It’s been awhile since I’ve had an update on this project, I actually completed it, but never got around to documenting everything.
Drew up a schematic and layed out a board, JLCPCB fabricated it with no issues. I used some existing board images that AGDR made available, so mechanically it’ll fit and clear whatever the O2 has existing on the board.
A set of 2 mil-max pin headers are used to interface to the existing opamp sockets, and then a couple of jumpers are required to tap onto the existing O2 board (ground, signal ground and lastly for the battery voltage comparator).
Each channel again is a bunch of OPA1688s paralleled (see previous post here for details) The Danger Zone: O2 Headamp output board, OPA1688s Part I (fillwithcoolblogname.blogspot.com)
Here it’s partially assembled, I use liquid flux (that’s kinda messy). Everything gets dunked in my ultrasonic cleaner afterwards though so I’m not worried.
View of the rear, there’s an output relay that latches and prevents that annoying O2 issue of dropping in and out when battery levels get low and creates that popping noise (which is probably not good for your headphones).
From the front assembled, here the signal ground jumper is visible.
Top assembled, here the comparator jumper is visible.
Last jumper for power ground, straight to the battery holder.
Offset voltages measure pretty good.
I stacked a whole bunch of capacitors on my breadboard to do some random capacitive loading tests (with a ~33ohm) load, it performed admirably (I can’t exactly remember the numbers, I think it was 2nF or so load capacitance with 33Ohms, typical headphone cables are maybe 30pF per foot, so this is plenty stable).
Finally this is it all assembled and cleaned up.
I haven’t gotten around to putting the schematics and board diagrams anywhere. If somebody is interested I’ll get off my ass and do it. Myself I’m fairly interested in what this measures properly in terms of THD+N, but I don’t have access to any cool equipment to do it.
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