[sdiy] Prophet 10 op-amp swaps?

[cheater00 .]

Hi Eric,
Great that you've got a scope. Use it and check there aren't any
oscillations happening on the output of the op amps while you've got
no sound playing. Use an 1 MOhm input and short cables. By setting the
timebase right you should be able to roughly count the frequency of
whatever wave encounter. Is it a DSO or CRO? What bandwidth? With a
CRO, anything above 20 MHz is great. With a DSO, you are looking for
at least double that.

One thing you might do is build an active, non-resonant, 2nd-order
high-pass filter fixed at 30 kHz. If without nothing playing you still
see something on the scope, you are having an issue. Just make sure
it's not the high-pass itself.

Alternatively: if you have a VU meter, plug in the synth and the
bottom most led lights up, but you hear nothing (so it's not noise),
then that might well be it. Start checking with a scope as described
above. However, electronics after the oscillating op amp might
actually reject the oscillation from the signal, therefore there's a
lot of ways for this test to give you a false negative (i.e. it seems
like everything is OK). And something else might be happening to the
VU so it might be a false positive too. But it's one typical symptom.

Cheers,
D.

On Wed, Jul 31, 2013 at 5:42 PM, Eric Frampton <eric at ericframpton.com> wrote:
> Parade: rained on.  :-(
>
> I don't know how to check for that sort of thing. I've got a scope and a DMM
> and the keyboard itself…how might I go about testing for this?
>
> e
>
> On Jul 31, 2013, at 3:25 AM, cheater00 . <cheater00 at gmail.com> wrote:
>
> Bear in mind when doing op amp swaps that new op amps have GHz bandwidths,
> whereas the parts you are replacing might have little more than audio bw.
> Often, such swaps result in sound that feels better, but in fact is only
> *different*. The difference is in those cases oscillation in the MHz range.
> This issue is often hidden by other improvements such as headroom,
> distortion, or highs and lows extension, but it is not something one should
> ignore. Consider checking your mods for that.
>
> Cheers,
> D.
>
>