[sdiy] Prophet 10 op-amp swaps?

rsdio at sounds.wa.com rsdio at sounds.wa.com
Wed Jul 31 21:20:25 CEST 2013

Depending upon how the op-amp is configured / wired, you can usually  
get rid of oscillation with a small capacitor in the feedback loop.  
However, I don't know whether there's a convenient resistor that you  
could strap a capacitor across in parallel. In some configurations,  
you wouldn't be able to add a compensating cap without cutting a  
trace, and I wouldn't cut a trace or otherwise alter a PCB for a  
different op-amp.

You could keep searching for an op-amp with less bandwidth. I  
recently designed a 32-channel DAC for ultrasonic frequencies, and I  
had to keep trying and testing multiple op-amps until I got one with  
enough bandwidth. Seems easy enough to go in the reverse direction  
and select for lower bandwidth.

... if there isn't an easy way to just add a cap in the feedback.

Brian Willoughby
Sound Consulting

On Jul 31, 2013, at 11:58, Eric Frampton wrote:
> 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)?
> e
> 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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