Beginner: OpAmp defective or not correctly wired

[Tim Age]

This is my very first pedal build, so please forgive any beginner mistakes I most likely made.

So I'm in the process of building a very simple bandpass filter pedal. The circuit is extremely simple: an active highpass filter, an active lowpass filter, a volume pot. I built a highpass filter before, albeit a passive one, but to make up for the volume loss caused by the filters, I planned on installing an opamp - either one after each filter or one after both of them. I was going to set the gain to 3.

I tested the circuit without the opamps on a breadboard and it worked perfectly, so I added in the amps and got total silence as a result. I checked the wiring thrice but only found that I had wired it correctly according to the datasheet, so I put just one opamp between input and output, wired for unity gain, to see if it was the amp or the rest of the circuit causing a problem.

As a result, I got an ear-deafening amount of buzz but zero signal. Everything was correctly grounded, leaving me with three possible explanations:

• the opamp is broken - either from the start or I accidentally fried it
• I wired it in a wrong way or the pinout deviates from the datasheet
• the one I got* is somehow unfit for the task at hand

*as I have zero clue about opamps, I simply went to the local parts store and asked the bloke to get me an opamp "that can do non-inverting" and got a uA741 in return

I'm pretty much at a loss what to do right now. In case it's any help, the IC has two pins titled "offset null" which I didn't connect to anything because I have no idea what they do. I can't imagine that both ICs I got would be broken, so I'm somewhat sure I wired something wrong and I'd be extremely happy if somebody could give me some advice.

Tim

[DrT]

Those are pretty common op amps.  Check the data sheet here for the pin outs. 

http://www.ti.com/lit/ds/symlink/ua741.pdf

Are you sure you are orienting the IC the correct way, i.e. you know which pin is 1. Try breadboarding a simple buffer such as this: https://www.eecs.tufts.edu/~dsculley/tutorial/opamps/opamps5.html

What pins are you connecting as input, output and feedback?  What voltage are you using to power the op amp?

[Tim Age]

Those are pretty common op amps.  Check the data sheet here for the pin outs. 

http://www.ti.com/lit/ds/symlink/ua741.pdf

Are you sure you are orienting the IC the correct way, i.e. you know which pin is 1.

Yep, I am. Front is where the little notch is, as far as I know.

Try breadboarding a simple buffer such as this: https://www.eecs.tufts.edu/~dsculley/tutorial/opamps/opamps5.html

That's exactly what I did for testing the circuit, I got ungodly amounts of buzz back.

What pins are you connecting as input, output and feedback?  What voltage are you using to power the op amp?

I connected input to 3 (non-inverting), output to 6 (output), fed that back into 2 (inverting) for a buffer, negative power supply to 4 (-V) and positive to 7 (+V). I use my usual power supply - the 9V supply that came with my pedal board.

[tonyharker]

If you are building an active high pass AND an active low pass you will need two op-amps or a dual.  What schematic are you using can you show us?

[Tim Age]

If you are building an active high pass AND an active low pass you will need two op-amps or a dual.  What schematic are you using can you show us?

I do have two amps, I'm planning on putting one after each filter, so it's HPF -> amp -> LPF -> amp

gimme a second to draw a schematic

edit

okay here's the schematic

[Tim Age]

There you go, I made a schematic. the rheostats are pots wired as variable resistors, the 0.25nF capacitor is actually four 1nF wired in series.

[MaxPower]

Are you using a dual polarity power supply?

[ElectricDruid]

MaxPower is asking an excellent question.

Your sketch and your schematic aren't the same. The second pot is wired differently.

The thing that jumps out at me though is "no bias". If it is a bipolar supply, that would be ok, and your schematic looks like the ones in the textbooks. If it's a simple 9V battery or wall wart, then you need something to bias the signal around the half-supply level - a "virtual ground". It won't work correctly without it. This is the difference between single-supply stompbox circuits and the "typical" op-amp applications circuits you see in books.

[Tim Age]

MaxPower is asking an excellent question.

Your sketch and your schematic aren't the same. The second pot is wired differently.

The thing that jumps out at me though is "no bias". If it is a bipolar supply, that would be ok, and your schematic looks like the ones in the textbooks. If it's a simple 9V battery or wall wart, then you need something to bias the signal around the half-supply level - a "virtual ground". It won't work correctly without it. This is the difference between single-supply stompbox circuits and the "typical" op-amp applications circuits you see in books.

First of all, the schematic is correct, my sketch is wrong.

I don't know what a bipolar / dual power supply is or what "biasing" it means, so I'm at a loss here. The one I'm using is this:

[DIY Bass]

Your power supply is effectively giving you 0V (or Ground) and a +V (Presumably 9V).  Your schematic calls for +V, Ground and -V.  Your power supply is not giving you a -V.  That is very normal for stomp boxes, where we are usually using either a 9V battery or a 9V power supply like yours.  The usual solution is commonly referred to as "virtual ground"  You need a pair of resistors matched in value, and usually quite high in value, such as 1M ohm in between the two power supply rails on your breadboard.  You connect the middle point of those resistors to ground on your circuit.  The +9V side gets connected to the +v of your circuit (and you circuit acts as if it is +4.5V, not +9V.  The 0V side of your power supply connects to -V on your circuit, and you circuit acts as if it is -4.5V, because it is 4.5V lower than the point that you have connected to the circuit's ground points.  That is way harder to describe than to draw, but I don't have time to draw it now.  Hopefully it makes sense.  If you Google virtual ground you will probably find a drawing easily enough.

[Kipper4]

Welcome Tim

It's at times like these that google cant teach you everything you need to know.
Stick around ask questions learn and partake in the community.
I redrew the eq with proper biasing (vb)

Any questions about this just ask

[Tim Age]

Welcome Tim

It's at times like these that google cant teach you everything you need to know.
Stick around ask questions learn and partake in the community.
I redrew the eq with proper biasing (vb)

Any questions about this just ask

Thanks!

So, I've two questions.

First, is the battery ground (the GND the IC is connected to) and the circuit ground (the one the filters etc connect to) the same ground? (and by extension, does that mean I can use the stereo input jack to disable the power if it's not plugged in, requiring the mono plug's sleeve to connect sleeve and ring to close the circuit).

And second, did I understand correctly that this way, the power supply only provides 4.5V? That'd be kinda inconvenient because the ICs require minimum 5V, according to the datasheet.

[ElectricDruid]

Nice work Kipper, but you missed a couple - the bottom of Pot1 at the input (serious) and the bottom of C5 (doesn't matter, since no DC crosses the cap). Pot3 can stay because again, it's separated by a cap.

The first pot is crucial though, because the two inputs of the op-amp are biased differently at the mo.

Tom

[Kipper4]

Yep well spotted. Thanks Tom.
So I would add a cap between in series on the input after c4.
Alternatively you can put the pot1 lug1 to Vb.

Just to clear things up a little for the op. I think it's worth saying that we created a bi polar power supply from our +9v 0v wallwart (battery) our new virtual ground (Vb) around which our Ac signal (guitar signal) can now swing both above and below the virtual ground.
Where as before the op amps where at 0v (btw that's the battery - ) and the Ac could not swing below the 0v.

I hope the virtual ground. Biasing now makes sense.

Just to add also I don't doubt the data sheet says min +5v.
Lucky for us we are using a +9v Supply.

It made sense in my head.......

I should be more vigilant. I drew it up in a hurry before venturing out in the garden to paint the endless fencing. Ho hum. Part of the honey do list.

I'm don't think it would hurt to put C5 t Vb too.

[ElectricDruid]

So I would add a cap between in series on the input after c4.
Alternatively you can put the pot1 lug1 to Vb.

I prefer the pot1 lug1 to Vb solution, because if you add the cap, you then also have to add a resistor to bias the +ve input.

There should probably be a 1M resistor to ground before the cap C4 to help prevent switch pops too.

Just to clear things up a little for the op. I think it's worth saying that we created a bi polar power supply from our +9v 0v wallwart (battery) our new virtual ground (Vb) around which our Ac signal (guitar signal) can now swing both above and below the virtual ground.
Where as before the op amps where at 0v (btw that's the battery - ) and the Ac could not swing below the 0v.

+1 agree. This process of conversion from bipolar to unipolar supplies is a good one to learn 'cos it don't half come up often!

Just to add also I don't doubt the data sheet says min +5v.
Lucky for us we are using a +9v Supply.

Yeah, it's not a 5V supply, it's now a +/-4.5V supply.

I'm don't think it would hurt to put C5 t Vb too.

No, it wouldn't. It doesn't change anything either way, but it's more consistent to take it to Vb too, like the others.

T

[Tim Age]

So how do I get this working? Do I need a different IC that will do with 4.5V and how will I have to alter the circuit / schematic to make this work?

[Kipper4]

I'll redraw it for you.

Observe the power supply. You will see Vb everything marked Vb join together.
Battery -- goes to Gnd
Battery (wallwart) + goes to +9 to the left above the diode.

Use the 741 if you want.

[Kipper4]

Don't forget a C6 it will keep the DC from the output.

Insert your values where needed.

Do you get why it's biased at half supply now?

[ElectricDruid]

Kipper's schematic looks great now - should work exactly as drawn.

HOWEVER - that TI datasheet that DrT posted *does* seem to suggest the minimum recommended operating voltage is +/-5V, so we're pushing the spec a bit.

http://www.ti.com/lit/ds/symlink/ua741.pdf

It's weird though, 'cos I'm sure I remember running 741 circuits off a 9V battery when I was younger (when the 741 was the bang-up-to-date op-amp!). Maybe TI's are more fussy, or maybe they're being very cautious with the spec?

Cross-checking their TL072 datasheet, they give the same figures (minimum 10V supply, recommended +/-5V) and I know full well that works fine at 9V and below. So probably they're just covering themselves.

http://www.ti.com/lit/ds/symlink/tl072.pdf

[Kipper4]

I dont recall having problems with +9v before with my 741 stash.