Question about diodes across opamp inputs.

[ayayay!]

Okay so I was looking at the schematic for the Boss CS-3 and was wondering aloud about diodes D2 & D3.  So follow me here:  In this thread http://www.diystompboxes.com/smfforum/index.php?topic=66485.0 we discussed at length the purpose of the output opamp of the BD-2 and it's associated diodes at the inputs.  I wish to discuss the form and function of the CS-3's D2 & D3 across the inputs of IC1a.

The Boss CS-3 has something quite similar, at least at first glance to me, as the BD-2 on that opamp.  So after all that, here's my simple question:  Are those diodes (D2 & D3) basically doing the same thing?  For the most part just protecting the opamp?  Thx!

[Ronsonic]

That's an interesting one. I look at those diodes as being between the noninverting and inverting inputs. So any signal above the diode drop becomes common and canceled and thereby limited.

Usual input protection runs diodes to the voltage rails to ensure that signal is always less than rail voltage to prevent latch up. This is different. I'd think of it as diode limiting rather than protection.

Sure don't want to hit this with too hard a signal whether turned on or not.

[R.G.]

Geofex - When Good Opamps Go Bad

[ayayay!]

Sure don't want to hit this with too hard a signal whether turned on or not.

Yeah Ronsonic that sums up what Jay Doyle said in that thread I posted above.  I think I'm sticking with simple protection for this one.  

I see you posted while I was typing R.G.  Heading to geo now...

[JDoyle]

It may be just me, but I don't see the engineers at Boss/Roland being the type to decide that they wanted to add a subtle bit of distortion to a chorus circuit.

This one is for protection for sure.

Regards,

Jay Doyle

[George Giblet]

I thought they used this method make the buffer stage sound better under overload.

The buffer output has a swing smaller than the supply rail.  Under normal conditions the output follows the input as an ideal buffer should.  As the input is increases the output stage starts to saturate and the output can't track the input when this occurs the diodes conduct.  Normally the internal stages hard saturate under clipping however the diodes prevent the input from being driven beyond +/- 0.6V.  As a bonus some current can dump through to the output which give a sort overload.  The diodes also prevent the input from excessive differential voltages - however some opamps cope with this anyway

If you look at the CH-1 it only uses one diode presumably to stretch out the more limited positive swing:
http://www.freeinfosociety.com/electronics/schemview.php?id=2290

If you look at IC1B of the BD-2 there is no chance of protection issues here the two diodes are to stop saturation alone:
http://www.freeinfosociety.com/electronics/schemview.php?id=116

[darron]

and geofex "What are all those parts for??" http://www.geofex.com/circuits/what_are_all_those_parts_for.htm