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! :).

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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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…)

A good idea…

Like many people, I use running and walking as a way to try and stay in shape (yes I admit, you can’t really see it ;)) and also to take my mind off work and daily life in general. I also very often use this time as a way of solving problems and generating new ideas. Some of the ideas are definitely good, some are probably not so good and some of them just turn out to be very expensive 😀

A couple of months ago while out, I had the idea for a way to build a balanced Beta22 headphone amp from AMB. I didn’t technically need that, but since I have a pair of balanced headphones I thought I’d give it a go.

The B22 is not a cheap project by any means, especially not in the balanced configuration – and when you then start adding a few custom-fabricated enclosure parts and transformers then it is really starting to hurt! But hey, I am not into DIY audio to save money (definitely not…) so here we are 😀

Still quite a bit of work left to go as you can see, but even the electricals are coming along nicely. And while I am confident that the result will end up being worth it, I have to admit it was a pretty expensive run I went for that day 😀