[sdiy] Prophet 10 op-amp swaps?

[Eric Frampton]

Hi Dave -

The op-amp(s) in question are U820 and U821, used for the Upper and Lower EQ's.

I've installed sockets for all of these, yep. I figured, worst case, I could always put the original op-amps back in if this experiment didn't work, or if someday I had to sell the keyboard and someone else wanted it completely stock.

e

On Jul 31, 2013, at 5:10 PM, Dave Manley <dlmanley at sonic.net> wrote:

> Hmmm, 'unsolicited agression' with a simple question.  Who knew.  Call the
> UN.  Let me help clarify with an example: "The opamp may not be the only
> thing having stability issues".  There.  That is an example of aggression.
> See the difference?  I accept you apology.
> 
> The reason why I asked about the 'GHz' bandwidth is because I didn't see
> anything in the mentioned opamps that were anywhere near that and it
> seemed like hyperbole.  I like to review datasheets before posting, and
> did so.  I also missed Eric's mention of the LME49710.  Which does have a
> much higher GBW than any of the other mentioned chips.  Not surprisingly,
> Eric reports the problem only occurs with the LME49710 - the LF356 and
> TL07x parts don't.
> 
> 1. Damian's suggestion he found googling to add decoupling isn't bad, it
> is worth a try.
> 
> 2. The suggestion about measuring the DC output voltage of the opamp
> should not be an issue.  The author was talking about opamps with high
> input bias currents, leading to input offset voltage, thereby causing a DC
> output, is not an issue with the LME49710 - it is measured in nA and it
> also has low input offset voltage.  A review of the datasheet shows this.
> 
> 3. Some of the other suggestions are nonsense if he had reviewed the
> Prophet-10 schematics.  Do not ground your scope probe on the 'negative
> supply of the opamp' - that will create a dead short to the -15V rail. He
> adds a warning, but a little research and less >>>typing<<< on his part
> would have eliminated this suggestion.
> 
> 4. He suggests putting a cap in 'series with the feedback resistor'.  If
> the opamp in question is the top opamp (U824) in the schematic, that opamp
> doesn't have a feed back resistor.  The opamp is connected as a voltage
> follower with the output directly connected to the negative input.  The
> usual method of reducing the bandwidth is to put a small cap (10's of
> picofarads) in parallel with the feedback resistor.
> 
> A question for Eric - are the opamps socketed?  Putting an opamp like the
> 49710 with a GBW of 55MHz (high but still ~1/20 of a GHz!!!) in a socket
> is asking for issues with parasitics.  If socketed: as Damian, will warn
> you - if you decide to remove the sockets take care not to damage the pcb
> traces.
> 
> If you don't remove the sockets, then Brian's suggestion is probably best.
> Look for a newer 'audio' grade opamp with a lower GBW than the 49710's
> 55MHz.
> 
> HTH,
> Dave
> 
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