DIY Headphone Amp - part 2

[Dave85]

Thanks bud will take a look at those kits

 

[Rroff]

Do not use the O2 with the ODAC at high (6.5x) gain, unless you want to control the volume at the PC. The full scale output voltage of the ODAC will definitely cause clipping in the input stage of the O2 at high gain, regardless of the what wall wart you use. The default 2.5x low gain should be enough, and will not clip.

This is true at any setting of their volume controls. The O2 was designed with the volume control “in the middle” for maximum performance (as explained in the O2 Design article) but this means some care may be required to not overload the input stage at gains above the default 2.5X. If you’re going to change the O2’s gain or use the high gain mode, be aware of the following:

Was thinking of persisting a bit with the O2 like design as when it works it sounds fantastic but looks like using my favourite op amps in that design the 8066 and 2228 which both require a minimum of 5 (realistically 6) gain to be stable is largely going to cause issues whereas the other design I'm using can handle input levels way above anything a consumer device can throw at it - I had some awareness of that before but thought I'd give it a try regardless to see how it worked out.

There is a nasty hack for this replacing the 100-270R input resistors from the input RF/EMI filter with ultra low noise 10K resistors (and swapping the 220pf cap there for a lower value to keep the filter working as intended) but that kind of defeats some of the reasons for the design being like it is to reduce the effects of resistor noise.