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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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.

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 😀