[sdiy] Prophet 10 op-amp swaps?

[Eric Frampton]

So…

After reading the posts and talking with some knowledgeable folks, I did finally replace those last two op-amps (U820 and U821, the Upper and Lower EQ summers) with 49710's as well. The magic bullet was (similar to what Tony suggested) putting 100pF ceramic caps across pins 2 and 6 of both of them, which diminished the oscillation down to the idle noise level (i.e. practically zero).

While sonically I can't tell any difference in the final audio with or without caps, I can tell that the output op-amps, while still slightly warm, are running quite a bit cooler now.

Thanks again to everybody for the many valuable comments. I feel like I just got a mini-education in op-amps!

e

On Aug 1, 2013, at 6:35 AM, Tom Wiltshire <tom at electricdruid.net> 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