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….
…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:
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.
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.
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 😀