I am trying to build a guitar amplifier circuit. I want to allow the amplifier circuit to be able to clip. I am testing it with a 5V circuit and a function generator and an o-scope. I use the function generator to push a 60 Hz, 1 Vpp sine wave into the input. I put a small capacitor (0.1uF?) on the output of the IC, followed by my o-scope. The IC is set to a gain of 20. But instead of a clipped 5V output, I get a 1 Vpp sine wave out - exactly what I am putting in.
I have no cap on the gain pins, so it’s internally set to 20. I grounded the inverting input pin, I put the signal generator (with no offset, btw) in the non-inverting input, with it’s ground connector shared with the power circuits ground. I left the bypass pin unconnected. Basically, this circuit without the pot nor the cap on the input pin:
Something is terribly wrong. I set the function generator to 1 KHz, but it puts out a 1/5 Hz wave. I set it to some other frequency (1.25 KHz?) and it puts out correctly. I added a cap to the input and it still has a zero gain. I isolated the power supply to see what it was doing. My DMM showed about 9.9 V. The supply is labeled 6 V. I measured it before and it didn’t reach 7 volts.
So my power supply is putting out almost four volts more than it’s supposed to. My function generator puts out 0.2 Hz when it’s supposed to put out 1 KHz. According to my o-scope, the amplifiers input pin is in the negative, even when coupled with a cap. I have no idea what’s going on.
EDIT
The function generator is working now. I still have a gain of 0. The power supply is still putting out 4 volts too many.
The input pins of this device, like all opamps or opamp like things, have a bias current. There must be a DC return path for this current or bad things will happen. You have a capacitor blocks this return path so it doesn’t work well.
Alter your circuit so that the capacitor is ahead of the variable resistor and the LM386 should be much happier.
But that doesn’t explain why the power supply is 4 volts too high
Move the capacitor, as UhClem suggested.
The power supply was completely disconnected from the rest of the circuit. I know the voltage read on a power supply is supposed to be higher that what's stated when it's not under a load. But not that much. That cap that UhClem suggested I move does not exist.
So the output of the signal generator is going straight into the positive input of the opamp? Doesn’t it need an appropriate load impedance (usually 50 ohms). The opamp input withou the potentiometer has a quasi infinite resistance, so it causes signal reflections back to the source distorting it. David Jones of the Eevblog had a video about this recently.
http://www.electro-tech-online.com/imgc … age005.gif