TL074CN High-Order Low-Pass Filter 2.8, 5, 7 kHz for AM Transmitter, Receiver, Transceiver
The low-pass filter turned out great! It's easy to replicate, no PCB required! It's stable and very low-noise! The main conditions for good operation are:
Power supply no lower than 9V, 12V is ideal.
Input an audio signal no higher than 1.5V, 1V is ideal.
The key is to select the capacitors and resistors very precisely, according to the schematic. This is the most labor-intensive process. But it's completely worth it.
We purchased the original microchips from our reliable supplier: https://alii.pub/6xbvu2 for about $4.50 for 10. Send him a private message, and he'll give you the link. If you find them cheaper elsewhere, they're guaranteed to be counterfeits, so don't get caught!
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You can order high-quality, original parts from LCSC and Mouser through this seller: https://alii.pub/74antq?erid=2SDnjc4JtPd Send him a private message, then an email, in English. Specify the exact name of the radio part from LCSC or Mouser, the manufacturer of the radio part, and the required quantity. If you don't see the part you need, try contacting this seller: https://alii.pub/6xbvu2 Both sellers are verified and reliable.
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AliExpress may block direct links to products. You'll see the message "There is no such page," but the product is still there. Send the seller a private message, and they'll give you a link to purchase.