A little upgrade to go with the Wolfson WM8740 dac. The amplifier under the dac is an "HD8-A1" class A headphone amp, from the usual place. Very nice amp, it has an R-core transformer-based regulated power supply and the output stage is built from 4 off BD139/BD140 pairs, two pairs per channel, with each channel driven by an MC33078 dual opamp (aka LM833) and a couple of small-signal driver transistors. The LM833 is socketed so you can change it for something more expensive, but I wouldn't bother, it sounds superb as it is, and the LM833 made it onto Doug Self's list of high quality audio opamps, which is a good enough reccomendation for me.
https://www.ti.com/product/LM833
The amplifier has the usual "ALPS compatible" volume pot, they're actually pretty good, it feels nice and smooth in use, channel balance seems fine, no crackling etc. A nice touch is they've included a power-on headphone protection relay. The sound quality from this is setup is approaching what I would call genuinely good, with both the DT990's and HD650's. Wide sound stage, holographic, good bass extension, gorgeous mids and sparkling highs, very musical, it doesn't sound dry and overly analytical like a lot of modern kit. Of course it sounds better when playing flacs. I think the Wolfson dac rivals Burr-Brown for sound quality, although at the limit I think BB still has the edge. Having said that, I haven't tried an AKM yet. Highly recommended for a budget system, you get a lot for your money. And a good demonstration of the results obtainable from freebsd's OSS sound layer.
Photo of headphone amp output stage. The semiconductors are all ST branded (ST-micro, used to be SGS-Thomson) and everything looks brand new, I don't think there are any recovered components here. Very nice build, of course you're not going to get Naim level build-quality (for example:
https://liquidaudio.com.au/gorgeous-naim-nait-amplifier-restoration-review/ ) at this price, but its not bad. Nice to see the shielded cable used for the input signal. The heatsink is bolted to the base of the case, so the whole case acts as a heatsink. It doesn't get very warm in use, unlike some other class A's I've listened to, which is probably a good thing for longevity.
Shown below is the R-core transformer from the amplifier's power supply. Nice to see the attention to detail in the heatshrink insulation on the terminals. I think this transformer is shielded, from the copper casing that can be seen underneath the paper label. I certainly couldn't detect any audible hum in my headphones.
I was slightly disappointed that there are no DIP switches to allow gain adjustment to match headphone sensitivity. There is plenty of power, driving my 250 ohm DT990's, comfortable listening volume is at around 9 to 10 o'clock, and at 12 o'clock its starting to get quite loud, I think the output level from the dac is actually quite high, in an ideal world it could probably do with an attenuator between the dac and the amp to drop the level a bit. The two big red coupling caps near the front are marked 'MKP-CYCAP', which is metallised polypropylene
https://www.amplifiedparts.com/products/capacitor-audiophiler-400v-mkp-metallized-polypropylene . They are obviously low cost parts but hopefully a bit better than the standard MKT/NP electrolytic types, this page
https://www.raypcb.com/audio-capacitor/ rates MKP caps as 'very good' for audio signal path, the next level down from true film capacitors which are normally expensive; they sound pretty good, anyway. I couldn't hear any noise or hum or hiss with the volume on full and no music playing, so the power supply seems reasonable. The overall build is nice and clean, and the all-metal case is good quality, everything is solid and stable, I really can't fault it for the money. Perhaps the headphone socket looks a bit cheap, and it's case mounting not quite as rigid as I would like, but normally I will just leave the phones plugged in all the time, so it's not under a lot of plug in/out stress. The lid comes off easily, there are 4 phillips screws in the corners, giving full access. All components are through-hole, no crappy SMD's, and the soldering looks neat and tidy. The case designs of the dac, it's psu and the headphone amp match up nicely too. Altogether the combination of the WM8740 dac and this headphone amp, with the thinkpad and some decent headphones, makes a pretty nice music system.
I guess there's not a huge amount of smoothing capacitance in the power supply, I don't have a scope to measure the ripple. I suppose they have to economise somewhere at this price. However I expect what they have used is perfectly adequate, I can't hear any noise anyway, and at least it's linear and not a cheap switchmode. There is a TL072 opamp and a couple of transistors next to the switch-on relay, which I assume forms the speaker protection circuit; it seems to work fine, after power on there is a delay of a couple of seconds before I hear the relay click to connect the headphones, and there's no audible thump in the headphones themselves. I get the feeling the whole thing is quite well designed, given that they are likely to be restricted by a very low cost BOM. I guess if you're making a couple of thousand of these a week, your costs come right down.
Well, it all sounds very nice, of course this is not a proper technical review using test equipment, but overall I can recommend this setup of the WM8740 dac and HD8-A1 headphone amp combination, as a low-cost add-on to get some nice sound quality from freebsd. I tested this setup with BeyerDynamic DT-990 PRO and Sennheiser HD6XX (aka HD650) headphones, it sounded pretty good with both types of headphones.
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