Everyone wants to sound louder and those who are wise would like to do so without objectionable distortion or splatter. Reducing the carrier on AM to add swing is not enough to accomplish this goal. That causes negative peak cutoff with distortion and splatter whenever the negative peaks hit 100%. There is a simple way to remove the distortion and maintain the huge positive peaks on AM. Connect this $40 compressor-limiter inline and modify it for NPC COMPRESSOR LIMITER FOR FM TRANSMITTER BROADCAST | eBay (JPL-995). I was amazed how well this device worked. Even being placed in front of many coupling caps in the audio chain it was still able to create asymmetrical modulation quite easily. The trick is those two little LED's on the board. One controls the limiting point for the positive peak and the other controls the limiting point for the negative peak. By experimenting with different LED's with various voltage drops, you can tailor the trigger points for the positive and negative peaks independently! Hint, infrared LED's drop much less voltage then standard ones and they can be wired two in series.
This is a Hi-Fi device intended for FM broadcast and you will want to remove the jumper on the board to disable the pre-emphasis otherwise you will have too much treble. There are many onboard controls to adjust things like attack and release time. In addition there are independent controls for both the low end bass and hi end treble compression levels. The limiter section is followed by several poles of audio filtering to remove any distortion created by the clipping. When the board is properly set, it is simple to achieve 200% positive peaks with no RF cutoff on the negative peaks. Its response is very smooth and only the trained ear will detect the compression effect because there are no obvious signs of the typical pumping and sucking sounds some compressors produce when they operate. The end result is approximately twice the modulation with virtually no objectionalable distortion from the transmitter. Not bad for $40 and a little install time. However, it does require line level drive so it will need to be installed after some preamplification.
This is a Hi-Fi device intended for FM broadcast and you will want to remove the jumper on the board to disable the pre-emphasis otherwise you will have too much treble. There are many onboard controls to adjust things like attack and release time. In addition there are independent controls for both the low end bass and hi end treble compression levels. The limiter section is followed by several poles of audio filtering to remove any distortion created by the clipping. When the board is properly set, it is simple to achieve 200% positive peaks with no RF cutoff on the negative peaks. Its response is very smooth and only the trained ear will detect the compression effect because there are no obvious signs of the typical pumping and sucking sounds some compressors produce when they operate. The end result is approximately twice the modulation with virtually no objectionalable distortion from the transmitter. Not bad for $40 and a little install time. However, it does require line level drive so it will need to be installed after some preamplification.