Yeah, the pinout is kind of suck. I prefer duals, like LM358.
Or since I no longer give a crap about cents of component cost, I reach for TLV2372s now, for breadboarding.
I don't agree, there is circuitry common to all four amplifiers. If one amplifier's inputs are floating and it's HF oscillating, I have seen other amplifier channels get affected.
Depends who made it. Different versions have independent biasing. Anyway, that should only be a problem if the inputs fall too high or too low. Which, open circuited should float too high if I'm not mistaken; unused inputs should be grounded for this reason.
On a breadboard there is more common circuitry than that. Wire is a 1nH inductor for every mm of its length. So a 10cm wire in the PSU rail connection plus a 10nF decoupling capacitor will resonate at what frequency?
A good reason to include a nice lossy electrolytic capacitor somewhere in the supply. 100nH and 10nF is a few ohms resonant impedance, well damped by a capacitor of the same ESR. If there's no smaller impedance directly in parallel with the capacitor, then its ESR will dominate and this damping will apply to any series LC branching off it (give or take the total parallel equivalent of them all, obviously). Or, place the electrolytic at the end point (i.e., in parallel with the 10nF, reasonably nearby), so there's effectively a big resistor in parallel with the tank, damping it all the same.
An LM324 doesn't have much impedance, or need much around it; just a few uF electrolytic alone is adequate.
An LM324 is much too slow to oscillate at HF
HF starts at 3MHz or so, near the GBW of the amp. The full loop (input to output) is pretty unlikely to run that hot, but a capacitive load on the output might induce that section to oscillate up there.
Tim