Op amp needs bias?

[YurkshireLad]

I hooked my electric guitar up to an op amp (TL082) to try to build a very basic distortion pedal with lowish gain, currently 2. I can crank that later once I've got the basic circuit working. I just wanted to make sure I understand this correctly. The output only has a positive voltage, the negative has all been clipped. I assume this is because my power supply is +5V/ground, not +5V/-5V? Am I right in thinking that I need to apply a bias to the input?

The circuit is shown in the attachment.

Thanks

[YurkshireLad]

Seems I reported that incorrectly. The new attachment gives a better picture, so I'm doing something wrong somewhere. Time to recheck my circuit.

[rstofer]

Yes, you need to bias the input to 1/2 Vcc.

https://www.ti.com/lit/an/sloa030a/sloa030a.pdf?ts=1636228792351&ref_url=https%253A%252F%252Fwww.google.com%252F

[YurkshireLad]

Yes, you need to bias the input to 1/2 Vcc.

https://www.ti.com/lit/an/sloa030a/sloa030a.pdf?ts=1636228792351&ref_url=https%253A%252F%252Fwww.google.com%252F

Thanks; I'm working on that change (in between watching the rugby).

[Terry Bites]

The opamp inputs always need a path to a DC potental. To over come your problem, bias this input to half rail 2.5V. The TL082 is "high voltage opamp" and not well suited to a single supply of just 5V. A low cost opamp such as an MCP6281 may better suit your deisgn. Note the AC coupling and cap in the feedback loop to ground. They are essential, otherwise you will be amplifiing the 2.5V DC bias.

I just gave it a quick route - you could make it smaller.

[YurkshireLad]

Thanks Terry and thanks for the reminder about the caps. Does it matter what resistor pair I use for biasing? I have a fairly small selection of resistors and I don't have 56ks.

Oh, and the TL082 is the only op amp I have. I haven't bought any others as I'm learning and didn't want to spend the money. Yet. 

[TimFox]

For audio with op amps, I recommend split power supplies (two 9 V batteries, maybe) since it makes the signal chain much easier.  (This shows that I was born in a previous millennium.)
If you insist on a single supply, then obviously the output cannot go negative with respect to the grounded negative power input of the op amp (ignoring some exotic devices with internal power supply thingies).
Therefore, the output should be somewhere between ground and the positive supply, and an output capacitor is required to drive a ground-referenced input.
Assuming your source is a typical guitar pickup, which is essentially an inductor, the low bias current of the FET-input amplifier can flow through it.  With a split supply, the pickup can be grounded directly.  With a single supply, you need to add a capacitor from the grounded pickup, and add a voltage divider to bias the amplifier side of the capacitor to mid-voltage.  Any noise or power-supply ripple at this voltage divider will be amplified by the circuit.
You also need to connect the "bottom side" of the feedback circuit somewhere:  the easiest is to use a large capacitor to ground, so that the gain at DC is unity although the AC voltage gain at audio frequencies is 10:1.
Even with split supplies, it is a good idea to have a capacitor at the output to remove the small DC offset voltage.

[YurkshireLad]

Thanks all, much appreciated.

[magic]

Dual battery supply is a nasty can of worms because it turns into a single supply when one of the batteries is depleted.

This may result in DC offset on the output. And it only gets worse when the opamp is prone to phase reversal: if TL072 negative supply drops down to -2V and the signal is at ground, its output will go to the positive rail which may still be a few volts.

[CatalinaWOW]

I am also old school and am more comfortable with dual supplies.  More dynamic range is often a real benefit of this approach.  And to avoid the drawbacks of dual battery approaches there are switch mode inverters available which can boost a single positive supply to a negative supply.   They are small, cheap and require only a couple of external components.

[Doctorandus_P]

Another method is to generate a negative supply voltage for your opamp. For example with an ICL7660. (There are many variants of that chip).

I do not see much use of adding DC blocking capacitors in each and every stage. If you use an ICL7660 and then use DC coupling for all pre amplifier stages, then that DC will (or at least should) be blocked by the power amplifier.

[rstofer]

For breadboarding, these Power Bricks work really well.  They take USB (or +5V) in and provide a dual rail output up to +-12V.  I wish they went to +-15V but that's the way it is

https://digilent.com/shop/powerbricks-breadboardable-dual-output-usb-power-supplies/

[YurkshireLad]

I biased the non-inverting input and the input signal looks great now. However, I'm getting 3.65V (DC coupled, 0V AC coupled) out of the output pin?  I think I need to pull the components and rebuild, perhaps tomorrow when I'm not so tired. I'm seeing references online to the input voltage being too low (voltage swing?).

[rstofer]

With a properly biased single supply op amp, you can expect the output to idle at Vcc/2.  For a dual supply setup, the output will idle around 0V.  I'm just guessing but it seems that your bias voltage may not be quite correct if you are getting 3.65V on a 5V single supply circuit.  Tie your signal input to Vcc/2 and make some measurements.  Remember, all measurements should be made relative to Vcc/2, not 0V.

Vcc/2 is the new ground.

Single supply circuits aren't my favorite...

[RJSV]

   Gawd, I pity you, with all this good info at once.  But a little more basic theory might help:
   OK, I view an OP AMP as a super gain device, (way more than the circuit it will be in). Think something like 100,000 X gain. So, the trick is to utilize feedback, from the output and consider this output as 'controlling' the OP AMP minus or negative input, which it does, but via some reduction.
So, if the positive input (feeble) rises a bit, the huge gain cause a big rise in the output, which also gives a feeble rise in your negative input (you connected it to drive a reduced output to the negative input).
   That's a control type response, where the OP AMP fights to eliminate the (voltage) difference, between the two inputs. That is the circuit, with the negative feedback is arranged to do this.
   So you never see that huge gain, directly, just think of it as strongly keeping the two inputs equal.
   Now, for DC, looking at previous examples, your desired negative feedback is for one to one 'gain', or just have your DC level at about half way, like 2.5 volts if you are using single 5 V supply. The OP AMP will fight, strongly, to control your negative input to also be at 2.5 V steady. Except for noise, there.
Your negative input, from a DC standpoint, should just follow the output, no reduction, and that is what the other posts do show. Therefore, you will see 2.5 V DC right there on your output, too.
You can see, the capacitor keeps the ground out of any DC feedback calculation, the negative input gets the DC 2.5 V since the DC part of the output is not reduced, in that negative feedback.
   One way to look at the AC (audio) is that it is a little disturbance, on top of the DC 2.5 V you've set up.
You can also appreciate, a pair of 100 k or a pair of 56 k makes no difference, as long as it's fairly large, so as not to start loading down the feeble AC. You can kind of think if it as two circuits, right there in same place!
   Now then, you have a DC level being controlled, putting those 2 resistors dividing the output AC, (because for AC your ground IS connected, so that divides) you have, paradoxically, actually TWO different gain situations, simultaneously.
And one last concept: by feedback having less than the output, you can say it the other way: In other words the output is MORE than the (negative) input, and, hey, same thing can be said about the positive input, your feeble guitar signal. That's because the circuit is controlling itself, to have the positive input be, also, a certain factor or fraction of the output.
   Very wordy, and round and round, but it's the key to all those variations being tossed at you.
   Question ? I will check back, hope my (limited) expertise helps...

[RJSV]

...the thing I wouldn't spend much time on, is any modification to basics, guitar-wise. Some mention, of 'input bias current' of FET input going through the guitar pickup(s).? And messing with the ground side, of the guitar / cable. ?
  Dude, put a DC blocking capacitor, first thing at input jack... Next thing, we get a half-hour discussion, on 'which' guitar type to use, for a bit of trickle current thru the pickups...and how to 'isolate' your ground sheild. Maybe a transformer, under the guitar plate...
All due respect, just thinking aloud.

[YurkshireLad]

Got it! I used the circuit that Terry posted above and rebuilt it. However, I do have a question.

The Capacitor C1 is reversed, so cathode connects to the guitar input and the anode connects to the voltage divider. Is this right, if so, what is the reason for doing this?

Thanks all.

[RJSV]

   I like the responses Terry has given.
My guess is the cap polarization just indicates which side is the higher DC potential. Op amp input device is powered, so cap reflects that, (but that doesn't cover the case for two batteries, so I'm over my head, in that water, lol).
Output, same thing. The OP amp has positive potential, so the cap polarization reflects that polarity direction, although again, my certainty factor is, like
perhaps 61.5 %.

   My philosophy is to generally cap isolate things coming into your 'box', and DC isolate outputs. That leaves room for a design, like for example, having 3 stages, DC coupled, no caps. Then a cap isolates the last stage.
   Wouldn't worry too much about 'noise' getting in, on distortion settings.
I might try this: First two stages with gain of X5, and a foot switch to insert a third stage with crazy high gain maybe X 12. Then, you pad down that output, with potentiometer, or resistor divider.  WAIT... I guess that's wrong, as you seek distortion effects. Perhaps try messing with that last stage, try put back to back diodes clipping the 3rd stage output.

   I've used a toy tape recorder circuit board, for great homemade distortion, probably due to overdrive the Fender Super Reverb.
BTW something like the Fender Princeton Reverb is a great mid-size.

You can also send PM for guitar chord info, I've taught some, and interested in music careers (Jimi Hendrix).
Friends eyes glaze over whenever mentioned too much...lol

[YurkshireLad]

Thanks RJHayward. I was confused, as other schematics I've seen had a non-polarized capacitor at the front.  Anyway, it works so that's progress.

[Zero999]

Got it! I used the circuit that Terry posted above and rebuilt it. However, I do have a question.

The Capacitor C1 is reversed, so cathode connects to the guitar input and the anode connects to the voltage divider. Is this right, if so, what is the reason for doing this?

Thanks all.

The capacitor symbols drawn are non-polarised.

Here's a drawing, showing the correct polarity for the capacitors, if polarised parts are to be used.

[HB9EVI]

there's nothing wrong using single 9V supply for a stompbox; all the professional manufacturers (Boss, Ibanez et.al.) are doing it already for years; that works well with all common audio opamps like tl072, ne5532 or lm833; bias to vcc/2 and everything is fine.

[Zero999]

Another option is to use the spare op-amp to make a virtual earth.

[TimFox]

The rail-splitter circuit above works well for small-signal amplifiers like yours, where the positive and negative currents drawn by the other amplifier are almost equal;  the splitter amplifier then supplies only a very small current equal to the difference between the two currents.

[YurkshireLad]

I replaced the resistor with a 10k potentiometer, the only one I have, to control the gain. I bravely connected the output to my guitar amp. If I turn the pot down, I get a clean tone from the amplifier. If I turn it up, the amplifier is overloaded a bit, but there's no distortion from the op amp. I don't know if the cable to the amplifier is of good quality or not. It might be crap. I'm not sure if I'm forgetting something in my circuit.

[HB9EVI]

which resistor did you replace with the pot?

distortion is mainly reached by 2 anti-parallel placed diodes which clip the signal; the way of over-driving one stage with the signal of previous one doesn't work in silicon the way it does in valve overdrive circuits