Thinking about paralleled opamps for a headphone output stage as I was, I suddenly remembered the Douglas Self power amplifier I have seen, which uses paralleled NE5532s. That was it, I had to draw up a few PCBs for a headphone amp.
Performance should be extremely good in theory. I can't simulate it for want of accurate models. If the PCB implementation is satisfactory defects should be well below the threshold of audibility. Components are all readily available and no exotic (or expensive) technology is involved. Since it's not intended for portable operation no battery failure protection is required, the only thing that might prove necessary is to add relay-switching of the output if there proves to be a large transient at switch-on or -off. The output will tolerate short-circuit indefinitely.
Here's the PCB layout:-
It uses 8 chips or 16 opamps per channel for the driver stage. I got 20 for $6 US, shipped, from Thailand. The datasheet makes reference to driving 600 ohms, 600/16 is 37.5, so this should cope with a 32 ohm load reasonably well. Power supply can be up to +/- 17V. This can be pushed to +/- 22V if LME49860s are subbed in for the gain and DC servo opamps. @ 17V it should drive pretty much any phones. Output short circuit current is ~600mA.
Each output feeds through a 1R resistor to discourage overloading of any particular amp and the final output is taken through a handwound 2u4 (Henry) inductor. The inductor is wound on a former made from a Bic Biro, which is mostly air and a little polystyrene. The DCR is about 0R035, the wire being 27 turns of 0.5 mm, about 8mm dia. and 600 thou long. The inductor is paralleled with a 10R resistor to kill its Q and should restrain any tendency to instability.
I have chosen to use an LM4562 for the gain stage. There is no local feedback on this as per the design in the LME49600 datasheet which uses LME49720s in the gain stage, these are comparable to the 4562. Global NFB limits the overall gain to ~4.4.
The input is AC coupled using PPS caps, despite this a robust DC servo is implemented using a DC precision OPA2277 integrator in conjunction with the same value PPS caps as are used at the input. This helps to keep component ordering simple. A low pass RC filter excludes RF.
The board was laid out initially as single-sided with links, hence the unusual grouping of vias, but making it double-sided greatly improves the grounding, so I may choose this for the second build. The DC distribution is nevertheless surprisingly satisfactory, the main concern is around the proximity of the tracks joining the 5532s inputs and those joining the outputs. Since these are quite numerous the tracks will necessarily be quite long, but the gain of this stage is <1. Most PCBs have some compromises, hopefully this will not degrade performance significantly, but of course that remains to be seen.
I'll build the first one socketed to ease debugging and recovery of the components. I haven't etched a board yet, I like to check the fit of all the components before I do that.
Here's the schematic:-
Obviously the values are difficult to read, but the general arrangement can be seen, and I will make detailed version and gerbers available on my website (http://wakibaki.com/techno.php#t6) if interest is shown.
A mezzanine board supplies power using an AC adapter, or optionally 2 DC adapters, I have a collection of these built up over a few years.
The main board is 4.75 inches square, I may not bother with finding an enclosure though, these things are more of a curiosity with me, and I have yet another mains/LiPo powered portable amp for which I already have a box in the pipeline. It's basically this amp with an AD8397 output stage in SMT + battery charging and protection features.