[sdiy] Prophet 10 op-amp swaps?

[rburnett at richieburnett.co.uk]

It's not really a good idea to just swap op-amps willy-nilly in a 
circuit without comparing their specs carefully.  You can get problems 
when replacing an old slow beast with a modern high-performance op-amp 
that is much faster.  The most likely cause of the oscillation is that 
the new high-performance op-amp is either not closed-loop stable for 
gains of unity, or at least has a low phase-margin when wired as a unity 
gain buffer.  If a high-speed op-amp has marginal stability when wired 
as a unity gain buffer then it will not take much capacitive loading at 
the output to push it into full blown RF oscillation.  I haven't had the 
time to pull the op-amp datasheet or plough through the lengthy P10 
service manual, but those are the things that I would be looking for.

If the oscillating op-amp is wired for unity gain then I'd simply swap 
it for one that is closed-loop stable for unity gain.  Or if it's 
stability is marginal I would try adding a 100 ohm resistor between the 
op-amp output and whatever it drives.  This will help decoupling the 
capacitance of any cable or whatever follows from loading the op-amp and 
pushing it over the edge!  The "few pico-farads across the feedback 
resistor" will not help if the problem is poor phase margin when wired 
for unity gain.

-Richie,



On 2013-08-01 11:35, Tom Wiltshire wrote:
> OPA134 has a Gain-bandwidth product of 8MHz, so that might explain
> why it works fine.
> 
> It looks like putting amps that are *too* fast in this circuit isn't
> the best idea since it doesn't include compensation we might consider
> standard, but that they didn't need for the older op-amps they used
> originally.
> 
> T.
> 
> 
> On 1 Aug 2013, at 09:01, Oakley Sound <oakleylist at btinternet.com> 
> wrote:
> 
>> I did my Prophet 10 swap out with OPA134. I typically check for 
>> things like oscillations, noise and offset problems. No such things 
>> existed with the OPA134 - although I could have been lucky with a 
>> marginal layout.
>> 
>> You could try an extra 33pF capacitor across pins 2 and 6 of each EQ 
>> op-amp.
>> 
>> Tony
>> 
>> http://takla-makan.bandcamp.com/
>> http://www.oakleysound.com
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