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…)

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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.

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 😀

 

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 😀 ).

Hypex UcD in progress…

It’s been quite some time since I have posted about a project that I have actually completed – and so I’m a little ashamed to say that this post won’t break that trend 🙂

It’s a class D power amplifier based on Hypex UcD400HG modules. As with many of my other projects, it started with a few leftover parts and some thoughts about what to make from them. In this case, it was the chassis and the “centrepiece” of the design, namely a gigantic 1000VA transformer. Trying to decide what to do with that lot soon led to some obvious choices:

  • It had to be class D because there was very little space for the actual amplifier and heat sinks once the transformer was installed 🙂
  • The “upstairs/downstairs” layout that I have mentioned before became a necessity to make use of the internal height of the chassis.
  • I wanted to reuse my PSU-boards because I already had spare PCBs, but they wouldn’t fit and so I had so make a smaller version.

I know Hypex has shifted to their newer N-Core technology and started building integrated modules with onboard SMPS, but the old UcD-modules still have a reputation as excellent amplifiers. The plan is to upgrade them with the Hypex HxR-regulators once I confirm that everything is working properly.

The “final problem” is how to provide heat sinking. I have my eyes on some pre-made heat sinks that should do the job, but they are from a supplier where I don’t normally order so costs are a bit higher than I would like. I might eventually break down just to get it over with, but in the mean time I have a couple of other options in mind as well 🙂

PeeCeeBees in a box…

Well, almost. At least the box is ready for the PeeCeeBees, even if they are not quite ready for it yet 😀

Apologies for the stupid puns, but I managed to (nearly) finish the chassis for the PeeCeeBee amplifier boards. The basis is a small ebay-sourced enclosure with side heatsinks, but I have replaced the rear panel and fitted the insides with custom mounting panels for the PSU and the transformer/softstart.

The “upstairs/downstairs” layout maximises internal space and is something I plan to reuse extensively on other projects (even if the cost of the custom panels from Schaeffer/FPX does begin to add up…)

Still to do is some transistor matching (bleh!) before I can finish and test the boards and I also need to order a new transformer – the one in the picture is the right size, but not quite the right voltage.

First Watt F4 (part 1)

I don’t normally build class A amps in the summer because my apartment gets really warm, but this time is an exception. Partly because this summer in Copenhagen has been much more “class A amp friendly” (i.e. a lot colder!) than usual, and partly because this is a design I’ve been wanting to try for a very long time now.

The First Watt F4 is a classic Nelson Pass/First Watt design with JFET inputs and MOS-FET outputs. However, as with the other FW amps there is a twist here, namely that the F4 has no voltage gain. That means it’s essentially a buffer than can provide a full 25W class A output. What’s the point of that you might ask? Well, one point is that it can help get a better gain structure and that it’s possible to use some sources (such as DACs) which have a very high output. There are various other applications in the F4 manual as well.

Some will have spotted that the F4 boards are from the diyaudio store. They are good quality and a well-proven design, so I decided not to bother doing my own.

The chassis is sort of the usual from Modushop, but then not quite anyway. Partly because the heatsinks are predrilled 4U types from diyaudio (which did cost a bit more, but saved me drilling and tapping nearly 30 M3 holes) – because they match the boards 100%, and partly because I have decided to do a bit of “hacking” to make a non-standard size chassis (teaser! 🙂 )

So, in addition to the chassis hacking, I am also thinking about which preamp to choose to provide the voltage gain for this and obviously there are plenty to choose from, so it should be possible to come up with an intriguing combination for you guys 😀

Soundwise I also have quite high expectations because of my past experience with the First Watt F5 design – which I still consider one of the best sounding amplifiers I have tried in my home system – but let’s see if the F4 delivers on that front as well when it’s ready 🙂

PeeCeeBees in the wild…

First of all, let me apologise for showing yet another half-finished board, but there’s a reason for that which I will get to later. Secondly, apologies as well for the stupid headline, but unfortunately this is the name of the design so I did not choose it 😀

The PeeCeeBee amplifier is as far as I can recall a version of the VSSA (Very Simple Symmetrical Amplifier) developed by diyaudio-user LazyCat and a precursor to his “FirstOne” semi-commercial design. The VSSA is (as the name sorta implies) a simple amplifier based on Lateral MOS-FETs which are excellent for audio and the PeeCeeBee circuit has built on the concept.

The PeeCeeBee design has gone through several iterations, but this week I received my boards from the diyaudio groupbuy of the v4 design from Shaan in India. The group buy boards are black (which would not have been my first choice because you can’t see the traces very well), but they are 2.4 mm thick(!) and excellent quality so I couldn’t really wait to start putting them together.

Now the reason for posting this now is that there is a second run group buy which runs until the end of the coming week, so if you missed the first round there is still a little time to secure your own boards 😀

Still short a few passive parts for the boards, but I have the LAT-FETs on hand, I have a suitable chassis on hand and probably a transformer as well. I’ve never built a LAT-FET amp before, so really looking forward to hearing this one “in action” in (hopefully) a few weeks when the last parts get here.