Op amp periodically fizzling out

[270fx]

I've got a simple optical tremolo circuit on my breadboard. The first stage is a simple noninverting op amp stage with a little gain. The opamp is setup like this:



The circuit works fine for a while but eventually the audio gets quieter and starts to crackle until I get no audio output at all. But if I take a jumper from vref and touch it to the +input for an instant, the audio comes back and everything is fine again for a while until the same thing happens again.

Could stray breadboard capacitance between the opamp inputs cause this, or do I just have the opamp configured incorrectly? Or something else entirely?

[antonis]

But if I take a jumper from vref and touch it to the +input for an instant, the audio comes back and everything is fine again for a while until the same thing happens again.

So, why don't you implement that permanently..??

More seriously, place a big value resistor (1M or so) between Vcc/2 and op-amp non-inverting input (+) and earn peace of mind..
(after Cin cap, of course..)

P.S.
It should be a good idea to place a cap between R1 and Vcc/2 to roll-off DC gain to unity..
You'll don't have to place neither OUT cap nor bias resistor (like above suggested) for next stage non-inverting input..

[anotherjim]

You (literally) touched on it. The +input needs a connection to Vref. Not a wire, of course, that would just short the signal out but a resistor large enough to have a negligible "shorting out" effect but still let that input "see" Vref. For guitar input, we usually pick a 1M resistor.
The reason it worked for a short while after touching Vref is that the opamp inputs have a very high input resistance (usually) and there's a little "stray capacitance" to hold the Vref voltage for a short time until it drifts too far out as you heard.

[270fx]

Ok the 1meg resistor to +input seems to have done it. It's weird because that diagram from the original post comes from a TI document about single supply opamp configurations. Don't know why they don't include that resistor since it seems to be essential to the functioning of the circuit.

Thanks for the help!

[anotherjim]

Manufacturers application circuit examples will catch you out if you take them too literally. It's just an isolated function as a building block they usually show and it will be assumed the circuit feeding it is DC coupled and referenced at Vcc/2.
Quite simply and definitely an opamp amplifier must have a referencing DC voltage applied to the +input. You can't use the -input to reference because that will invert it.

[antonis]

Quite simply and definitely an opamp amplifier must have a referencing DC voltage applied to the +input.

Just to add for that DC voltage to be "real"..
The above posted configuration can't work neither for symmetrical dual supply (Vref = 0V = GND) 'cause Cin needs a path to GND for discharge.. 

[PRR]

Manufacturers application circuit examples will catch you out if you take them too literally.

What's interesting is that TI's drawing is Just Wrong. Which may be why it is not on the TI site anymore?
https://mil.ufl.edu/4924/docs/TI_SingleSupply_OpAmp.pdf

Missing grid resistor and wrong formula:


No resistor but correct as-is (it gets bias from being connected to a source):