JISBOS/Alpha20 buffer clone…

I’ve got a few projects that are now very nearly finished and also a couple of new PCBs in the mail that I am pretty excited about, but before we get to any of that I just want to show a bit more about the buffer design that I presented in my last post about the integrated ICEpower amp.

The Alpha20 buffer by AMB was originally called the “JISBOS” because of its “JFET input stage, bipolar output stage” and its original design pages are still online here. Sourcing the JFET inputs are of course a constant problem, but fortunately I bought some with my original boards from AMB. The first version of the design was a pure buffer, but since then AMB added the option to have gain as well – not something I need right now, but definitely adds to the versatility of the design.

Technically there is of course nothing wrong with AMB’s original board layout, but a couple of things were annoying me. One is that the original board is intended for very small resistors which I would have to buy, instead of just being able to use the RN55s that I have already (yes, I know it is possible to make the RN55s fit a 7.5mm lead spacing, but for me it always ends up looking like crap and a real 10mm LS is much easier to work with anyway).

Another niggle is that power and I/O connections are just holes in the board. That makes it pretty easy to solder up a permanent design, but it is a royal PITA for testing and also in case something ever goes wrong. Last but not least, I really like having LEDs to give some indication that the board is powered and operational. Of course this is not bullet proof in any way, but as a quick indication that everything is OK I find it works well.

The original plan was to run these boards without heat sinks (because they are only for line-level applications and not supposed to deliver a lot of power) but at the last minute I chickened out and put some small heatsinks on anyway. I’ve actually got another layout version with the output transistors turned 90 degrees. Then there is space for heatsinks to extend over the sides of the board, which for headphone use and other high-power applications would probably be better.

Now as I wrote in the previous post I don’t normally set out to make my boards twice as large as the original, but in this case I am willing to take that tradeoff for the improvements I have made – so let’s hope the finished amp will sound as good as I expect it to! :).

Starting the 250ASX-int…

For a while I have been thinking about doing an “all-in-one” integrated amplifier and therefore I’ve been looking for a suitable class D amplifier module as the “centrepiece”. A few weeks ago the perfect candidate showed up in a local classifieds ad and so I picked up a single unused ICEpower250ASX2 module at a fair price. Conceptually this build is quite simple – two switched single-ended inputs and a buffered ICEpower module with a volume control inside. However, just doing that would have been a little bit boring, so I added a some complexity to make it interesting 🙂

Part of my reasoning to build this at all was that I wanted try out an ESP8266-based amplifier monitoring & control board I developed based on my IoT-T design. This control board was actually intended for ICEpower-modules so that I was lucky enough to pick up a 250ASX was really good. The original inspiration for the control board wasn’t even the ASX-modules but rather the Pascal-module which has the ability to output quite a lot of monitoring and diagnostics signals. However, as I only have one Pascal module and no reliable way to get more I decided to build a first version to suit the ICEpower ASX modules instead.

I don’t really have working software yet, but when completed the finished amplifier should have the option of basic web control and monitoring via the ESP’s Wifi connection as well as driving a “local” front panel LCD display via I2C. I haven’t fully decided if I want to use this feature for this particular build, but at least the option is there. A potential problem is that the ESP8266 is going to be enclosed in a aluminium and steel box and the Wifi-connction might not like that. Obvious solution #1 is to use a ESP pro module that can be fitted with an external antenna on the back but my mechanical layout is suboptimal for this purpose to say the least. Obvious solution #2 is to ditch the steel lid in favour of acrylic or something else – we’ll see where I end up with that.

Another goal of this design was to try using a discrete buffer such as the JISBOS/Alpha20 with the ICEpower amp as I’ve never really done that. However, once I started looking at the A20 boards from AMB that I already have I decided I preferred to do my own version instead. Normally my goal with clones is to make stuff smaller but in this case I ended up making it about twice as large as the original… Still, I think it was worthwhile to do and I’ll probably do a separate writeup on this design later. For input selection I have a basic design that works (I only need two inputs), but once I got the boards I have out of storage I couldn’t resist messing with them a little, so I can’t finalise this until the new boards show up (which may take a while if all the factories in China stay closed due to Corona-virus…)

For volume control I ended up with a very difficult requirement, namely that it had to be controllable by I2C from the control board. That’s a surprisingly difficult one since the “usual suspects” for high-quality audio (e.g. a PGA23xx or similar chip) requires SPI, so my solution ended up being something else – we’ll see if that works 🙂

Wooden amps…

Well, not completely wooden of course! Some years ago I showed a plan to make a set of amps with wooden front panels, because I picked up some pre-cut wood in the right dimension on one of my trips to Japan. A while back the plan was resurrected, but then immediately brought to a halt because I had to enlarge some already-drilled holes and managed to destroy one of the panels in the process – bummer!

However, in the mean time I’ve found a local place that sells wood trim pieces for professional craftsmen and in their assortment I found a pre-made profile in the right dimension for a 1U panel and in several different kinds of wood. I chose oak as it is more my thing than the darker wood types and it goes well with both black and silver fittings. I’ve recently invested in a better drill press, so redoing the panels were without accidents this time and I also managed to overcome my fear (or is it loathing) of doing cabling to finally complete the set 🙂

The set consists of a DCB-1 preamp (on a clone board), meaning a DC-coupled version of the classic B1 buffer circuit, and a Hifimediy T4 Tripath-style class D power amp. The power supply is a surplus N2 XL375-type which I bought a small stash of some years ago.

No detailed listening impressions from my main system yet, but on test speakers it sounds excellent as I expected from what is basically an “evolution” of my old B1/125ASX bedroom system. Both components include relays to minimise turn-on and turn-off thumps and so as a set they are well-behaved enough for daily use (which is always one of my success criteria). As far as the looks are concerned, I could honestly see this being something I will want to try again in the future…

Project files: The BalBUF and PSU

I’ve made a little bit of progress on my balanced mono block 700ASC-amplifiers lately, so now is probably a good time to release the project files for the balanced buffer input board and the accompanying PSU that I used in that design. It’s a pretty obvious clone of AMBs Alpha24, but since I did my own board and ditched some of the configurability I figured it’d be OK. Big thanks to Ti Kan though for actually showing how to build this circuit which I previously attempted a couple of times without getting it right.

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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 🙂

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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? 😀

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.

Project files: The last of its kind…

…for a while at least 😀

What is it?
The last (and smallest) version of my EL2k buffered headphone amp using NOS Elantec 2008/2009 buffer ICs. This is the smallest version designed for 1.5″ heat sink profiles as described here. The two other versions are of course also still available (here and here):

How big are the boards?
The board measures 3.95″ x 1.5″ (app. 100 x 38 mm.) and is obviously a mono amplifier channel.

What is the status of the boards?
I’ve called this board version 1.5 as it is a redesign. Apart from the redesign work described in a previous post, the circuit is identical to the other published files.

Does it use any special/expensive/hard-to-find parts?
Yes.

  • The EL2008/EL2009 buffers have been out of production for 10+ years. They can still be found and I don’t think you need to be especially concerned about fakes – there can’t be a lot of demand for these anymore – but of course no guarantees. The main risk is probably that instead of NOS parts that you get used parts that have been pulled from old equipment. This is annoying, but should be OK.
  • The heat sink profile is the same as the original, Fischer SK68, in 37mm length. Easy to get in Europe, but I’m not sure about elsewhere.

Anything else I need to know?

  • I’ve had to mount the buffers on the side of the heat sink that has an M2.5 slot and not an M3-slot. This isn’t a problem as such because there’s no need to isolate the tab, you’ll have to remember to buy M2.5 screws for mounting 😀
  • Otherwise this is a bog-standard buffered opamp circuit and there isn’t much that can go wrong 🙂

Downloads:
Download design files here

Related information:
Be sure to read the original posts for additional information and tips.

Note: Always read the “intro post” for additional important information about my designs.

Project files: The (modified) EL2k headamp

What is it?
The board files for the new “medium-sized” version of the EL2k buffer/pre as shown a few weeks ago. The smaller 37mm board version will follow in a while.

How big are the boards?
The board measures 3.95″ x 2.0″ (app. 100 x 51 mm.) and is obviously a mono amplifier channel.

What is the status of the boards?
I’ve called this board version 1.5. Apart from the redesign work described in the last post, the circuit is identical to the originally published v1.1 files.

Does it use any special/expensive/hard-to-find parts?
Yes.

  • The EL2008/EL2009 buffers have been out of production for 10+ years. They can still be found and I don’t think you need to be especially concerned about fakes – there can’t be a lot of demand for these anymore – but of course no guarantees. The main risk is probably that instead of NOS parts that you get used parts that have been pulled from old equipment. This is annoying, but should be OK.
  • The heat sink profile is the same as the original, Fischer SK68, in 50mm length. Easy to get in Europe, but I’m not sure about elsewhere.

Anything else I need to know?

  • I’ve had to mount the buffers on the side of the heat sink that has an M2.5 slot and not an M3-slot. This isn’t a problem as such because there’s no need to isolate the tab, but some swearing will likely ensue when you sit there on Sunday afternoon and realise you don’t have any M2.5 screws to hand 😀
  • Otherwise this is a bog-standard buffered opamp circuit and there isn’t much that can go wrong 🙂

Downloads:
Download design files here

Related information:
Be sure to read the original posts for additional information and tips. You should be able to reuse the linked BoM as well.

Note: Always read the “intro post” for additional important information about my designs.

Evolution of a design…

Sometimes when looking at a design I was originally quite happy with new ideas come up and I start to rework the design, either as an optimisation of the original or simply as a “branch” that I hadn’t originally considered.

One such example is my “EL2k” buffer/preamp/headamp design. I was fairly happy with the original layout, but when I contemplated putting four boards in the same box for a balanced configuration the original board size started to look a bit big and so a redesign-attempt was in order.

Originally the ambition was a “space no object” design which had room for the best quality parts possible, but aside from that the original design goals were simple:

  • Through hole parts where possible
  • Short signal path and good decoupling as per the component datasheets
  • “Overkill” Fischer SK68 heat sink profile because I like the way it looks and because it provides solid mechanical mounting to the board.

Mostly because the heat sink profile comes in predefined sizes (which means that there are some natural steps in how the board should be shrunk), I thought this could be an interesting way to showcase the evolutionary process of what I ended up with 🙂

On the original version I was pretty happy with the basic layout and most of the traces are as short and as clean as the physical layout allows (at least I think so…). The only real exception is the unsightly top layer trace that links the negative supply to the buffer with the negative supply pin on the opamp. The first step was to try and tackle that….

el2k-evo-1

…and it’s not easy. There isn’t really a lot of space to begin with, and even with tricks such as physical jumpers and SMD decoupling caps I wound up more or less back where I started (see below).

Next came trying to actually reduce the board size. The next step down in heat sink size is 50mm, so that becomes the target. End result:

el2k-evo-2

The 50mm version actually looks good to me and there are very few actual compromises here.

  • The input cap has been moved and it has been changed to a 27.5mm lead spacing box cap (with a 15mm option). It’s a small step down in quality vs. the axial cap on the original board, but probably still fine for most people/applications.
  • The power LED arrangement has been changed. The original “1 LED per rail” replaced with a single LED connected between the supply rails. In return, the LED resistor was changed to a slightly bigger package that allows for resistors up to 1W.

Now, the next step down in heat sink size is 37.5mm.

el2k-evo-3

Now we’re seeing some actual compromises 🙂

  • The basic layout is still the same, but the input cap has been shrunk considerably to a 15mm type. However, that in itself is not enough and one of the mounting holes had to be removed to provide space for the input connector.
  • The output connector also had to be removed and replaced with solderpads.

Other than that, it’s pretty much identical to the 50mm version. This led to a bit of thinking – what if the input cap was removed altogether? – and either omitted or mounted off-board? That would allow the fourth mounting hole to be kept. However, since the cap can easily be bridged and the board still has two mounting holes on the “heavy” end, this was deemed unnecessary overall.

el2k-evo-4

Both of these versions have larger compromises as far as I am concerned, but still not unacceptable if I had an application that required the smaller PCB size. Suddenly it becomes possible to take the idea of a balanced-bridge amp and realise it in almost the same space as the original stereo amp. Also, it gives an excuse ahem, opportunity, to design a backplane for the amp boards to keep the wiring tidy and make it look better 😀

It also becomes clear that it isn’t really possible to shrink the design further without making substantial changes. A stereo board version would of course be possible, but looking at the configuration of parts around the EL200x IC and it became clear that I couldn’t have two amplifier blocks side-by-side and keep the original arrangement of power supply, decoupling, signal routing etc. Also, when deciding between a stereo 75mm version and a mono 37mm version, I would normally choose the latter as it is cheaper to manufacture and more versatile in use.

So, all things considered the original 75mm version is still good but the “modified” 50mm version should be almost as good. The 37mm version doesn’t give up the overall flavour of the original design and it’s definitely still viable, although the exact application would have to decide exactly which compromises to make. Not bad if I do say so myself 🙂

So with that done – expect to see revised prototypes in about a month or so 😀