I have some knowledge of the mechanics arround how op-amp works but never applied it to audio, i designed a prototype but I’d like to get feedback from people who have done it before to improve the circuit.
PS R6 is a dummy load to reppresent a headphone;
PPS I’m not sure how to scale R10/11 so that it can be used with lower impedance headphones too
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From my limited experience with analogue circuits (only recently started to tinker with analogue stuff), you want to make sure that your U1 is fine with the resistance for R4 (and indirectly R5). Often the spec sheet has a note about feedback resistor choices somewhere.
You got C1 in there to block DC but no provision to shut down on DC (or block DC) before the headphone. Any specific reason for that?
More of a question, but why not pull the feedback for U3 and U4 after R10/R11 ?
You should try reading the papers Burson and Sparkos have on their op-amps to maybe try to get a better idea…
Also, I’m sure if you email them they’d be happy to share some knowledge…
I was looking to use OPA4134 for this project as it is just a prototype to test the feasibility, the gain resistors as long as the value is less than 2k I should be all right.
C1 Is not so much for DC but more to filter high frequency distortion from U1 that is where all the gain is basically, U2/3/4 is just a huge buffer with very little gain
For the output stage, I had great success with NJM 4580’s in overly simplistic (but very compact) settings and am currently working on a headphone Amp based on two of those per channel (thread soon).
Input/Gain stage, the 4134 looks good.