Then I started playing with the Transmit audio circuit capacitor values... you know, the basics of Audio electronics.. bigger bypass caps equals more bass...
And here it is today, as received on a bone stock Uniden Pro 510XL. The only processing done on the audio is from a Behringer VX-2000 Mic Preamp:
For a comparison reference, here is a comparison between me and my BME modified Cobra 29 and Coast To Coast AM's George Noory as sampled from a Local AM station:
Me and my Bastard 29 (Purple) vs George Noory on Coast to Coast (Green)...
So what is the point, you may ask?
Well, the CB band could be used for Pirate endeavors, if you want to deal with the idiots the roam the CB band...
or
as the Solar Cycle picks up, and 11 meter propagation improves, the 11 meter Shortwave Broadcast band is very under utilized... and wide open... and 11 meter shortwave frequency compatible CB's are not that hard to find...
11 meter International Shortwave Broadcast Band: 25.670–26.100 MHz
That's neat. The problem I have found with CBs is they are usually low level modulated and sometimes even a combo of low/hi level modulated causing all kinds of odd audio characteristics. One thing I tried once was taking an older CB and putting a modulation transformer between the output transistor and the positive rail to directly high level modulate it. The audio sounded really good just doing that but of course power output was dropped some because of the resistance the transformer caused.
Really it is not too hard to take an old CB crystal and juice it up with a 6F6 tube and plate modulate it like a simple oscillator transmitter. You would get around 5 watts carrier using that setup I think. Or use a CB as the exciter and modulate a PA amp.
The biggest problem is the bad audio circuits used in CBs. I remember putting an audio sign wave into a CB and monitoring the RF on a scope. You'd be amazed at how awful some CB circuits are at getting that same sign wave to appear on the RF unaltered.
Asymmetric audio helps a lot also to get those high positive peaks. Almost makes me wonder what an Orban optimod would sound like through a decent CB. You are right though, the CB bands are wide open and maybe if the sun decides to help out we could get some decent skip again.
Another quick tip, if you find an older CB that is crystal controlled usually there will be two sets of crystals that are mixed to generate the 27mhz frequency. If you pop out one of the crystals on the tuner section and replace it with a slightly lower frequency and retune the RF section to allow for it you can get it up on the international 25-26mhz shortwave band
Oh one more thing.. aren't most CBs 70% modulated? I recall reading somewhere that unmodified CBs are 4 watts carrier and 12 watts peak which is why SSB on CBs is 12 watts PEP. Maybe some were built to handle 100% modulation? I don't remember seeing one that could do the full 100% as is?
Post by Reverend Bow Voodoo on Mar 2, 2010 15:47:16 GMT -6
Nah.. most Cbs run about 95% from the factory, and it is a slight adjustment to get them to 100%
I ran into the same problem with this radio and teh modulation transformer, anything under 200Hz and the sine wave went to hell... so I pulled the transformer out.. completely, the modulate the driver/final with a darling transistor...
After that, it would modulated a 30Hz tone 100% with no problem...
Of course the carrier dropped a bit, to like 2 watts, but that is fine, it drives an amp great... the carrier is currently set and 0.75 watts...
Reviving an old thread but I did a few modifications to the CB I own for audio also..
This Teaberry "five by five" had tinny sounding audio on receive, and on transmit was telephone quality at best. I am sure most other stock CB radios are no different. The microphone it came with it sounded terrible. Using a broadcast quality microphone didn't make it sound that much better which astounded me. I swept the microphone jacks input directly with a sine generator and monitored the demodulated RF on the scope. Response was right around what I would expect for a 2 way radio being from 300hz to 3kHz at best. Anything outside of that range was poorly modulated or just plain distorted.
First I started with doing what the original poster did, I went through the audio circuit and replaced the capacitors with more beefy ones to allow passing lower frequencies. I also looked for places in circuit where it was cutting off high frequencies taking care to not mess with any RF blocking capacitors. This did very little for helping the audio quality.
Getting a bit frustrated I went full on and traced the signal entering the microphone jack right to the point where I noticed it was being distorted. This led me to the modulation transformers directly. The radio has two transformers, one acting as a primary transformer before the push pull modulator, and the secondary bigger transformer acting as the actual modulation transformer going out to the PA transistors. I removed the first transformers input legs and injected audio directly into it and found that it was indeed being saturated at low and high audio frequencies only leaving the mid range for voice only.
So I dug through my part box finding every transformer I could made for amplification input stages and found one that was 3x the size of the one the radio came with that was very similar for input/output ohms. Popped it in place temporary and voila! The audio was now really good sounding on air, covering all the way down to 100hz easily with almost no distortion or roll off. Below that point there is some distortion but almost inaudible and by far much more natural sounding. Best of all this also made the receiver in the radio sound fantastic! The audio coming out of the built in speaker was rich and full.
I swept the audio again and get a flat response from 100hz to 5kHz. With a tiny bit of EQing I can make the response flat out to 10kHz with no distortion, but of course since CB radio channels are only 10kHz apart that only leaves 5kHz of modulated bandwidth before it starts splattering onto channels next to the one operated on. I think most international shortwave stations only use 5kHz of audio bandwidth anyways.
So it is not too hard to get a CB setup for HIFI transmission by just replacing the smaller of the two modulation transformers, or if the unit only has one transformer replace it with a much larger transformer of the same impedances so that it can handle the broader audio range required for music or quality voice reproduction.
Best of all most CB radios use the modulator also as the receivers AF amplifier. This means making the radio sound good on transmit will also greatly improve the received audio!
The older radios like mine also don't take much effort to convert to the lower 11 meter international shortwave broadcast frequencies by simply replacing the mixing crystal with a lower frequency, but probably with slightly reduced power output. It may even be possible to get on the 13 meter band with some units but more than likely not worth the effort since this would render the radio useless as a regular CB transceiver again without a major tuneup. It would be cool to add a VFO to it and a toggle switch to flip between VFO use and regular CB crystal use.
It is sad though, the AM mode of CB is really wasted as far as quality audio goes, 80hz tkhz sounds 5khz so much better than the standard 300-4500 hz
Well you have to question what is wasted. If you are talking strictly voice communication then nothing is really lost by the small audio bandwidth. It can actually be beneficial when band conditions are terrible. On the other hand we have lots of people running high gain antennas or/and linear amps so fidelity rarely comes into signal penetration anymore.
The honest truth is and we all know this is that a narrow nasally voice will get through the muck long before a broad HIFI voice signal. This is why these radios were designed to limit the audio in the first place to provide the most bang for the buck on voice range.
You have to realize listening to channel 6 or 11 that when these HIFI CB operators get on board they rarely use that extra bandwidth for communication, but instead use it for playing songs to their friends, running sweeps of audio range, and general jacking around just to prove who has the flatter and more wide bandwidth without going outside the bounds of the channel.
This I see as a great thing as long as the principal of not splattering and using quality AM, but sometimes that narrow crap does come in handy lol.
Either way I am having a lot of fun running my radio wide open and using outboard processing to handle the limits of regulations while still maintaining AM broadcast quality
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