Please clarify an LM741 mystery for me

[Mark Hammer]

I bought an older Morley wah/volume a week ago, and the thing works fine, except for the hissiness of the wah.  Seeing that it used a 741, I figured a simle op-amp replacement would cure that problem, and sure enough it did.  I think the 5534 is a little quieter than the TL071, but I may switch back.

In any event, it is the identical circuit to this one: http://www.morleypedals.com/bwaes.pdf

What I can't for the life of me figure out is why pin 8 on the 741 is tied to pin 2 via a PCB trace, particularly when the datasheet for the 741 says "No connection".  If it was soldered to the board just to secure or physically stabilize the chip, why pin 2?

(NOte: I made sure to lift pin 8 when subbing the TL071 and 5534)

[waltk]

Just making a wild guess...  Is it possible that the original designer had actually intended a different opamp - maybe a predecessor of the 741?  For example, the LM(uA)709 had the same pinout as the 741, except that pin 8 was an additional frequency compensation pin.  Depending on the age of the thing, maybe the 741 wasn't yet available, or was just becoming available when it was designed.

[Mark Hammer]

A decent wild guess, but this dates from the mid-80's so there were plenty of chip choices.  The chip was soldered directly to the board, not socketed.  And rummaging through the backlog of Morley wah and other pedal circuits, a lot of them used 741 op-amps.  So, a decent wild guess, but you'd need to present a lot more to convince me.

[artifus]

709's! *has sexy star trek related thoughts* haven't thought of those since i tried to track one down for a crackle box years ago. never did find one.

[pinkjimiphoton]

most likely it's tied the - input to it to prevent oscillation maybe?

[Steve Mavronis]

Beats me why they did that. The only place I've found that mentions using pin 8 on the LM741 at all (besides as a "tie" point to simplify a layout) is on this webpage under figure 2:

http://www.cs.wright.edu/~phe/EGR199/Lab_2/

"Pin 8 is not connected (NC) to the op-amp electrically, but can be used as a heat sink."

More... the "74l" is a typo below

http://www.ski.org/Rehab/sktf/vol04no2Spring1983.html

"The compensation capacitor comes built-in for most op-amps, including the 741. Others, called uncompensated op-amps, require an external capacitor, usually around 30 picoFarads. These units are versatile in that different compensations may be used to increase or decrease bandwidth to match particular applications. The uncompensated 74l is a 748; the external capacitor goes between pins l and 8."

LM748 vs. LM741 info from a pro audio forum:

http://mcirecording.com/forum/index.php?topic=325.0

[StephenGiles]

There is a video on youtube of Ritchie Blackmore talking about some of his guitars. He pointed out a piece of metal fixed to one of his Strats, which he said did absolutely nothing, other than to wind people up!!

Could this be just that??

[Paul Marossy]

What I can't for the life of me figure out is why pin 8 on the 741 is tied to pin 2 via a PCB trace, particularly when the datasheet for the 741 says "No connection".  If it was soldered to the board just to secure or physically stabilize the chip, why pin 2?

Maybe the PCB was designed around an LM709 (probably because they were dirt cheap and maybe they got a large quantity) and that was never addressed when they switched over to the LM741? So they just left it alone because it doesn't hurt anything. Pin 2 has no connection, so it's kind of obvious that it can't really serve any purpose.

[Mark Hammer]

I'm beginning to think that Steve Mavronis may have solved the mystery.  In this thread - http://www.diystompboxes.com/smfforum/index.php?topic=98057.0 - you can see that the PCB has the components soldered on the copper side.  I put in some machined sockets to try out different op-amps for noise purposes, but the original 741 was soldered directly to the board, such that the assembler would be pressing up the soldering tip right against the entire IC pin.  If pin 8 doesn't connect to anything inside but is able to sink away heat, AND the layout requires some components on the board to find their way to the other side of the chip and pin 2, then maybe it was intended merely to draw heat away while soldering the other pins.

[waltk]

Sometimes I wonder who designed things, and where they are now.  This pedal isn't THAT old.  Somewhere out there, there's a guy who was sitting at a desk/workbench one day making the layout for this PCB.  Wouldn't it be cool to know who it was, and just ask him what he had in mind?  It's too bad that the originators of most of this stuff are anonymous.

[Mark Hammer]

Absolutely.  I love finding out about the thought processes of people who come up with things.  Personally, I don't read fiction of any kind anymore.  But we have a weekly radio show here where the host interviews writers, and finding out how they come up with stuff, and solve the challenges that arise with writing, provides much greater pleasure for me than actually reading the stuff itself.

[Paul Marossy]

If pin 8 doesn't connect to anything inside but is able to sink away heat, AND the layout requires some components on the board to find their way to the other side of the chip and pin 2, then maybe it was intended merely to draw heat away while soldering the other pins.

That doesn't make sense to me. That means the only pin on the opamp that they cared about was pin 2. Doesn't add up for me...

Unless you are saying that all the heat from soldering is supposed to go to pin 8, but I don't see that happening either. The whole opamp is going to get hot with the heat being distributed evenly throughout the whole body of the thing. Why would it all go to pin 8?

[Mark Hammer]

It may be more a thing of "Well, I/we need to get a trace from those components over there to this pin over here.  Do I really need to make the trace go around pin 8 to get to pin 2?  I/we need to solder the damn IC to something, so if it has no electronic consequence, may as well just make the solder pad for pin 8 part of the trace that goes to pin 2."

[Perrow]

In any circuit I design a PCB for from now on, I'll make sure to tie leg 8 of any 741 to a randomly selected trace

[Paul Marossy]

It may be more a thing of "Well, I/we need to get a trace from those components over there to this pin over here.  Do I really need to make the trace go around pin 8 to get to pin 2?  I/we need to solder the damn IC to something, so if it has no electronic consequence, may as well just make the solder pad for pin 8 part of the trace that goes to pin 2."

Put that way,it makes sense. But in the case of your Morley wah, is that is what's happening?

[Mark Hammer]

Put that way,it makes sense. But in the case of your Morley wah, is that is what's happening?

Pretty much.  As the schematic shows ( http://www.morleypedals.com/bwaes.pdf ), it's a single op-amp tunable bandpass, so the feedback loop  is between pin 6 on one side of the chip and pin 2 on the other.  They placed the components for the feedback loop on the 5-8 side, so a trace needed to find its way back to pin 2, and the "highway" goes straight through pin 8 territory, rather than around it.

[Paul Marossy]

Put that way,it makes sense. But in the case of your Morley wah, is that is what's happening?

Pretty much.  As the schematic shows ( http://www.morleypedals.com/bwaes.pdf ), it's a single op-amp tunable bandpass, so the feedback loop  is between pin 6 on one side of the chip and pin 2 on the other.  They placed the components for the feedback loop on the 5-8 side, so a trace needed to find its way back to pin 2, and the "highway" goes straight through pin 8 territory, rather than around it.

Well, that makes sense in some ways. That way they secure pin 8 and since it's not being used by the IC chip, it just becomes a "node" on the PCB for other stuff to connect to.