I checked out the AGC microphone on Techlib website, and I like the circuit. I have some LF353 opamps I got from Radio Snack a long time ago (I used to look in the blister packs and see both LF353s and TL072s), and I could use it in place of the TL072. One thing he said that makes me more than a little bit leery: ¨..the performance is fantastic.¨ That is not an objective statement. Every circuit has some compromises made to prevent costs going through the roof and to make it doable with reasonably priced parts. I thought about the circuit and found a few issues that I would deal with differently.
Both of the wires to the microphone are 6.8 volts above ground. I guess this is not a problem as long as the preamp is located where the microphone is and is hard wired to it. But if you want to put a jack on the input to allow changing to different mikes, then there is a problem, since you will have to deal with three wires, the two for the mike and the grounded shield over both. I would solve this by putting a DC blocking capacitor between the input and mic, and a resistor to allow bias current to reach the input. Also, the way the circuit is, it cannot be used with an electret condenser microphone. Another solution is to go professional and use a matching transformer on the input.
Being that this could be located near or at the microphone, it would be really handy if it could be powered from a battery. But a 12V battery is really expensive and hard to find. It would be much easier if it was a 9 volt battery. But reducing the supply voltage is a real problem. Once the supply passes through the diode on the input, it´s down to 8.3V. Then the voltage is so low that there will not be enough current through the 6.8V Zener to keep the voltage at 6.8V. Not to mention that the 6.8V will be too close to the supply voltage, so the opamp will not be biased at its center point, which is normally half the supply voltage. Actually, most opamp bias circuits use a simple resistive voltage divider, which has two equal resistors. I´m not sure why he chose to use a Zener here when it would be easier and simpler to use two resistors. Changing to resistors lets the bias point stay at half the supply V as the battery voltage drops with use.
Another thing I like to do is, instead of using the series diode for reverse V protection, put a 1 amp rectifier across the input, cathode or banded end to positive. A backwards battery will cause the diode to conduct and drain the battery. But there is no V drop across a diode so the full battery voltage is used. This is especially important when the supply voltage is below 5V. Adding a 1/4 amp fuse before the diode will then cause the reversed battery to blow the fuse and leave you with a good battery but a fuse that needs replacing.
One other thing. If the output of this is to be plugged into the microphone input of an amp or recorder, then the output should have a resistive voltage divider to reduce the output level to the optimum for the preamp input. A simple High-Low level switch would take care of this.
I noticed that the diode on the supply input is not labeled. A 1N4002 rectifier should work, or even a 1N4148. But a 1N5817 Schottky diode would save a third of a volt or more.
Back to experimenting…