[sdiy] Prophet 10 op-amp swaps?

[Eric Frampton]

Hi all -

After way too long I bought a tube of OPA124's, and tonight I finally found the time to A/B/C those with the original LF356's and the LM49710's.

Now, to further show my ignorant ass in front of a bunch of brilliant EE's, I thought I'd post one last note in this thread.

So,

Although I understand the argument that the LM49710's have such a stupidly wide bandwidth that they could go unstable, I've yet to see or hear that actually happen in all my banging around on the P-10. That said, I certainly don't have the test equipment to formally measure into the GHz range where strange stuff like that might occur. All I know is, with either the 49710's or the OPA134's dropped in there, the veil totally comes off of that machine. To me, it finally becomes inspiring to play - which is all I was after when I began this damned exercise anyway!

In the final 6 output op-amps (the ones directly driving the balanced upper/lower/mono outputs on the back of the unit), I can't see or hear any PRACTICAL difference between the 134's and the 49710's, though I grant that there's likely an electronic one that I'm oblivious to. The very small (100mV P-P) waveform I was describing before is, as best I can figure, a noisy but periodic wave from the switching power supply on my halogen bench lights coupled (perhaps) with the motor noise from my dehumidifier.

I only learned that after discovering what a proper op-amp oscillation looked like tonight:

Dropping an OPA134 in the output of either EQ section presented a crystal-clear sine wave up in the 2MHz range (a 2uS period is 2MHz, right? I failed math!) at, like, a 10x greater amplitude as the noisy wave described above. Whether the chip was socketed or not made no difference, I tried both. When going back to the 49710 with a cap strapped across the top (now that I know what a proper oscillation looks like) I can say that that oscillation completely disappeared. I'm sure the same thing would happen with a cap across a '134, but (frankly) the other works, so until something blows up I'm going with it.

Thanks again, everybody, for all your help. I'm going to go back to making music now.

Eric