This is an extraordinarily simple headphone amp, and has essentially reached legendary/history status at this point. I decided to build the original design (there are countless mods and totally different amps that hark back to this one), though opting for a wall-wart desk form factor instead of the original 9v battery tin.
I am building from the the 2008 "williamneo" blog post, as I like that point to point layout of his. (I also could not easily procure the now ancient radioshack proto board- yes, the one shaped like a crab) 🙂
opting for a desktop form-factor and 12 volts, my little amp works fine for easy to drive headphones.... But does it actually sound better than, say, an iphone?
The truth is, while it gets much louder than a phone, the OPA jfet op amp powering the whole thing is simply NOT high end. it will distort with too much input (after a few clicks on a phone before the phone is maxed out for reference- well below standard line/"dac" level in both home and pro audio voltage wise) and will incrementally break up with hard/loud songs on bigger headphones, as the volume and rocking out goes up- in my case, the current fostex RP evolution I have been mangling is the big cheese candidate. the planar drivers in the RP series are definitely "very hard" to drive in the scheme of things, but anything "heavy weight" will simply not do when pushed to an accessible limit. I CAN use smaller dynamic/efficient headphones with ease, such as my ported and open hifiman edition S without experiencing distortion. Still here though, it is clear this amp is a "diy/cheap access to power"- slated against the fiio e12 for example, the CMOY seems a bit.... Loose? Not any better, that is fore sure! (granted, the e12 is a fantastic budget amp)
Note of op amps: the NTE and other non-OPA brand-name look alikes and electrical analogues sound terrible. I was fussing around with the NTE variants after needing the short out resistors "R5" (very important when building: do not use R5) when they actually blew out from my using a inversely-poled power supply by accident, and was getting frustrated as the amp was working but sounded like a making TIDAL streaming gurgle out from a cheese grater. eventually, after a ebay shipment of brand name OPAs came, I popped one in cautiously and it turned out the opamp was the source of the.... cheese grating.
In all, a good project for skill building but falls short of anything "hifi".