Great Mic Sound for your CB or Export Radio-part 4

[Robb]

Previous article: http://www.worldwidedx.com/installa...itizens-band-radio-export-radio-part-3-a.html


Hooking it up

The cable between the microphone and the preamp requires a male XLR to female XLR. The female XLR will plug into the base of the microphone; the male end plugs in the mic input of the preamp. No modification needed.

I recommend that you get two 10 ft pre-made cables; they are relatively cheap and can be bought at most music stores that carry pro sound equipment. One will be for the mic/preamp connection intact/unchanged; the other we will now modify to use between the output of the preamp to the radio itself.


XLR cable from the preamp to the radio . . .

The second cable will now be modified. We only need the female XLR end of this cable to plug in the output of the preamp.

1) Remove the male XLR end; we won't need it.

2) Open up the female plug of this cable for a just a moment, and solder a jumper wire from Pin 3 to Pin 1 inside the XLR.

3) Leave the white wire on Pin 2 alone.

4) Re-assemble the female XLR connector.

FYI - What we have done, is turn this preamp-to-radio XLR cable into an unbalanced cable by joining the ground/shield to the negative wire of this cable. CB/Export radios only use two wires for their mic; so this modification made the cable go from a balanced to unbalanced state. The output of the preamp is balanced; and the radio has an unbalanced input. Don't worry; be happy . . .

We will now focus on the radio end of this cable. I had to take my cable apart so I can give the pinout for the radio's mic plug for the push-to-talk and the mic '+' and '-'.

Mic plug to radio:
Pin 1 - mic wire ' - ' and shield/shared with push to talk
Pin 2 - mic wire ' + '
Pin 3 - push to talk
Pin 4 - N/C
This is how the Heil cable is wired. This works on Galaxy, Ranger, and Magnum. Cobra will require a ground wire on Pin 4 for receive to work. I didn't hook it up to a Cobra, but that is a fair guess. The white wire on the cable will be your Pin 2 connection on the radio's mic plug, Pin 1 will be the shield and black wire. Pin 3 will be the push to talk along with using Pin 1 for its ground point. Of course, all of this is subject to the wiring on the radio you have. Just keep in mind that the white wire on the XLR cable coming from the mic preamp is the mic " + " wire to the radio.

You can wire this up to your favorite radio, so long as you can get the pinout for it and are willing to play with it in order to get it right.


The last episode for this project will be this Tuesday.
http://www.worldwidedx.com/installa...r-citizens-band-radio-export-radio-final.html




(Top picture) This is a Heil cable. If you don't want to build your own cable, you can call up Bob Heil and he will make one up for you that will work for what ever radio you want. Cost is about $40. Or - build your own for less than half the cost.

(Center picture) Look at the left side of this picture. You will need to use a 1/4 inch female plug wired into the plug end of the cable for the 'push-to-talk'. This will be needed to use a pre-made footswitch or handswitch from Heil or MFJ. On the right side of this picture, is the plug for the radio - notice that the 1/4 push-to-talk plug is wired into the mic plug as well.

(Bottom left picture) In case you need to have it, the wiring at either end of a XLR is the same for the mic to preamp cable.
1 - Shield/ground
2 - White/ ' + '
3 - Black/ ' - '

(Bottom right picture) This is the diagram of the layout of this project.

 

[loosecannon]

just wanted to point out that there are other ways of doing this.

the one i chose was to add an external audio input jack to the radio.

this is a 1/4" jack with a couple of resistors and a shielded cable going right to the input of the audio amplifier chip inside the radio.

then you just make up a simple switch box to key and unkey the radio from the mic jack.

this allows me to plug any microphone, CD player, electric guitar, etc... that i want, right into the audio amp of the radio.

when the mic is unkeyed, your voice, or whatever else, comes through the speaker, like talkback.
when you key the mic, it goes out over the airwaves.

im not trying to step on Robb's toes here or anything, just wanted to put out an option that has worked out very cool for me.
LC

 

[Robb]

It is better to bypass the entire preamp in the radio and use just the external preamp if you know how. I was hoping someone would pipe up and make some suggestions like this - thanks LC.

I have a lot more info to add to finish the project up. But please go ahead and add whatever you want. I'd appreciate it!

 

[CTStallion]

This is really GREAT.... I've been following all parts thus far and it really helps in bringing 'High Fidelity' audio to the Amateur & CB airwaves. It's always a pleasure to work someone who has nice, clear, brilliant and articulate audio; just makes the QSO that much more enjoyable. Because I'm an 'ICOM guy', it's often diffficult to find the right microphone. Fortunately, through trial-and-error, I've finally found an acceptable mix for the different rigs under different conditions (i.e., ragchewing vs. DXing vs. contesting vs Net Control duties.)

 

[Robb]

Update . . .

Been reading countless reviews on tube condenser mics. Today, I got my courage up and made a selection. I went to 'Guitar Center' (a 'shameless plug') and asked them if they would match 'Musician's Friend' (yet another shameless plug) for the price of this mic. They agreed, and sold it to me for $219 instead of $299. Which cinched the deal for me. So I pulled out my plastic and handed it to the nice man behind the counter. With tax, it came to ~$240.

What I bought is the MXL V69 Mogami Edition Tube Condenser Mic. It comes with an acoustic mic suspension, foam wind screen, the power supply, a special 7-pin Mogami cable (mic to power supply), a Mogami 3-pin XLR cable, and a nifty aluminum flight case. Plus - a three year warranty from MXL.

I hooked it all up and ran it into the Behringer 2200. But then, I realized there was a RCA 5-star 5751 tube in my tube collection that didn't have anything to do. So taking the mic apart became the next high point of the day. Never can keep my hands off a screwdriver - it seems. It came apart easily enough. Rolled the stock 12AT7 tube out carefully, and in went the 5751. The original 12AT7 sounded quite nice in the mic, and for recording - which I don't do any more - that would be leaving well enough alone. The stock tube has a nice bass roll-off, but the Mullard 4024 would be the real upgrade replacement giving it an overall better recording quality. But the GE 5 star 5751 gives it a real punch without losing the mics subtle qualities on the highs. The bass has been much improved without making the mic lose any quality whatsoever.

Got some audio reports, and everyone noticed the difference in my sound without me fishing for compliments. Not to mention, I put another tube back into the audio chain. I may update this as the week goes on, as I need to see how well it works out with DX. Clarity and punch were easily recognized over my favorite mic until this time, the MXL V88. The V69 has got more audio punch, and may well work out to be part of my audio Voodoo.

We will see . . .

 

[Robb]

A sightly better match between the radio input and the preamp output . . .

It is better to run a matching transformer between the radio input and the output of any preamp. This will allow for the signal to pass into the radio without allowing too much potentially damaging voltage int the radio's own mic preamp. You don't have to get/use an expensive Jensen-brand transformer; any 12:1 audio transformer will work. If a transformer is used; then the mic gain on the radio can be adjusted upward - rather than keeping it low in order to protect it.

It is far better still - to have a the mic preamp inside the radio bypassed - and use an external pro preamp. The difference and quality of sound will easily be noticed and recognized. A shop can do this for you, that isn't something that a beginner can do. They can put a plug on the back of the radio so that you can plug your external preamp right into the radio, as LooseCannon suggested above.

If you try to use a dynamic mic into the radio w/o using a preamp; it will work. But it just won't work very well. It will have really low output; so this is why a preamp is necessary to get real hifi audio. Pro mics work really well with a decent preamp and should not be avoided.


Graphic: The 'Bal Input' is where the output of the preamp would be. The 'UnBal' would go to the mic input of the radio plug. To use this arrangement with the transformer, you just use a stock XLR cable between the mic preamp and the transformer - that hasn't been modified - as previously mentioned in the article at the top of the page

 

[Captain Kilowatt]

This is all you need for a preamp Robb.

http://www.rdlnet.com/pdf/Data_Sheets/stm-1.pdf

or this http://www.rdlnet.com/pdf/Data_Sheets/stm-2.pdf


Radio Design Labs has GREAT audio products but they come at a price. What kind of price? Well lets just say that the broadcast industry uses them so ..................

I have a handfull of their STA-1 amps and a pair of STM-2 microphone preamps as well as a pair of older Altec Lansing mike preamps that I want to make use of in a future project. I want to build an interface box that will accept various mikes and feed various radios each of which is isolated from the other. That might get done this winter I don't know but hopefully it will get somewhat completed. Not really looking for HiFi audio but just decent quality audio.
BTW I don't know too much about the Jensen transformers you speak of. I use Hammond 800 series especially the 804, 812, 842, and the 850G series. Thier frequency responce is perfect and no ringing even when run up to their power limit, which nobody would ever want to do anyway in a microphone preamp application. They are meant to handle up to +30 dBm.

 

[Malcontent297]

I had a friend wire up my microphone set up and it works with one major exception. I now have talkback, which isn’t all that bad except to get the modulation up, the talkback is very loud. Loud enough to cause a serious feedback issue. The wiring instructions look to be for a 4 pin plug. My Cobra 142GTL has a 5 pin plug. Here’s a diagram of how it was wired. I’m using ¼” balanced and unbalanced audio plugs. The footswitch is a Boss FS6 connected with a ¼” TRS stereo cable. I’ll try to post a picture of my setup for clarity.

Pin| Radio | PreAmp |PTT
_________________________________
1 | Audio / White |X Tip |
2 | Shield |X Sleeve |
3 | RX / Black | |X Sleeve
4 | Common / Blue| |X Sleeve
5 | TX / Red | |X Tip & Ring

I’d like to add a toggle switch to defeat talkback whenever I don’t want to hear my voice (often).
I can't get the formatting for the diagram right so I attached a Word.doc file.

Thanks,

Malcontent

 

[Robb]

Are you using a linear amp?
Seems that it might be stray RF that is getting into some of your cables. I'm not sure that your wiriing is correct; but I think it is. I'm pretty sure you cannot get talkback in your radio by manipulating the mic wires. I would make my own 1/4 inch jack cables out of high quality cables that are RF approved...

If you have any ferrite clip-on beads, I would put them on the poorer cables and see if it doesn't change things. This may be an indicator of an RF leakage problem. A linear amp will only compound this condition - IMO.

Maybe someone else may know about this talkback dilemma.

 

[Malcontent297]

I get the talkback with or without the linear, even on my friend’s mic tester.
I think it must be something I have wired wrong. I really don’t think it has anything to do with stray RF since it does it on a mic tester where there is no RF. Maybe it’s time to take this to a good radio tech for review. I certainly don’t have the expertise to sort this out.

Malcontent

 

[GnG8d]

This will be needed to use a pre-made footswitch or handswitch from Heil or MFJ.

This is incidental and surely well know, but you can make a PTT out of just about anything. I made this one with a 2 cell AA flashlight, 3 miniature momentaries and some aluminum flashing. It's hooked to a mic and 2 different amps.

 

[tecnicoloco]

You might need this too

 

[CM-4441]

Very good info! I will be following your suggestions ASAP. Now...what about the mobiles!

Terry

 

[jazzsinger]

Very good info! I will be following your suggestions ASAP. Now...what about the mobiles!

Terry

Buy an astatic 575-m6 if you can find one,save you hundreds of dollars on all that studio shit and sound every bit as good if not better on almost any radio. it won't flatter the ego as much but will sound just dandy.

 

[CDX-007]

I've been recently amazed by the sound quality of the RF Limited 2018 Extreme, when the effects are set to <OFF> of course.

I have a 575 M6 and as good as it is, I like the sound of the 2018 better, probably due to it's condenser element.