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Great Mic Sound for your CB or Export Radio-part 2

Discussion in 'General CB Services Discussion' started by Robb, Oct 29, 2010.

  1. Robb

    Robb Yup

    Dec 18, 2008
    Likes Received:
    Part 1 is here:

    When one wants to get the best broadcast audio sound possible out of their CB, it is time to understand all of the factors involved that will influence the outcome. We must also have a reference point to understand the components and why we choose them. You might already have some knowledge with mics and preamps; but if you don't we will go over what you need to know. The rest will just be a matter of making a few solder joints to hook it all up. This will involve a small amount of soldering and having a modest budget to complete and perhaps some materials you might already have lying around. The soldering will be necessary to fabricate one of the two cables. The budget can be expected to be anywhere from as little as $100 or up to $300 or more - depending if new or used items are purchased.

    Let's consider the transmit bandwidth of your radio...

    A local friend of mine was running an Icom radio that was using a very high quality mic (an ElectroVoice RE-27/ND - similar to the classic RE-20 - but improved) and external preamp (a Behringer UltraGain Pro 2200). The radio had a failure at one point, so he sent it in to be fixed. He then had to substitute a Galaxy/General Stonewall Jackson for his station in the mean time - he chose to use his same quality mic and external preamp to run this Galaxy radio. I was astonished that the Jackson sounded just as well as the Icom did. Wheels began to turn.

    WE must understand first - that a radio's ability to transmit does so in a narrow portion of what the human ear can hear. The human ear can hear between 20 cycles per second ('low frequencies tones') and up to nearly 20,000 cycles per second ('high frequency tones'). Most CB radios can only hear and transmit from 100cps to 3000cps of audio bandwidth. So we have very little to work with as far as transmit goes. Most of what we understand of speech is contained in this narrow margin; so we can maximize what we DO have to work with here. It is possible to make this narrow bandwidth sound astonishing.

    However, the midrange that exists can easily become too cluttered with an abundance of undesirable freq's that can mask its true potential. In effect, the voice will sound anything but natural. What is best to strive for, is an even or more flat response that can represent all of the available frequencies we have to work with! If one were to look at the frequency response of a stock Cobra mic, it would have a large peak in the middle and very little else being used of this 100-3000cps bandwidth spread. The result is transmitting a metallic, nasal sound that is unnatural. What does one expect from a mic that cost the mfr $.25 to produce? It works; and that is the best you can say for it. What we want to do is to make the best of this narrow zone. We can do this by maximizing our equipment used before it enters the mic plug. The radio itself has filters in place to limit this frequency width, but they can also be altered to provide and even wider bandwidth. More on that later after we get the basics down first.

    You can't get quality sound out of your radio without a minimum of 3kc wide on SSB. Many CB radios will provide this much width. Although it won't compare to some Ham rigs that can go as wide as 4kc or even 6kc's on SSB! For CB, the CB radio still can be greatly improved with the 3-3.5kc's it does have. The difference can become the difference between day and night. If we can make it sound great on SSB; just consider how much better it will sound on AM - or even FM for that matter . . .

    A pause for the cause . . .

    If you are already using a power mic on your CB, it is a simplified form of what we are going to expand upon here. They have a mic element, preamp, battery, case, cord, and switch. These mic elements are cheap; their freq response contains all kinds of peaks and valleys that will sound anything but natural from your transmitting radio. Their preamps are also very noisy, and this subtracts from the quality we want to give it. By the time most common power mic preamps get to their best gain curve, they are infested with harmonic distortion. Maybe even as much as 10 to 20%; there were few quality controls in place when built. They weren't built to be studio grade either - to be fair; they were built to work and to be manufactured at a low cost.

    Using a dedicated pro quality mic and a separate high grade preamp, the loss if valuable signal to distortion is minimized - and the advantage of a mic that more closely mimics true sound with a decent freq response nets a tremendous gain in audio transmit performance that can be realized by the CB radio enthusiast.

    The transmitted sound will change from a 'cheap squeaker' - to a 'chief speaker'.
    Our goal is simple to achieve and the cost and time are well spent to attain much in return.


    The microphone is the most essential player here. We have choices and price ranges that vary. We must understand how these mics work, and choose those will work best four our purposes. Technology today has made quality audio very affordable that was impossible ten years ago. In particular, the abundance of real condenser mics that require preamps can be found easily and at a surprising low cost. Some of these inexpensive condensers rival the high end $3000 mics - and with some homebrew modifications can nearly match them in performance. Not to bad for a mic that can be bought new for $200 and even found used for about $100! So we can afford to spend a little and get a lot. Even professional-grade dynamic mics can work if chosen properly, and can cost as little as $50 used from eBay if you know what to look for.

    I'm not going to go into the various polar patterns of mics - such as omni-directionals or figure 8/bi-directionals - we will instead focus on the cardioid directional type. These polar patterns of microphone pickup have little to do with operating a radio - except for the cardioid pattern mic. Cardioid pattern mics are built to be most sensitive in front of the mic, and then sensitivity then falls off sharply from side to side and none directly to the rear of the mic. If choosing either a condenser or dynamic mic; we will want a cardioid pattern type of mic.

    The 'proximity effect' is the term used to describe how a mic will change its frequency response when we get very close to the mic. Most every mic will have a stronger bass output as we get closer; and even out as we move away from it by a few inches. We will want to get about 1-3 inches away from the mic that we choose. Using a wind screen will help even out this distribution of detected frequencies and balance the bass and midrange frequencies while eliminating wind blast from out mouth. More on this later.

    There are three basic designs of microphones: condenser, dynamic, and ribbon . . .

    The ribbon mic uses a thin ribbon suspended between two points of a magnet then picked up by an inductor coil which delivers and adequate freq response - but is very fragile. It has traditionally been used for recording brass instruments - not very friendly to the human voice range. It lacks a freq curve that is harmonious to voice; the nasal sounds of the 1930's recordings is an example. Todays ribbon mic are far more hardy and are used for their ability to reproduce hight freq's with great resolve. One can use these mics; however they are used more often for female voices and instruments than for male voice characteristics. They tend to be strongest in the upper register of the spectrum, which we cannot use anyway. We need a mic that is flat between 20 to 3.5kc.

    The dynamic mic is essentially a moving voice coil and magnet - like a speaker - but is far more sensitive. Today's high-end dynamic mics incorporate rare earth metals - like neodymium - for better overall characteristics. They tend to be used most often in live performances because feedback is easier to control, have a high output, and they have a wide freq range as well. Their freq response curve is peaked at various points and will color its sound. There are many different brands with various response curves; some maximized for voice, instruments, or even both.

    A condenser microphone uses a charged diaphragm/capacitor that can detect very slight changes in in freq and volume. They require +48v to operate that is supplied by one of the preamps we will choose. They have an extremely wide and flat freq response and offer little to no coloration whatsoever. They are used extensively in the commercial broadcast and recording industry because of their ability to be extremely accurate. There are two basic types of condensers - large and small elements or 'capsule'. The large element capsule mics will have the specs that we want for optimizing sound in this project.

    The condenser and dynamic mic will be the mics of choice here for our purposes. For this article, I will be using and discussing both. But I will focus on the condenser, as this is the mic of choice for the most natural sounding audio available for the money spent. The cost of a high quality condenser mic is more affordable than ever, and this will work for you - the CB operator. We will discuss the options between the condenser and dynamic mic, and why one may be chosen over the other.

    There are two other type of mic elements - the crystal and the ceramic - but we are going well beyond them in this study. These elements are most commonly found in many of your favorite power mics in the past. Such as the Astatic D-104 Silver Eagle with the crystal element, and the Turner +3 that has a ceramic element. These mics of the past were built and sold for convenience to the consumer utilizing a cheap mic element and a simple but noisy preamp. We like them and use them because they work much better than a stock mic; but we will be doing here with this project will prove to be so much better in every way.

    Tomorrow we will cover the basics of mic preamps and then choosing a mic and preamp:

    Pictured: Ribbon mic, Dynamic mic, Condenser mic, and the condenser element itself . . .

    Attached Files:

  2. unit_399

    unit_399 EL CAPO

    Jun 17, 2008
    Likes Received:
    ROBB -

    Yes there are two sidebands on AM. However, the lower sideband is a "mirror image" of the upper sideband, and they each contain exactly the same audio information. So there will actually be no difference in audio content between AM and SSB, and the available bandwidth for audio will be the same for both modes...approx 3.5khz. Since human voice frequency range is from 300 to 3000 hz, this is all that is really required from a stock mike. Using a replacement mike, like the ones you pictured, sounds better for two reasons:

    1. The microphone's response curve is linear, so the voice reproduction is much more accurate than with a stock mike.

    2. The wide frequency response allows the mike to pick up the harmonics of the speaker's voice. Even though they are attenuated by the 3.5khz bandwidth of the transceiver, there is still enough there to make the audio signal richer and more natural-sounding.

    - 399
  3. Robb

    Robb Yup

    Dec 18, 2008
    Likes Received:
    Got some info wrong.
    Thanks for straightening that out.

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