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 🙂

Nudging projects forward…

Sometimes my projects are “stationary” for a very long time, and then a small nudge from another project is just what was needed to get them going again.

The current example is this (eBay) clone of an Ray Samuels Audio XP-7 headphone amp, which has been needing a custom front panel for several years now. Originally I tried drilling the case myself with a stepped bit, but that simply wasn’t accurate enough and so I needed to remeasure and do a custom panel instead. I’d been dreading this because of the frustration I feel when it doesn’t work out (and custom front panels are not cheap), but spurred on by my recent success of doing the back panel for the ES9038 dac I decided to give it a go, and this time it actually also worked on the first try 🙂

The XP-7 is a pretty straightforward headamp design with an opamp and a BUF634 output buffer in the feedback loop, as was the “state of the art” around 15 years ago 😀 . The only things that stand out in the clone design are the feedback resistor values that are lower than usual (which means lower noise) and then the chosen opamp which is an AD797 that also has very low noise. Together they should make a very dynamic amplifier and I remember being pleasantly surprised when I tested the bare board. However, putting it in a case should make is easier to try again (although to be honest I do next to no headphone listening these days).

The battery supply is two 9V batteries which is normally very impractical, but it does also have some benefits and I actually use a battery-powered CMoy-style headamp for a lot of testing (no risk of ground loops etc.). There’s still a bit of wiring to do here, but it’s always a nice feeling to move something along that has been sitting on the shelf for far too long 🙂

Minipre in a box…

A while ago I presented my “MiniPre”-project of a simple op-amp based preamplifier. Now I’ve had occasion to put it into use as a small standalone preamp/active monitor controller.

The design is very simple, so not much to be added there (whatever you need is probably already in the original post), but it’s basically a standard dual op-amp in non-inverting configuration.

The power supply will be in the form of a small DC-DC converter (a continuation of my previous experiments) so that I can feed the box from a single 5V supply and keep the case size down. Because of this I’ve managed to cram everything into the smallest available hifi2000 case, so it will fit nicely on a desk 🙂

The advantage of this simple design is the the selection of opamp tends to have a noticeable influence on the sound signature, so this is one place where there is room to experiment whether different options have better synergy than others.

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

Edit 26th Aug. 2020: Download file for v1.5 version intended for SMD opamps is here. Apart from the change to the SMD opamp footprint, the only thing that has really changed is that the output connector can be 3 or 4 pin (instead of just 3-pin). The original BoM should still be good as well.

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.

RIP…

Just read over on Innerfidelity that the founder of the Headwize site and namesake of the CMoy headphone amplifier, Chu Moy, has passed away recently.

As a relatively early member of Headwize, I can remember discovering a site with an active discussion forum and a lot of inspiration for a novice audio diy’er like myself. Apart from the direct contribution of the Headwize forum (including the projects gallery that I have mentioned several times in the past), Headwize of course also spawned head-fi.org and many other international and local headphone forums that still run today.

Headwize was also the birthplace of the first community headphone amplifier build projects (like the ones from Tangent and AMB) that have helped others get started in this new hobby. I am sure there are quite a few people out there who first experienced the elation of playing music through a self-built amp via one of these projects 🙂

Although I never knew Chu Moy personally and he was out of the “spotlight” for many years, I am very grateful for his contribution to the audio hobby and of course for being one of the reasons why I am in this at all 🙂

Full story by Tyll Hertsens