Introduction: Modify the RadioShack Amplifier to Power a Condenser Microphone Element
Here is a very simple modification to the Radio Shack 277-1008 mini amplifier (one trace cut and the addition of a capacitor and resistor). This is a nice little battery operated amp that uses an LM386 amplifier IC. It also has a transistor pre amp. The output is a little low with a dynamic microphone so I have had to use an external 9 volt power supply circuit to power a condenser microphone element. But not any more.
The first picture in this step shows the unmodified circuit board.
1. Cut the trace between the tip connection of the input jack and the potentiometer as shown. Also scrape off some of the insulating coating down to the bare copper on both sides of the cut.
2. Solder in the 0.47 uf non polarized capacitor as shown.
Step 2: The Resistor
1. Insulate the 4.7K or 5K ohm resistor with shrink tube.
2. Solder one lead to the pad that connects to the 0.47 uf capacitor.
3. Connect the other end to the pad that connects to the positive side of C6 as shown.
Power is supplied through the resistor and the capacitor blocks the direct current and allows only the AC signal to pass through to the preamp.
You are ready for testing. Just about any inexpensive condenser microphone element will work fine. The tip of the mini phone plug that plugs into the jack is positive. and the sleeve is negative. The current through the microphone I used for testing was about 0.3 milliamps. Other resistor and capacitor values will work but these worked very well for me.
You can also connect a photo transistor (I prefer photo Darlington types) as well a CDS photo cells for visible and infrared light listening adventures. The 0.3 milliamps should not damage a dynamic microphone but why take any chances. I own two of these amps. So this one will be dedicated to non dynamic mic uses.
Step 3: MIcrophone Elements
If you don't have a condenser microphone element wired up see below. The element pictured was purchased at Radio Shack but is no longer available. Try digikey.com. You can get these starting at about $0.93 each.
The negative lead is typically connected to the metal case.
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