Need Help With OpAmp Circuit for Cell Phone Recording

Question

I'm trying to make a simple circuit that will allow for the combining and recording of both sides of the conversation by tapping into the connection between a cell phone and a headset.

For convenience sake, I want to also use a spare cell phone as the recording device due to portability,etc. I am using iPhones for the cell phones in this application, although that shouldn't matter.

NOTE: The iPhones provide approximately 1.8V to the microphone to power it, thus my decoupling capacitors on both the input and output. The 4.7K on the output is to trigger the recording iPhone to switch over to external mic mode, otherwise it will only record the internal mic and ignore the input signal.

I've tried designing this to the best of my knowledge and understanding, however I am not getting anything on the outputs. I've checked the input with an oscilloscope and I'm getting a good signal at the opamp input pins, but nothing on the other side. I've tried changing out the IC and still nothing, which I'm guessing there's a flaw in my design since I don't fully understand each component's role/effect in the circuit beyond basic theory. Any help would be awesome and much appreciated.

Answer 1

At the very least, you need to bias the + input of your opamp to the virtual ground you want to use. Use a resistor divider between your 9v rail and ground to get 4.5V. This will bias the input to the midpoint of your rails.

In this configuration, you will need two separate biasing dividers for the two op-amps.

A more rational approach might be to configure them as inverting op-amps, and you can use a single voltage divider to both + inputs of the op-amps.

Answer 2

The TL07x op amps are not rated for rail-to-rail operation. In connecting pin 4 to ground, you are forcing the inputs much closer to V- than they can support. The data sheet specifies limits on the common-mode voltage input (which means the input voltage in this case) as 4 volts above V- to 4 volts below V+, which means that it won't work as you've connected it. The simplest change is to get another battery, and connect + to ground, and - to pin 4. This will provide +/- 9 volts to the amp. Also, I recommend a 0.1 uF to ground at both pins 4 and 7. This is called decoupling, and you should always do it. You already have a C3 of 1000 uF, but this is much bigger than you need, and will not respond quickly enough to keep you out of trouble. 0.1 to 10 uF is much better.

Also, it's possible that the iPhone outputs are AC coupled. If so, your inputs will drift around, and may be unusable. I recommend connecting your + inputs to ground with a 10k to 100k resistor.

Finally, there's no need for the 22k resistors. Just connect the output of each op amp to the - input.

Answer 3

Alright, so I created a stripped down version on a breadboard using only one set of I/O pins and connected it to a function generator (100mVpp - 1.5kHz) and an oscope. My function generator can go as low as 50mVpp (which is closer to what the input signal will be), however this low of a signal conflicts too much with noise and the oscope, so I chose to work with a higher Vpp for this test. I tested it with and without the voltage divider and the results are self-explanatory. The yellow is the output signal, the blue is the input signal.

The circuit:

Without the voltage divider:

With the voltage divider:

Notice that there's a drop in the signal output amplitude compared to the input signal and I'm not sure why. (There's also a phase shift occuring, however I'm not concerned about that unless it's indicative of some other problem of concern.) The output amplitude increases when I increase the frequency, however 1.5K is well within the vocal range and this will be a problem. I saw no difference between a resistor across pin 1 & 2 vs a jumper across them, as per @WhatRoughBeast suggests. I guess I don't grasp the negative feedback to set the gain of the opamp.

My reason for choosing an opamp was to simplify amplification (among other benefits of using an opamp) and to increase the amplitude of the input signal for better mixing on the output. Am I just not understanding this correctly?

EDIT:

Ok, I took Daniel's suggestion and swapped it to an inverting configuration and presto! Here's the revised schematic:

And, as you can see, the results are as expected:

Now I don't quite understand why this works and the non-inverting layout doesn't, but the fact it works is what I was trying to achieve. If I understand what Daniel said, I can connect pin 3 to pin 5, using the voltage divider for both Pin 3 & 5, correct?