Best sounding op-amp

The little differential small signal amplifiers, packaged as integrated circuits, and commonly called "Op Amps" are called that because they are not often designed or used as linear amplification devices. The important characteristics of an op amp are very (absurdly) high open loop gain and very high slew rate. Linearity is of no importance. The op amp drives its output so as to match its input and follow changes of the input. The "signal" that drives the op amp is not the signal that you feed into the circuit. The signal that drives the op amp is the difference between the op amp output and what you are feeding in. The high gain makes sure that this difference is driven down to zero, and the high slew rate makes sure that rapid changes of the input can be followed closely. The characteristics of the op amp circuit are overwhelmingly determined by the design of the feedback loop within which the op amp works, and the electrical components of this loop. In most cases substitution of an op amp with superior specs will have no effect on how the circuit performs.

As always, there are some exceptions. Soime op amp ICs are more noisy than others. In some high gain applications, such as a phono preamp, the op amp may play a significant part in overall circuit performance. (The practical solution to this problem is not to use op amps, even $50 items, in high gain phono preamp stages).

So, don't waste money replacing all the op amps, when few, if any, will have any effect. Make a review of the circuit before you "upgrade" op amps.

Well, I've 'rolled' the opamps in my headphone amplifier, and came to the following conclusion after a few days to a week with each one:

OPA2134 - flat and lacking soundstage, but decent overall since it didn't do anything wrong in particular, just failed to come through in those areas.
OPA2227 - similar, but warmer overall, slightly larger soundstage
OPA2604 - more detailed than either of the two but slightly cold sounding. Very wide soundstage though.
AD823 - punchy bass, but so detailed in the highs it sounds grainy. Constricted soundstage at times.
AD8066 - comparitively warm and lush, and not as detailed as the 823, but still very good. However had distortion when the input or volume as above a certain leve, like there was a 'step' in the volume knob - I don't think the voltage supply was correct. Upon going back to it the second time it let out a faint stream of smoke and was dead, maybe I plugged it in wrong though...
OPA627 - had to wire pair of these to work as a dual-channel, but by far the cleanest, richest sounding of them all, with the largest soundstage. Details are all there, but not offensive. It's staying.

Another one I may try: AD825.. and of course the AD8620 - I've heard some prefer it to the OP627.

Hi, I'm having Brice @ Sound Odyssey(phone717-846-6043 or e-mail ads on Audiogon) put the warm sounding AD825 op amps in my cdp(they just snap in)along with the (9) upgraded diodes, Cardas RCA's and damping sheet. He is very reasonably priced for the service and value minded. I'll update when I get the player back. Jerry

Op-amps do vary a lot in sound, particularly because of the power delivery on-die and on-package. Some parts are power starved from the get-go and need a lot of decoupling capacitance very close. Others are designed well for power delivery and need very little decoupling cap. These tend to be more dynamic and clean-sounding. OPA627 is my personal favorite. Packaging also has a big effect.