I am in the process of moving (a lot of) DIY stuff out of my apartment, so what I should definitely not do is buy more things. However, sometimes an offer comes up that you just have to jump on 🙂

This time it was a set of unused ICEpower 700ASC-modules which is one of the ICEpower models I have not yet tried. They came up on a local classified page last week where I just spotted them by accident. The price was reasonable and since this is the ASC-version of the 700 with a few extra useful features then they should be quite versatile in use.

For now though, they are going on the shelf while I look for a suitable project for them (and move them to the new house 😀 ).

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.