Project files: Subwoofer filter/preamp

Time flies when you are having fun – or when you are just very busy with a big project at work!. Here are the project files for the subwoofer preamp I showed in the previous post.

Project files: TDA7297 amplifier

As I mentioned in the original post I want to share these files so you don’t have to wait for my experiments to find out of this amp is as good as the internet leads me to believe 😉

However, there is a caveat – I seem to have made a bit of a layout faux pas so read on and decide for yourself if you want to try it…

Project files: B1 R2 buffer/preamp

It’s been a while since my last project files-post, so it’s a good feeling to get this published – and on Christmas eve to boot! 🙂

Project files: A different F5…

Would love to say that I am extremely busy with loads of interesting DIY projects and that’s the reason why I haven’t been posting much lately. Unfortunately it’s mostly just the usual “pre-christmas” panic at work that is keeping me busy. Here’s something from the “archive” though. Not sure why I never published these files before, but it looks like I didn’t and I really should – so here they are 😊

Project files: Single-supply experiments…

After a short delay caused by me finishing the HF-lineamp writeup first, here are the promised project files for the single-supply minipre and the railsplitter mentioned in the previous post about my USB-C experiments. Although these two designs are pretty different, I’ve decided to group them together because they also have some things in common that would otherwise be a bit repetitive.

Project files: The HF Line Stage…

Like the NE headamp this is another ancient amplifier design by a (now-defunct) Danish magazine – “High Fidelity” – that I wanted to try out. I’ve therefore redone the original PCB layout and since it is a bit of legacy design for a fairly narrow audience, I am publishing the files directly in case anyone else wants to give it a go 🙂

Is it an exceptional design? – probably not. However, while I haven’t tested this in a big setup I have to say that I was pretty impressed with the clarity and level of detail on offer when I did a quick test on my desktop, so it’s very likely that I’ll try putting the prototype boards in a case at some point.

Project files: SDS/AMB headphone amp

The other half of my recent board shipment (see the previous post) was this design, which I’ve always liked but never tried. I can’t really remember what led me to start doing a PCB for it, but probably a combination of boredom and a flash of inspiration – at least that is what usually works 😀 Anyway, here is the design for anyone else who wants to have a go at it 🙂

Project files: Pascal I/O adapter

After a longer-than-expected delay in shipping, I received some boards. Among them was the revised version of my Pascal adapter board, so after a quick check that it still works, time to release it into the wild… 🙂

Update 18th July 2022: A reader was kind enough to point out that there are actually two versions of the “universal” Pascal 26 pin I/O interface. There is one for the L-Pro (that I was looking at) and one for the S-Pro (which I didn’t have access to previously)

Basically the RX programming resistor is not used on the S-Pro modules and instead pin 14 can be used to put the module into a lower voltage mode suitable for running the S-Pro module in BTL with a 4-ohm load.

The download file has now been updated so there are now two versions of the adapter-board with some minor tweaks for each module type. If you have been using the “old” version of the board with the S-Pro modules then don’t worry. If you want to be sure then remove the RX resistor by cutting it off the board, but if your amp already works then it is unlikely to make a big difference.

I’ve also included some additional information in the file that I did not have access to before so you can see what the differences are (I wouldn’t normally, but this is pretty annoying so I’ll make an exception…).

Update 30th August 2022: Another reader sent me the pinout for the T-Pro modules and we’re now up to three versions of the same “standard” interface…

The T-Pro and S-Pro interfaces are very close, except that pins 14 and 15 on the T-Pro are reserved for internal use. If you have boards already, don’t mount or cut off the RX resistor. If you do not have boards already, use the S-Pro version and don’t connect anything to pins 14 and 15.

Update 13th November 2022: So, the saga of the Pascal-modules and the documentation continues. A reader pointed out to me that the input of the Pascal-modules isn’t actually a balanced interface, but merely has a split of different grounds broken out onto the connector. After reading the datasheets very carefully again, this seems to be correct and I just missed that.

The adapter boards below are still usable, but basically the modules have two SE-inputs and you’ll not be able to run a true balanced signal from a preamp unless you a) use the modules in BTL-mode or b) add a way to convert between BAL and SE in front of these modules. In this regard, the configuration is exactly the same as the ICEpower ASX-series. See further comments below.

Project files: LME49600-based headphone amp

When I built the LT1210-based preamp I thought I set a record for longest wait between making a PCB and actually trying it out. I am happy to say that the record has been broken with this – a headphone amp based on the LME49600 buffer IC, which I apparently did back in 2013 but haven’t managed to assemble and test until now (!).

Moving with the times?

I regularly complain about the declining availability of DIY-friendly parts – as do quite a few others if you look around. An obvious case in point is opamps, where (more or less) you can say that on one hand high-quality parts have never been better or cheaper, but on the other hand these parts have never been available in less DIY-friendly packages. Even the classic SO-8 SMD footprint is starting to look big and many newer devices are only available in smaller packages, some even “leadless” and/or “powerpad” packages that are very hard for the average diy’er to solder.

However, the many available opamps are actually obvious candidates for some experiments, especially with basic preamp/CMoy-style circuits where the sound quality and sound signature of the opamp really shines though. Some years ago I made a PCB design specifically for that and a few months ago I decided to revisit this layout and update it specifically for use with SMD (SO-8) opamps.

Of course you could use the old design with DIP-adapters, but fast opamps and DIP adapters are usually very bad for stability, so it was way better to create a new version of the layout intended for SMD packages instead. This naturally means you lose the ability to “roll” opamps (i.e. experiment with replacing them), but to most people I think that is an acceptable trade-off.

I haven’t really kept up with the development on the opamp scene the last few years, but what I would look at myself are the various OPA16xx duals. Here you can get both bipolar and FET inputs, “standard” high-quality opamps as well as “audio-specific” devices and more – and the prices are a small fraction of what you paid for top-of-the-range models such as the OPA627 and OPA2107 ten or fifteen years ago. AD also does a few interesting variants by to be honest here I am even less aware of what is available than for TI – I am sure diyaudio.com has answers if I ever need them.

The prototype here is made with an OPA1656 dual opamp and the initial impression is that I was on to something with this idea, because it does sound very good. I also have both the OPA1642 (FET-input) and the OPA1612 (bipolar input) in stock and I think these would be interesting candidates as well. Another interesting thing is that the power consumption is lower – in fact so low that without the LED my lab supply is showing a consumption on 0.00A for both rails (it’s not very sensitive). That also means that battery-powered versions would definitely be viable.

The download file for the new version is added to the original project file post because that’s way easier, so go and grab it from there if you want it 🙂