Audio Limiter Circuits

Hello everyone and Happy Holidays from all of us at BellsCB.

     This month I thought I would do a little editorial about audio limiter circuits, their use and misuse, in CB radio and what it means to you.

     An Audio limiter is a circuit that senses the amount of modulation at the output of the radio. It is adjusted to output to a limiter transistor so it can reduce audio levels when over-modulation is about to occur. This is where the FCC would like to see it adjusted so that we don’t exceed 90% or so. A good technician can bring you up to 100-130% by making proper adjustments here.  Many people like to just remove the circuit all together and rely on their operator skills to keep a good limitation on the mic gain.

     The last of those options of coarse delivers the loudest possible audio but on many occasions there is some distortion to the waveform. This is where the operator has to be in good control of his station. He needs to keep good control on a 1:4 carrier-to-peak-power ratio and also a good track on his mic levels.  Since there are so many variables that change that control level the invention of automatic audio limitation circuits became very necessary. 

     Many times a CB radio is described as LOUD when a finer ear would say there is distortion causing a particular raspy sound…that many CBers interpret as a LOUD thing. When the signal is allowed to flat top and “BOX CAR” the waveform is clipped and has sharp edges and is non-sinusoidal. Some people like this, some don’t.

     That’s what some people like I suppose, but that’s not “Screwdriver Audio”.  My “so-called” secret is to allow the radio to exceed 100% to some degree (115% on average) so that it is loose and loud without restriction to available sound level.  Just enough level of AMC (Automatic Modulation Control) to keep the signal from flat-topping and causing unnecessary distortion. The best combination for me over many years of adjusting for many types of people is to set the radio up for about 120-125% modulation capable at maximum. Then I adjust the customers microphone and talking distance to produce approximately 95-110% final out of the radio.  Clean and loud without distortion.

STOCK Radio	Screwdriver Audio	Box Car Audio

     This can be a much larger topic, but I don’t want to bore you.  I would like to briefly mention that the distortion caused by signals at 150-300% modulation puts some of the radios energy wasted in spatter energy, on frequencies other than where you are talking, the actual watts on-channel will go down.  So an over-modulated waveform puts out less RMS (average) power and less power on-channel.

I hope I didn’t make this too long a topic and I didn’t bore you.  This is a heavily debated topic among Cbers. I have given you enough knowledge to decide for yourself what’s best for you.