Here I wanted to show how you can listen to AM radio station using simple power amplifier. This power amplifier is built using the popular LM386 audio amplifier IC.
The circuit diagram of Power Amplifier for AM radio is shown below.
You can connect this audio power amplifier circuit to the output of a simple diode AM demodulator circuit. In fact, this schematic is specifically tailored to act as the audio power amplifier stage right after an AM detector. See the following circuit:
The input section of this LM386 circuit perfectly handles the real-world issues we discussed regarding your detector circuit. Here is how they tie together beautifully:
How They Connect
The "From Detector" Terminal: Connect the output node of your diode detector circuit directly to the top terminal labeled "From Detector" on this schematic.
Common Ground: Make sure the ground (0V reference) of your AM detector circuit is connected directly to the GND rail of this amplifier circuit.
Why This Combination Works So Well
This specific LM386 design includes several clever optimization features that fix common AM radio noise and signal problems:
Solving the DC Offset ($C_c$): Remember how we noted that the diode detector leaves a generic DC offset on the signal? The capacitor $C_c$ at the input of this amplifier serves exactly as that DC blocking/coupling capacitor. It strips away the DC voltage and allows only the pure, raw AC audio signal to pass onto the volume control. A value between $1\,\mu\text{F}$ and $10\,\mu\text{F}$ works great here.
Volume Control ($V_{\text{OL}}$): The 10kΩ potentiometer acts as an adjustable voltage divider, letting you easily attenuate the detected audio signal before it gets amplified.
RF Filtering ($R_1$ and $C_1$): Because your diode detector output might still have tiny, leftover traces of the high-frequency RF carrier wave riding on top of the audio, this circuit places a low-pass filter ($R_1 = 10\,\text{k}\Omega$ and $C_1 = 2.2\,\text{nF}$) right before Pin 2. This completely strips away any residual RF carrier noise so it doesn't overload or distort the LM386.
High Gain Setup (Pins 1 and 8): The $10\,\mu\text{F}$ capacitor connected between Pins 1 and 8 configures the LM386 for its maximum voltage gain of 200 (instead of its default internal gain of 20). This is excellent for AM radios, as the audio output coming out of a simple diode detector is typically very quiet.
Ferrite Bead
In this specific classic National Semiconductor application design, the output arrangement at Pin 5 is designed to kill parasitic VHF oscillations. The LM386 is prone to high-frequency instability, and the combination of the parallel $47\,\Omega$ resistor and the ferrite bead forms a lossy choke that dampens RF feedback before it can radiate down the speaker wires.
The exact specifications depend on how you choose to implement it:
1. The Original Vintage Flipped-Spec
In the original datasheets, this wasn't an off-the-shelf SMD component. It was a DIY part wound manually:
Core Material: Ferroxcube K5-001-001/3B (or an equivalent high-permeability MnZn/NiZn material designed for EMI suppression in the 1 MHz to 30 MHz range).
Winding: 3 to 5 turns of thin enameled copper wire (around 24 AWG to 28 AWG) threaded directly through the center of the bead.
2. Modern Equivalent Substitutes
If you are ordering a modern component from a supplier like Mouser or DigiKey, or assembling this physically on a PCB, you can use a fixed, off-the-shelf part. You want a high-current power ferrite bead with the following parameters:
Impedance: $60\,\Omega$ to $120\,\Omega$ at $100\,\text{MHz}$.
DC Resistance (DCR): Very low, ideally less than $0.1\,\Omega$, so it doesn't drop your audio power or get hot.
Current Rating: $500\,\text{mA}$ or higher (to comfortably handle the peak current traveling to your $8\,\Omega$ speaker).
Package Examples: An axial through-hole bead (like a Fair-Rite 2743001112) or a 1206 surface-mount power bead works perfectly.
3. The Popular "Hobbyist Shortcut"
Because specific vintage ferrite beads can be a hassle to source for small builds, you can build it using classic alternative approach that achieves the exact same electrical result:
Take a standard $1/2\text{-Watt, } 47\,\Omega$ through-hole resistor, and tightly wrap 20 to 30 turns of fine enameled wire directly around the resistor's body. Solder the ends of the wire directly to the resistor leads.
This creates a parallel inductor-resistor ($LR$) combination that blocks the high frequencies via the coil while dumping the remaining stray RF energy safely into the resistor.
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