Bacon Bits schematic question

[Om_Audio]

Hi,
Can someone please explain IC2 and how it sits on its own with 2 connections going off down the rabbit hole to "4" and "8"? I tried to see if it meant pins 4 and 8 of IC1 looking at the board traces or if the numbers referenced points on the lower/main circuit but I'm a novice and don't know what the hell I am doing.

Thanks,
Clifford

[nzCdog]

The 4 and 8 to nowhere are the power pins of IC1... 8 is V and 4 is G

[Om_Audio]

Damn I was sort of on the right track. I could see the traces on the layout but could not fully understand. Are the red traces all V? I can see IC2 pins 1 and 8 connecting to pin 8 of IC1, but pin 5 of IC2 goes to non-red trace to a hole, turns red, then goes to pin 4 of IC1. I am comparing the schem with the layout. I don't understand how to read the layout obviously. Like is the neg pole of C6 connected to pin 5 of IC2, and to a hole under D1 and onwards to pin 4 of IC1? I don't see any component using the hole below D1 so can only assume it is to connect maybe backside and frontside or ground plane? Errrr.....

Shit man, now that I look at it, why not just have them resolve to the 9V and ground to the left, why have them hanging free like that? Why not show they connect to IC1 on the schem?
And damn, where is IC1 on the schem??
Does IC1 have 2 transistors in it thus the numbers on the 2 trannys in the schem?
Now I also see IC1A and IC1B by those 2 transistors... ok, getting warmer.
Thanks for the tip-
Eternally curious,
C

[gtr2]

Because that's how eagle does it.  On many schematics power and ground are not even shown on the IC's.

Josh

[oldhousescott]

Those triangles with 2 inputs and 1 output each are the schematic representation of an operational amplifier. IC1, the TL072, is a dual op-amp so it has two amps in one package, hence the labeling IC1A and IC1B.

[midwayfair]

Here's an excellent resource:
http://www.beavisaudio.com/techpages/SchematicToReality/

Let's go through the schematic, because it's a fairly simple one and therefore good learning opportunity. What we have is a single OP-amp booster with a +9v/-9v power supply (aka "split rail").

Let's start with the power section, because it's the easiest. +9v power is applied at pin1 of a charge pump IC. The charge pump can do a few things, including providing a way to invert the DC (this is one of the uses of Bean's Road Rage). The datasheet for the chip tells us what each pin does, and even provides some recommended schematics:
http://www.datasheetcatalog.org/datasheets2/14/145160_1.pdf

Why do we want -9v instead of 0v as ground? More headroom. This provides an 18v swing, making clipping less likely.

The power from the power section is then applied to the power ( and ground (4) pins of a dual op amp.

Look at many schematics with dual op amps and you'll see one of two things: Either 4 and 8's connections aren't shown, or the pins aren't even shown on the schematic. Usually they're the same. If you're using a chip you've never seen before, just check the datasheet. Always check the datasheet.

The audio circuit itself is fairly simple.

--Our input encounters a pulldown resistor, followed by a bandpass that cuts a small amount of highs and sub frequencies.

Why do we want to cut frequencies? Because it helps reduce noise, electronically isolate the circuit (you don't want DC getting into your pickups!), and prevent OP-amp distortion from the guitar input. Controlling bass between gain stages is important -- here we're mostly concerned with making sure the signal sounds "clean," and the best way to do that is to avoid amplifying information that is very far outside the frequency spectrum of a guitar. A balance must be sruck between keeping some harmonics (and thus making it sound interesting) and the shortcomings of amplifier circuits (noise and unwanted distortion). Something to keep in mind if you ever decide to design something on your own.

--Our audio signal is then applied to the non-inverting pin of IC1A, then passed on through the chip with no intervening circuitry through the inverting output, into Pin1. This is a simple IC buffer.

It might be a good idea to open up the datasheet and have a look at what the INSIDE of the op-amp chip looks like. Op-amps are pretty simple ... they house multiple transistors (a single op-amp can be constructed with FOUR [not one or two] transistors -- clearly it's much easier to use a chip!), but it'll help you understand why there's an inverting and non-inverting pin.

What's the deal with "inverting" and "non-inverting"? It revers to the polarity of the voltages in the circuit. Here's a mathy explanation: http://en.wikipedia.org/wiki/Op_amps#Non-inverting_amplifier

And here's how it applies to our stomp box: Remember how we made the voltage in this pedal +9v and -9v?

--A 10K resistor is placed between the buffer and the next op amp stage.

This could have many purposes, and I don't know which was Brian's intention, but among the things it can do is keep the high frequencies in check (we just buffered our signal ... don't want to take anyone's ears off when we boost it many, many times over), or simply to keep the overall volume in check.

-Our audio signal again enters the non-inverting output of an op-amp. IC1B has other stuff going on. Increasing the resistance between pins 6 and 7 will increase the gain, with minimum and maximums set by R5 and R6. That 100K, incidentally, is another place you can see gain and filtering controls (when it's in series with a capacitor) -- it's before the IC's gain. It's where the Tim/Timmy and Zen Drive cut the bass and where the Afterlife sets the gain. There's also some filtering with a small (82pf) cap, which affects the "presence" of the overall sound. This is meant to be a pretty sparkly effect, so that cap needs to be small. There's a pot that controls the gain. It will probably give you something on the order of x40 here, but I suck at math and this is just based on how much gain similar circuits have.

--Finally, our audio signal passes through the AC coupling cap (again provides some filtering by cutting some lows and can also protect the next pedal in line the same way the first one protects your guitar), a 100K resistor to ground and 1K resistor in series with the output (this shaves off a very small amount of volume and highs ... it's like having a 100K volume pot almost wide open).

[Om_Audio]

Wow, thanks. Yes- I felt silly after you all pointed out the op amps were not transistors- I knew that!

Jon, thank you =so= much! I really need some mentoring and you have been most generous here.
I read the whole beavis article, perfect timing. I have recently bought a breadboard and some perf and gathered parts for a testing rig. The article covered a lot of things I am just beginning to approach and is great. Studying the breadboard and also the wiring and 3PDT was great. I can visualize what is happening when you throw the 3PDT now that I have taken some apart and also had some help on my Macheen build thread- but I can "see" how center row is connected to one or other outer rows and look at that each would do at that moment. I am slowly understanding everything more and more but in tiny little bits- its a curious process.

This is a lot to digest but I printed out the schem, put it up on my 37" TV blown up, got a pen, and sat down and read the article and walked slowly through what you wrote while comparing scem w layout.

--> Is this a 2 sided/layered layout/PCB or something? Finding it very difficult to trace things out. this also touches on the fact that i am still struggling to have an internal model of how current behaves in a circuit. that will be a sweet ass "A Ha!" moment I hope soon!

--> On the schem, regarding perpendicular crossings with a point like R6/R5/IC1b pin 6/C2--- does it matter if a connection goes across this intersection in a straight line or not in terms of actual PCB traces? In other words, should the connection between R6 and C2 and onwards to pin 7 be a direct trace and R5 to pin 6 be direct? Like do straight lines have priority over perpendicular crossings?

The ICs are complex and will require further study to be sure. i am so surprised at all the tricks the TC1044 can do! I will be learning more first hand as I want to make an 18V power supply and also the Mangler uses a Road rage.

- IC2 preps the power to what IC1 needs to operate. Always check the datasheet.
i really do not understand + and - voltage yet but i get the idea that and 18v spread across -9V and +9V will give more headroom.
- pulldown resistor -I dont understand impedance, but I don't need to right now to know what it is for and that it is very common in guitar pedal circuits and to be familiar with it. I have also heard about "loading" guitar pickups. will learn more on this later.
- C1 is also a curiosity as i always just imagined caps as little batteries and not like a diode where it can somehow prevent DCV from travelling back up to your guitar. I do know about polar and non polar caps and their symbols. Your explanation is very useful as I know for a fact i will be interested in trying to design something myself at some point- it is just my nature. the info you provide will be really helpful.
- inverting and op amps datasheet- on my to do list.
-filtering, pots, a cap at the end for DC isolation like the beginning- cool

Anyway, there is more I could comment on but I have plenty of food for thought and will come back to this in the future for sure. way more than a single lesson in this post!

Thanks again!!

C