A modern take on the CMoy Headphone Amplifier

Anybody who has browsed these pages in the past knows that I keep an eye on certain people’s projects, one of those being Uffe at Slowdiyer. Recently he has been experimenting with USB-C power supplies, one of the offshoots from this being an updated version of the’mini-pre‘, a variant of the classic Chu Moy headphone amplifier. I’ve used an earlier version of this design before as a preamplifier in my IcePower integrated amp. A while ago, Uffe began using some SMD parts for the optional bandwidth limiters, and this iteration also allows for the latest generation of high specification audio grade op-amps in SOIC (8) packages, with the OPA1656 from TI being used here. Additionally, the standard ‘virtual ground’ on the original CMoy HPA has been replaced with a TLE2426 rail splitter chip.

Following a comment I left on Uffe’s post, he was kind enough to send me PCBs of this project. In essence, what we have here could be seen as a modern take on the CMoy and although Uffe designates this as the mini-pre SS v1.5, I prefer to title it the Slow-Moy (sorry Uffe!).

You can see the posts detailing the rationale behind the project here:
Post 1 – Moving with the times?
Post 2 – Comparisons…
Post 3 – Project files: Single-supply experiments…

The third post has both the mini-pre and a rail-splitter project contained within the folder as well as BOMs for both.

I built the amp up about a month ago and luckily I had all the parts in my stock, with the exception of the Alps pot which had to be scavenged from one of my earliest CMoy builds. The OPA1656 seems to be out of stock in most places, like many other ICs, but I had a couple from last year. I did nothing further with it though as the initial test showed there was a problem with one of the channels and I didn’t have the enthusiasm to go troubleshooting. Fast forward to today and I started to further dismantle the old CMoy so I could utilise the enclosure for this, having forgotten about the problem with the board. A test with the headphones again revealed the problem so I put it through my scope, which showed the problem below. The solder on the inputs of the opamp (pins 2 and 3) looked a bit problematic and so I resoldered them and that solved it.

Whilst I had the dummy load out, I ran the power tests on the amp and you can see the results from the fully functioning unit on the dedicated page I have for the headphone amplifier power outputs. Like an idiot, I forgot to capture images from the scope, so I’ll update that page when I’ve redone the readings. (EDIT 28/09/2021 – Images are now up).

In terms of audio quality, I was really surprised at this. I used ripped 16/44.1kHz FLAC files located on my phone (Galaxy S7) as a source, playing through HiFi Cast directly into the amp via a decent lead and outputted to my Sennheiser HD569. I tried three very different genres of music (links to Spotify below – note, obviously these will not be the same versions as what I listened to). I was instantly impressed, the sound is crystal clear and detailed, in fact the AC/DC track seemed a little bright compared to how I ‘remembered’ it, but this might be the recording or my memory of course, I will have to try it with other setups. The other two tracks sounded excellent to my (middle aged, tinnitus blighted) ears. It really is an excellent amp, with stellar performance (note the hearing related caveat above) and a price/quality ratio that is outstanding. This new version, with a state of the art OPA1656, is even cheaper to build than the previous iterations using opamps such as the OPA2134. I really do think that an amp such as this would likely satisfy many people. Obviously there are many other considerations besides cost that drive builds/purchases and it would be interesting to see some measurements such as those conducted at ASR. But for now, I think this will go on my bedside table, at least until the next one comes along!
(For information, theSlowdiyer is not a commercial site and Gerber files for many boards are available for everyone to make for themselves – free of charge).

In the pipeline I have another HPA that uses the OPA1656 and it will be interesting to compare this simple build with a more complex one. This will have to wait though as I’m missing a couple of crucial components which I will need to order.