Op amp needs bias?

[Zero999]

I'm using a modern digital multimeter to measure the bias voltage at R1/C2.

The junction of R7/R8 is 4.5V. This is what I'm seeing.

That's a little ambiguous. Are both of those voltages measured, with respect to 0V, or are you actually measuring 20mV, across the 1M resistor?

If both voltages are measured, with the meter's negative/common probe on 0V, then something is badly wrong: either the op-amp is completely busted, or C2 is short circuit.

If you're measuring 20mV across the 1M resistor, that's a bias current of 20nA, which is quite high for the TL072 and is a sign it's fake, or broken. Where did you buy it from? A proper distributor, or a random ebay/Amazon seller?

Have you tried swapping the op-amp, for a different one?

[YurkshireLad]

I'm using a modern digital multimeter to measure the bias voltage at R1/C2.

The junction of R7/R8 is 4.5V. This is what I'm seeing.

That's a little ambiguous. Are both of those voltages measured, with respect to 0V, or are you actually measuring 20mV, across the 1M resistor?

If both voltages are measured, with the meter's negative/common probe on 0V, then something is badly wrong: either the op-amp is completely busted, or C2 is short circuit.

If you're measuring 20mV across the 1M resistor, that's a bias current of 20nA, which is quite high for the TL072 and is a sign it's fake, or broken. Where did you buy it from? A proper distributor, or a random ebay/Amazon seller?

Have you tried swapping the op-amp, for a different one?

Sorry, I meant with respect to 0V.

I've tried different TL082s with the same results.

It works if I use a voltage divider on the input signal as in this diagram (not my diagram, I found it online).

Edit - I rebuilt the gain circuit and it appears to now buss the op amp, but there's no gain.    haha! That must be progress of some kind.

[Zero999]

I'm using a modern digital multimeter to measure the bias voltage at R1/C2.

The junction of R7/R8 is 4.5V. This is what I'm seeing.

That's a little ambiguous. Are both of those voltages measured, with respect to 0V, or are you actually measuring 20mV, across the 1M resistor?

If both voltages are measured, with the meter's negative/common probe on 0V, then something is badly wrong: either the op-amp is completely busted, or C2 is short circuit.

If you're measuring 20mV across the 1M resistor, that's a bias current of 20nA, which is quite high for the TL072 and is a sign it's fake, or broken. Where did you buy it from? A proper distributor, or a random ebay/Amazon seller?

Have you tried swapping the op-amp, for a different one?

Sorry, I meant with respect to 0V.

I've tried different TL082s with the same results.

It works if I use a voltage divider on the input signal as in this diagram (not my diagram, I found it online).

The two circuits are very similar and both use voltage dividers. The one with the separate 4.5V node, as you've drawn it, is the better one, because the capacitor in parallel with the lower part of the divider, smooths the 4.5V node, do it doesn't fluctuate with the power supply voltage. It means noise on the power supply, isn't coupled to the output of the amplifier. The one you've posted there, which just couples the signal onto the voltage divider, is simpler, but any noise on the power supply rail, will go through the potential divider and to the input of the op-amp and be amplified.

Have you checked the coupling capacitors aren't bad?

How are you building it? Is it on a stripboard? Are you sure the tracks have been cut where they need to be and none have been cut, where they shouldn't be?

Edit - I rebuilt the gain circuit and it appears to now buss the op amp, but there's no gain.    haha! That must be progress of some kind.

Which circuit? Note the circuits have different gains. The most recent one has a gain of 11 and the other one, a gain of 101.

[YurkshireLad]

ok, some success. I rebuilt the circuit from scratch and I can get gain from the op amp. I played my guitar through my amp (minimal tone circuitry) and it worked. However, the output was around 7V (relative to ground), so I put in a 1k resistor and a potentiometer in series on the output. I also replaced the gain resistor with a 50k potentiometer. Now I'm getting around 0.7V (relative to ground) on the output but it's saturating my amp, even when I turn the volume pot and gain pot down.

Edit:

I added a high pass filter with a cutoff around 80Hz and that improved things. However, if I roll back the guitar's volume control, the output signal from my "pedal" loses all of the positive signal. That's weird. I probed around and it's the op amp. Time for some more reading.

[Zero999]

You've obviously got far too much gain.

Do you know which amplifier is saturating?

Do you have an oscilloscope and signal generator? If not, there's some software to enable you to use a PC's sound card as a poor man's oscilloscope and signal generator.

What sort of guitar pick-up do you have? You probably don't need much gain, just an amplifier with a high input impedance.

[HB9EVI]

did you check if the bias on the output is correct, so +/- the voltage applied to the positive input?

also i think you're running too low impedance on the input for a guitar with the 56kohm voltage divider; in stompboxes you normally find a voltage divider with 2 10k and an electrolytic cap and bias to the opamp is done from the middle of this voltage divider over a 470k resistor. If you go too low with the input impedance you lose signal level in the upper frequency spectrum, so you get a very dull sound out of it
if you're not just playing a guitar with a builtin active electronics, you have to have an eye on the input impedance; if it's too low it's like you use a far too long cable - the frequency response of the guitar starts to suffer.

[Zero999]

did you check if the bias on the output is correct, so +/- the voltage applied to the positive input?

also i think you're running too low impedance on the input for a guitar with the 56kohm voltage divider; in stompboxes you normally find a voltage divider with 2 10k and an electrolytic cap and bias to the opamp is done from the middle of this voltage divider over a 470k resistor. If you go too low with the input impedance you lose signal level in the upper frequency spectrum, so you get a very dull sound out of it
if you're not just playing a guitar with a builtin active electronics, you have to have an eye on the input impedance; if it's too low it's like you use a far too long cable - the frequency response of the guitar starts to suffer.

I would use a bootstrapping circuit to give a much higher input impedance, but it's more complicated, so I wouldn't recommend it to someone who's struggled with a simpler circuit.

Here's an example, for educational purposes. It uses positive feedback to eliminate the effect of the input bias resistors on the input impedance. The voltage where R1 and R2 join, follows the input voltage and has a relatively low impedance of  R1||R2.  C1 couples this to one side of R5, so the voltage at this node follows the input, thus very little current passes through R5. R3, R4 & R6 bias the input to half the supply voltage. C2 can be two 47µF polarised capacitors connected back-to-back.

The circuit will be more stable if built with two op-amps: one bootstrapped stage, with a unity gain and the second one with the required gain.

[HB9EVI]

sure, technically a good option, but it always seems to me a little bit complicated transporting the 'magic' of bootstrapping to beginners; its function is not so evident at first glance

[Jan Audio]

gain_stage.png is good.
Post scope movie and schematic.

[YurkshireLad]

I re-built a new circuit without gain to make sure the op amp is correctly biased. However I'm seeing a DC voltage of approximately 2.3V at the non-inverting input of the op amp. It should be 4.5V?

[TimFox]

To clarify, is that voltage measured on a physical circuit, or just simulated in Spice?
If measured, what is the input resistance of your voltmeter?

[YurkshireLad]

This is a real circuit - I haven't measured the input resistance of my DMM.

[TimFox]

The voltage you measured is consistent with a voltmeter resistance of 1 megohm, which is less common than 10 megohm, but I have some with that value.  There is 1 megohm feeding that node, so a 1 megohm load would divide the 4.5 V source by a factor of 2.  The manufacturer’s spec should include the input resistance, which may be different on AC and DC modes.  Oscilloscopes usually have 1 megohm input resistance, in case you were using that to measure the DC voltage.

[YurkshireLad]

The voltage you measured is consistent with a voltmeter resistance of 1 megohm, which is less common than 10 megohm, but I have some with that value.  There is 1 megohm feeding that node, so a 1 megohm load would divide the 4.5 V source by a factor of 2.  The manufacturer’s spec should include the input resistance, which may be different on AC and DC modes.  Oscilloscopes usually have 1 megohm input resistance, in case you were using that to measure the DC voltage.

Thanks, that makes sense. How would I verify that the op amp is correctly biased?

[TimFox]

With that circuit, the output DC voltage should equal +4.5 V if you move the voltmeter (with any reasonable resistance greater than, say, 10 k, to the op amp output (pins 1 & 2), so long as you don’t have a voltmeter at the non-inverting input pin (3) to load the bias voltage.
Note that I did not proofread the pin numbers on your circuit drawing.

[YurkshireLad]

With that circuit, the output DC voltage should equal +4.5 V if you move the voltmeter (with any reasonable resistance greater than, say, 10 k, to the op amp output (pins 1 & 2), so long as you don’t have a voltmeter at the non-inverting input pin (3) to load the bias voltage.
Note that I did not proofread the pin numbers on your circuit drawing.

Good point  I forgot to note the output pin's DC voltage.

[YurkshireLad]

I re-measured and I'm getting about 3V on the non-inverting input and the output pins.

[TimFox]

Assuming no wiring errors or shorted capacitors, and further assuming that no AC generator is connected, that implies about 1.5 uA input bias current, which is far too high for a FET input op amp.  Perhaps you have one of the notorious forgeries, where a cheaper BJT amplifier has been relabeled.

[YurkshireLad]

Maybe it is a forgery - I do have some LM358s (perhaps they're LM393s, must check) I could try instead, though maybe they're forgeries too!

I am using a Wemos D1 R32 ESP32 as a simple signal generator, as I don't have anything else to use - except my guitar.

[TimFox]

1.5 uA is also high for a 358 (which is higher than a TL082).  Check the DC voltages on both ends of R2.  The LM393 is not an op amp (dual comparator), with the same pinout.
Make sure the AC generator is disconnected when you measure the DC voltages.

[YurkshireLad]

4.5v and 3.1v with the signal disconnected.

[YurkshireLad]

Maybe it is working; I put in a gain loop with a gain of 11 (10k + 1k resistors) and I'm getting a clipped output signal of about 7.8vpp. I'll plug the guitar into it tomorrow.

With a gain of 1, I'm getting an output signal of about 3vpp, which is the same as the input signal.

Thanks for your help.

Edit: I thought I had some LM358s but I can't find them at the moment. 

[nigelwright7557]

You can either use split rails or bias the inputs around half rail voltage and use a coupling capacitor on input and output.