[sdiy] Prophet 10 op-amp swaps?

Eric Frampton eric at ericframpton.com
Wed Jul 31 20:58:17 CEST 2013

D -

I've now swapped out all the 356's except for one in the Lower EQ out.

I've got a CRT scope, 30MHz.

Sure 'nuf, you called it. There's a super-high-frequency oscillation happening at the Upper EQ out that isn't happening at the Lower one. You can't hear it, and it doesn't register on VU meters, but you can see it on the 'scope. It seems to only be happening at that one stage; the final outputs do amplify it (no rejection, darn!), but they don't appear to be adding any problems of their own.

So, aside from building an active filter circuit (which I'd rather not get into), might this be solved by a well-placed bypass/filter cap, or maybe a not-quite-as-fancy op-amp (though I don't want to drop that 356 back in there)?


On Jul 31, 2013, at 1:12 PM, cheater00 . <cheater00 at gmail.com> wrote:

> 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.

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