[sdiy] Prophet 10 op-amp swaps?

[Dave Manley]

I'm looking at page 27 of the PDF which has the upper and lower eq, but not U820 or U821?

Are we looking at different schematics?  The ones from cem3374.com?

Eric Frampton <eric at ericframpton.com> wrote:
>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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