Okay, I guess it's not as robust as I had hoped

These are my suggestions on some of the critical nodes to probe when debugging this circuit.  The list is arranged in order of what depends on something higher in the list.  In other words, step 1 requires nothing other than the circuit and power to work.  If step 1 gives a bad result, then step 2, 3, 4, 5, etc will probably fail.  This list of potential problems are endless.  However, if a certain step fails, it probably means there is a bad connection or bad part.  The components near the node you are checking should be checked.  (Ie. swap out the ICs with other ones and see if the numbers change).

1. Verify that VDD/2 is right. It should equal about 4.5V and should not vary. (Non-dependent on other items).

2. Short IN+, IN-, and BODY together. Verify the voltage of the shorted nodes is the same or close to VDD/2 (4.5V)

3. Keep config of step 2.  Check pin 1 of IC1A. It should be somewhere around VDD/2, but not exact (close to 4.5V)

4. Check pin 7 of IC1A just like step #3 above.

5. Check pin 1 of IC2A. It can be anywhere between 1 and 8 V (we're just checking to make sure this amplifier has not railed).

6. Check pin 3 and pin 2 of IC2A. These should be nearly the same and equal to VDD/2 (4.5V).

7. Check pin 7 of IC2B. It should be around VDD/2 (4.5V). Give or take a little.

8. Check pin 6 of IC3B. This should equal pin 5 or VDD/2 (4.5V).

9. Finally, look at VOUT. It should be VDD/2 (4.5V) or somewhere close.

After all this, the circuit should output a good signal.  One way to check is to apply a signal with a function generator or some other device.  It will saturate the audio input, but you will see a signal.


Good luck, and I hope you all have fun!