> Where is the opamp output connected ?Yeah, I had a "???" moment too, even though I have seen this trick before.
The most common op-amps can be broken into three stages:
1) differential input
2) high voltage gain stage ("Vas")
3) current gain output stage
For easy use with feedback, we must make one stage much slower than the other two. In the LM301 you connect a pFd "compensation" capacitor across two pins. In the LM741 this cap is internal. The dual NE5532 has caps built in. The single NE5534 has small internal cap plus pins to add an external cap.
The red dots in the plan above are the compensation pins.
Note that one of them is also the point between the voltage-gain stage and the current-gain stage.
The actual 5534 plan is a lot messier than the one above. And because it is an old design, from days when integrated PNP transistors had poor performance, the 5534 uses a very tricky current-gain output stage which acts like a complementary emitter follower, however all the hard work is done with NPN transistors.
While tricky, the 5534 output stage really IS good. However since it is on a small die with a lot of other parts, including quite large input transistors, it is limited to 30mA-40mA maximum. It was intended to drive 600 ohms very well. It will drive lower loads, but 32 ohms is a real strain, and very little power (25mW) is possible in 32 ohms.
What 00940 and folks at DIYaudio have suggested is: keep the excellent input and volt-amp stage, but attach a "better" output stage. Buying discrete, we can get PNP as good as NPN, we can get devices larger than Rupert cared to squeeze into the chip, and high load currents in exteranl devices do not heat the input devices and cause bass distortion.
00940 shows a very ordinary complementary emitter follower, using high Hfe transistors. It will drive over 100mA, even 200mA.
The internal output stage is still connected, but drives nothing, and does no harm.
My only comment is that it isn't utterly short-proof. There is no separate protection for the output devices, and on paper I can find a situation which will exceed their rated dissipation. But heck, I've run a lot of crude speaker amps with no protection and much higher worst-case over-spec. Worst-case rarely happens. And headphones get shorted a lot less often than bare-end loudspeaker wires. And it IS a DIY project.... if you can build it, you can re-build it.
posted 03-20-2009 08:56 PM CST (US) 
