[sdiy] Prophet 10 op-amp swaps?

[Dave Manley]

There's an inverse relationship:

           Freq = (1/period)

So 2us = 500kHz.

Think about an LFO at 2Hz.  You get two cycles per second.  So each cycle has to be half a second.  Make sense?

Dave (is it key-lo-Hertz or kill-a-Hertz) Manley

Eric Frampton <eric at ericframpton.com> wrote:
>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
>
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