Opamp mystery

[WildMountain]

Hey guys, I really need some help here. I've started on my very first project (woohoo) but I'm already at a loss.
I'm trying to build the MXR distortion (schematic from tonepad.com). When I read the schematic for the Opamp wiring (4558 in this case) I can see what to do with pin nr 4,5,6,7 and 8, but what do you do with nrs 1,2 and 3? It doesn't say so in the schematic, so it's probably a general rule that I haven't heard of yet. To me this is all a big mystery, to you it is probably nothing new.
Thanks a lot, from an excited first time pedal builder!

[Paul Marossy]

That circuit uses a single opamp. Pins 1, 5 & 8 are not used, and therefore, not connected to anything. Good luck with your first build!  

[WildMountain]

Thanks for a VERY quick response
But, now I'm even more at a loss. My schematic tells me to actually DO connect pin 5 and 8, so I assumed they are being used. So what about nrs 2 and 3? :?
And, why would someone use a dual opamp as a single opamp?

[Paul Marossy]

The Tonepad schematic I'm looking at shows only Pins 2, 3, 4, 6 & 7 as being used. Apparently, your layout/schematic is using a dual opamp. Numbers 1, 2 & 3 would not be connected to anything in this case.

There's no reason why you can't use just one opamp of a dual opamp. As for why someone would do that, sometimes it's easier to get a dual opamp of a certain series than it is to get the single opamp version.

[Aharon]

Sometimes unused (inputs?) pins are tied to ground to avoid oscillations.
Aharon

[WildMountain]

Here's the schematic I'm using:
http://www.tonepad.com/getFile.asp?id=6
From the PCB it seems that 1 and 2 are connected to each other and that 3 is connected to: R2, R4, R5, R6 and C3? If that's correct, why can't I read that from the schematic? (probably just ignorance :oops: )
Thanks again.

[Paul Marossy]

Oh, sorry, I was looking at the wrong schematic.  :oops:
Hmm... I am not sure why Pin 3 is connected the way it is - it's tied to the 4.5V supply. You interpreted the schematic correctly AFAIK.

Don't have an answer for you on that one.  :?

[WildMountain]

In general: Are the pins that are not mentioned in a schematic tied to ground, or should they be left alone?
It must be me, because I'm not getting this IC wiring either:
http://www.runoffgroove.com/tubereamer.html
(tiny voice): help :cry:
What did I get myself into?

[Paul Marossy]

In general: Are the pins that are not mentioned in a schematic tied to ground, or should they be left alone?

Not usually, but on something that uses a hex inverter chip (like a 3-Legged Dog), all of the unused pins are grounded.

On the Tube Reamer layout, that is a single opamp, and the unused pins are simply not connected to anything.

Don't worry, it's part of the learning process. BTW, I believe some of this might be in the FAQ section of this site.  

[WildMountain]

I'll look into the FAQ (again). Thanks.

[Aharon]

http://www.maxim-ic.com/appnotes.cfm/appnote_number/1957

[Aharon]

Oops
Aharon

[Paul Marossy]

Thanks for the link Aharon, that's interesting stuff that I didn't know  :!:

[Torchy]

crp

[Aharon]

Thanks for the link Aharon, that's interesting stuff that I didn't know  :!:

Hey no problem.
Aharon

[WildMountain]

What's that saying again about the one fool that can ask more questions than a thousand wise men can answer?
Why don't they mention in the schematic that the unused opamp should be nulled out (if I read the above posts/links correctly, you can get a lot of noise if you don't)? How are you supposed to know how to do that?
Torchy: What do you mean by the + input connected to 1/2 Vcc?

[Transmogrifox]

What's that saying again about the one fool that can ask more questions than a thousand wise men can answer?
Why don't they mention in the schematic that the unused opamp should be nulled out (if I read the above posts/links correctly, you can get a lot of noise if you don't)? How are you supposed to know how to do that?
Torchy: What do you mean by the + input connected to 1/2 Vcc?

I think often times those of us who are more experienced with this learn to simply assume these things, like, "Yeah, of course you would do that".  I do remember back to when I didn't even know how to wire my own guitar pickups in phase, and the Fuzz Face schematic made absolutely no sense to me at all--so severely I couldn't have built it then...So that's what this forum is all about.  As you can see, even Paul M, who seems to be quite experienced in this learned something new from your questions.

Ok, let's see if I can make this simple without having to write a novel:

"1/2 Vcc" is half the supply voltage, in this case is 4.5 volts.  Look for a junction between two resistors of equal value that are in series between the power supply and ground.  Usually there is a capacitor to ground at that junction, and a resistor of about 470k or some large value connected from there to the + terminal of the op amp  That point there is "1/2 Vcc".  The supply voltage is divided in half by the two equal-valued resistors.  Using "Vcc" as a name for "supply voltage" is just a convention used in electronics--the "cc" part refers to the collector on a transistor, therefore this convention is used for BJT circuits, and "Vee" refers to the negative voltage, in this case, ground.  In MOSFET (CMOS) circuits, generally the convention is to use Vss and Vdd for negative and positive supply voltages, respectively, refering to source and drain.

Ok, now this is getting long and I hope I don't overload you and confuse you.  Let me wrap it up:

The simple thing to do is to connect pins 1 and 2 together, but NOT to anything else.  Then take a wire from that "1/2 Vcc" point to pin3.  Pin 4 should be connected to ground, pin 8 to Vcc (9Volt battery).  Pins 6,7,8 (if I remember from your first post) were used in the circuit and shown correctly.

Otherwise, I think you picked a good project for a first build--it has enough of the standard distortion pedal components to be able to learn something, but simple enough that you will be able to get it working.

[Paul Marossy]

As you can see, even Paul M, who seems to be quite experienced in this learned something new from your questions.

Well, I still learn at least one new thing a day.  :wink:
To be honest, I really never thought about the effect of the unused pins. That application note was a eye opener for me.  

[col]

I always ensure that unused connections on op-ams are not connected by using a socket and cutting off any legs that are not needed, usually 3 on 741 type. I have not had any problems with them with just 5 legs soldered. When I first started making pedals I found a really good guide to op-amps but I can't remember where I got it from, it might have been off General Guitar Gadgets. If I can find it tonight  I'll post the link tomorrow.

[WildMountain]

Thanks all you guys.
Transmogrifox: I'm not getting all you said, but probably will in time. I CAN see the way the circuit supplies 1/2 Vcc. It's exactly as you described with two resistors in serial and a capacitor. And thanks for reassuring me for picking a good beginners project, because I really started having doubts about that.
Col: Does this mean you can either connect the +input to 1/2 Vcc, or just not use the legs that don't show up in the schematic and cut those off? I guess a guide to opamps is indeed what I need. I'll look into it myself as well.
Paul: That's what I meant with one fool's questions. I can probably "teach" you a lot more in a similar way :wink:  How did anybody get this stuff learned before there were forums like this where experienced guys like you answer a lot of questions within about five minutes. (enough sucking up for now)