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 😀

Project files: Universal Mini-preamp

A few weeks ago a reader was commenting on simple buffers/preamps and also asked about ebay-kits to use since I haven’t posted anything with a volume control yet. That got me searching to see what was actually out there and very quickly came the realisation – “I can do this better” 😀 Not sure if I did, but I at least tried 🙂

What is it?
A very simple opamp-based buffer/pre with an onboard volume control that can be used as a “buffered volume control” with a power amplifier module, a real preamp with or without gain or even a “CMoy”-style headphone amp. The board has space for a DIP-8 dual opamp, polypropylene input caps and a full-size Alps volume control and still manages to be very compact. I’m showing the board now as I already have a couple of applications for it in the pipeline myself which you will see later 🙂

How big are the boards?
2″x2″ (app. 51×51 mm) – a theslowdiyer standard size (TM) 😉

What is the status of the boards?
The board file is v1.0. I’ve built a prototype and everything seems to be fine.

Does it use any special/expensive/hard-to-find parts?
None, really. You can get what you need from Mouser/Reichelt and similar places and most of the component values aren’t that critical anyway.

Anything else I need to know?

  • The opamp should be a dual-type with standard pinout. My recommendations would be either the LME49720 (sadly discontinued in DIP) or the OPA2107 (still available but fairly expensive), but there are loads of other options. The board layout should be suitable for using adapters as well (for DIP/SO-8 singles or SO-8 duals) and if you want to go all-out there’s even a discrete option from Burson that should fit as well.
  • The only surface mount components are the optional (but recommended) 1206 bandwidth-limiting caps on the bottom – otherwise it’s through-hole all the way.
  • The PCB should be happy with just about any (regulated) dual power source – linear PSU, switching PSU or even a pair of 9V batteries.

Downloads:
Download design files here

Related information:
Even though this is a basic opamp circuit and I can just about draw the schematic and recite the parts values from memory, I went back to look at it once more to try and read up on the theory behind. If you aren’t very familiar with the basic schematic already I can absolutely recommend the old but still excellent articles from Headwize/Head-fi member Tangent here and here. Tangent’s pages also have a ton of other useful information and although the site isn’t updated any more (and it’s quite old) there’s still plenty of good stuff even for inexperienced diy’ers.

If you are more technically inclined then probably the best resource is the “Opamp Applications Handbook” from Analog Devices and edited by Walt Jung.

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

The Great B1-binge…

I’m obviously a fan of the Pass B1 design but the last couple of weeks have seen the arrival of no less than three more B1s which is a bit much, even by my standards… 😀

Not sure how this really came about, but it must be something like this: A while ago a saw an ad on a forum for a B1 clone board (the original type which is pretty much the same as the original pasty board). It was quite cheap and so I bought it. Since I now use both analog and digital I thought I could build one with two inputs (because my previous one only has a single input).

Shortly after I received this board and had started populating it, I realised I already had a partially-assembled board of another clone design that was basically only missing the input and output caps in order to be ready (yeah I know, I should keep a list or something… 😀 ). Because of the size of the onboard electrolytic caps, the board I had would fit nicely in a 40mm high enclosure, whereas the new board would require a bit more internal height, i.e. an 80mm chassis.

More or less the only thing I don’t like about these B1 boards is the fact that you have to “air-wire” the input switch, which means criss-crossing the inside of the chassis with long wires carrying the input signals. The obvious solution is to use a relay, so I went ahead and made a couple of small adapter boards to accomplish this. With a 24V relay all that is needed to switch the relay is the supply voltage to the B1 so it makes for very simple connections.

The last design was prompted by a reader email asking if I knew of a B1 with more than 2 inputs. The answer was “no”, but I then decided to build a source selector to match my own B1-board. The selector is a simple relay-based type with four inputs but it should work just fine. As I had some PCB mounted RCAs I made a board version for those, but also a more universal one without onboard connectors (not shown).

I’ve tried to put all of these in fairly nice cases, but it’ll still be a bit of a challenge to decide which one to keep as my “personal reference” 🙂

Project files: GP-PSUs v2

What is it?
Two boards for general-purpose LM317/LM337 power supplies with two rails, useable for many low-power applications (preamps, buffers, filters etc.). There are two versions, one where the +/- voltage is derived from a single AC-voltage via a voltage-doubler and one where it comes from a traditional dual-AC, two-bridge rectifier circuit.
These boards are effectively an update on the old GP-PSUs and they are based on the triple-PSUs I posted a while ago. In fact they are just the three-rail designs with the third rail removed 😀

How big are the boards?
Both board versions measure 3.925″ x 1.8″ (app. 100 x 46 mm.) and they are mechanically interchangeable.

What is the status of the boards?
Both boards are in v1.0. I haven’t actually prototyped these in this format yet, but since they are the same as the three-rail version (which I have tested) I don’t mind publishing them.

Does it use any special/expensive/hard-to-find parts?
Nothing, really. As always with these circuits, you can use standard LM317/337 regulators or splash out on more expensive (low-dropout) types like the LT/LM/LD108x-series. My experiences with the latter parts aren’t the greatest though (instability), so unless your applications require the low-drop capability I’d just as well stick to standard 317/337-types from a reputable source. If your application requires a higher performance PSU than this, you are probably better off looking at entirely different circuits and regulators anyway.

Anything else I need to know?
Yes, pretty much a repeat of what was mentioned for the three-rail circuits:

  • The diameter of the main filter capacitors is 18mm, but the dual footprint means that anything between 10mm and 18mm should be fine.
  • The DIP rectifier bridges exist in versions up to 2A rated current although anything more than 1A can be a bit difficult to find. Realistically though, if you plan on drawing more than 1A from either supply the SK104-type heat sinks are probably going to be a limiting factor anyway.
  • Mounting the regulators and heat sinks is a bit of a faff because there is not much space, especially if the heat sinks are 38mm or taller. My suggestion (as always) is something like this:
    • 1) Loosely assemble the regulator, the isolation components and the heatsink.
    • 2) Mount the combination on the PCB and solder the heatsink in place.
    • 3) Tighten the screw holding the regulator to the heatsink.
    • 4) Solder the regulator in place.

Downloads:
Download design files here

Related information:
Even though the regulators used here are generic types made by many manufacturers, there can be small differences in recommended parts values etc. I suggest you always consult the regulator data sheets from the specific manufacturer.

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

Project files: The J-Mo Headphone Buffer

What is it?
The project files for my version of Richard Murdey’s  J-Mo mk. 2 buffer with gain.

How big are the boards?

  • Amp: 2.45” x 1.975” (app. 62 x 50 mm.)
  • PSU: 2.35” x 1.975” (app. 60 x 50 mm.)

What is the status of the boards?
Both boards are version 1.0, meaning I have prototyped them and they work. However, I am still waiting for some mechanical parts for my own build so this isn’t final yet which means I have only done very basic tests.

Does it use any special/expensive/hard-to-find parts?
Well, the J-FETs are getting harder and harder to find but it isn’t impossible yet.

Anything else I need to know?

  • Can’t really think of anything. Be sure to read through the article on Richards website though, that contains most of what you need to know.

Downloads:
Download design files here

Related information:
See the original post for some more information and links. There is also a big discussion thread on diyaudio that may be of help.

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