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Experimental Audio Mod: Cobra 2000 Covertered to Low Level AM

cbphreaker

cbphreaker

Ganbatte Kudasai "Success depends your effort"
Feb 26, 2016
49
58
28
I had a wonderful time this weekend messing about with my 2000 last weekend and just about finalized the conversion to Low Level AM with the addition of a new Jfet audio buffer amp (see attached pic). A few other misc items need to completed but none pertaining to TX audio (I think anyway).

Here is the setup:

Transmit:
Cobra 2000, Jfet audio buffer between the computer sound card and the balanced modulator.
A Modest PC running Ubuntu Studio - using Calf Plugins and JackD
An Audio Clip

Receive:
Cobra 148 with a precision diode detector as the receiver and Line-Out direct from the detector to the recording PC.

Other Stuff:
DSO-2250 PC based Oscilloscope
Another old PC for recording audio and using the o'scope.
Camera to take video.

The audio clip is from the Motor Mouth Maul talking on his modulator with a Shure SM58 mic. Since my D104 is no match against a studio mic, I used it as the audio source for this test. This audio clip is freely available on his website. I needed a nice sounding audio clip and it fit the bill.

Anyway, you can judge for yourself if you think this a worth while mod after checking out the video; complete with recovered audio.

What really makes me happy is to completely remove and retire the use of the TA7222 as it's services are no longer required and all the bandwidth robbing associated passive parts can kick bricks.


 

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I couldn't help myself so I had to mod the mod. I was looking to get more peaks so I added a much more precise and independent power control circuit.

Here is the result.


I was thinking of making a web site with way more details. So all can enjoy modding at their leisure . It would be all 100% free. Do you think there would be any interest in something like that?
 
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cbphreaker said:
I was thinking of making a web site with way more details. So all can enjoy modding at their leisure . It would be all 100% free. Do you think there would be any interest in something like that?
Click to expand...

There's a guy in my town named Tim that would love for you to post all of your technical information for free.
 
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Personally I think anything over 100% modulation is a waste of time.
Even though it's possible to do 300% modulation on the positive Peaks and 95% modulation on the negative Peaks it really doesn't serve any useful purpose. I'm not too Keen on the quality of that audio because it's actually kind of distorting the sound of John's voice. I actually like the sound of this much better. Not to mention it's a more efficient method of transmitting
 
High level modulation is indeed a more efficient method since the RF stages can be operated class C with the modulation applied to the final stage. This is why plate modulation was used in tube type AM broadcast transmitters. Efficiency means money.If the modulation is applied to a low level RF stage all RF stages following it must be class AB or B in order to remain linear. I hope the OP changed the bias settings on the finals in his Cobra to reflect this otherwise it is just another class C splatterbox.
 
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rabbiporkchop said:
Personally I think anything over 100% modulation is a waste of time.
Even though it's possible to do 300% modulation on the positive Peaks and 95% modulation on the negative Peaks it really doesn't serve any useful purpose.
Click to expand...

For once, I'm forced to agree with rabbiporkslice. Anything over 100% modulation IS a waste of time. I'm not dissing the work that phreaker is doing, because I think any successful experimentation serves a useful purpose. Trouble is, most of these mods only work on AM, and really nothing he's doing is new. Simply proven AM broadcasting techniques applied to a cb. If his work was in Essb, I would gobble it up, because (let's face it) ssb is where the action is these days. Ever since I switched to using ssb exclusively, my "Ancient Mary" equipment sits in a corner and draws dust.

- 399
 
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With all the videos I have heard of MMM, I never think he sounds good. Overmodulated and scratchy, IMHO. I am all for experimenting though!

I know this clip is not c.b., but to me, these guys have great AM audio, and is what I would be shooting for if I was experimenting. Watch the positive peaks on the meter, and refer to Captain Kilowatt's post. :)

73,
Brett

 
Don't know if it's truth or CB Voodoo but back in the day the better sounding old timers around here told me that with the right mods you can put out a clean signal with some pretty high positive peaks but most receivers couldn't demodulate anything over about 125% modulation without adding its own distortion to it, they recommended no more than about a 5:1 peak to DK ratio even with asymmetric modulation.


PS: those REA monitors look really nice.
 
Last edited:
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unit_399 said:
YUP !! It's ssb for me. And, hey, we grow coffee on our farm. You have no idea how good coffee can be until you drink some made from fresh roasted beans only a week out of the field.

- 399
Click to expand...

Yes I do. :D Had some Jamaican Blue Mountain coffee, the REAL stuff, while on a bicycle tour in the Blue Mountains. Fresh roasted and ground the traditional way with a wooden club in a hollowed out stump. :D Sadly the best we could do after that was pay $35 USD a pound to bring some back home. :(
 
StrangeBrew said:
Don't know if it's truth or CB Voodoo but back in the day the better sounding old timers around here told me that with the right mods you can put out a clean signal with some pretty high positive peaks but most receivers couldn't demodulate anything over about 125% modulation without adding its own distortion to it, they recommended no more than about a 5:1 peak to DK ratio even with asymmetric modulation.


PS: those REA monitors look really nice.
Click to expand...

Commercial AM broadcast station generally run positive peaks of 120-125%.
 
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2RT307 said:
With all the videos I have heard of MMM, I never think he sounds good. Overmodulated and scratchy, IMHO. I am all for experimenting though!

I know this clip is not c.b., but to me, these guys have great AM audio, and is what I would be shooting for if I was experimenting. Watch the positive peaks on the meter, and refer to Captain Kilowatt's post. :)

73,
Brett

Click to expand...


Don't tell me. Let me guess. 3885 AM right?All you needed in the mix was WA1-H-Yell-R. LOL

Really nice sounding plate modulated rigs there although some are going class E stuff now.
 

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