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

[ExitThirteen]

The bottleneck is the audio circuits. That's why direct injection makes a big difference. I thought about switching between direct injection for AM and then running the radio's audio circuits for SSB... just switching the audio path thru a toggle. Dunno if I'm gonna do that yet. With the MRF477 final I have installed, my current obstacle is beefing up the power supply... then I'll think about dabbling around more with direct injection.

~Cheers~

 

[Shockwave]

Direct injection is certainly one of the ways to achieve Hi-Fi and it can produce exceptional results. Although I've noticed some major differences between "outboard audio" and changing the audio caps in the AF stages. Having run the outboard modulating equipment in several setups before, I have to ask have you not noticed extra attention was required to keep RFI feedback out of the external mic pre-amps? That can be a particularly stubborn problem with AM.

The mic amp stages in CB radios almost always use discrete transistors that have their individual base to emitter junction RF bypassed with a ceramic cap. Most modern studio pre-amps and "export" radios use an IC chip as the pre-amp which presents a situation were you cannot bypass the many internal transistors used in high gain stages. You also have to be very careful about not creating any ground loops and providing the correct gain distribution between stages otherwise you get hum and inside noise.

You will need an external compressor / limiter in order to keep the peaks from flat topping or reaching RF cutoff. Modifying the internal audio circuits allows you to repurpose the original AMC circuit in the CB. While we often notice aggressive compression even when the stock circuit has its potentiometer wide open, it's very easy to increase the point at which compression begins. That will make the AMC circuit a bit more desirable to leave intact.

Perhaps the biggest difference is the fact that once you modify the internal AF stages you actually get to hear the improvement in your receiver where direct injection only allows you to transmit in Hi-Fi. For those who question others who have an interest in Hi-Fi, many of us see it as a diverse frontier for experimenting with the hobby. Once you've put the effort into the signal and antenna what do we have left to experiment with if your interest in radio goes beyond ragchewing?

Today there are so many different ways to achieve Hi-Fi from analog to digital that it allows people with different backgrounds to apply their knowledge in accomplishing the same goal. I'm somewhat amazed how a bunch of ones and zeros arranged in the correct order can now reproduce the same sound analog equipment has in the past. That opens the door for everyone from old school audiophiles to computer buffs to join in.

 

[Shockwave]

Exit 13, one last comment about SSB. Changing caps in the mic amp stages will make some improvement in bass response on SSB but no increase in treble or high end "brilliance". If you look at the service manual for the 8719 board you will see that AM audio and SSB audio take a very different path through the radio after the two mic pre-amp stages. SSB audio is switched to the balanced modulator. The output is mixed to obtain the 7.8 MHz. IF frequency where it's then passed through the crystal filter. That is the component that defines the SSB bandwidth on both RX and TX.

To complicate things a bit more, we also have the SSB carrier point alignment to deal with. You won't notice much improvement in bass unless you recenter the carrier point so that the extended bass now fits into the crystal filter passband. Since that bandwidth is determined by the filter, you are forced to give up some of the stock treble response to realize any improvement in the low end bass. If it's not one thing, it's another.

 

[ExitThirteen]

Shockwave,
Yep, I was aware that the audio takes a different path on SSB, and since you're going to be transmitting on technically narrower bandwidth, you're not going to gain as much from the cap modifications as much on SSB than you will on AM, since the audio goes into the balanced modulator stage instead of the audio IC. That's why I haven't gone too crazy with changing out/modifying all the caps in the mic preamp stages... just some minor treaks on a couple of caps. I do have a small increase in bass response on SSB, but a fair bit of that comes from the mic itself. Besides, I don't want to have too much bass on SSB, as that can make the SSB audio sound too bassy and flat, like you're running too much compression. That's why I am happy with what I have done currently with my setup.

~Cheers~

 

[Bullwinkle]

Forgive the necro-threading here but I got a couple things.

First, ribbon mics, even cheap ones, are wonderful on the human voice. I have a lovely old vintage RCA 77DX that just has a silky quality that's hard to beat. I'd dearly love to be able to use it on my station.

Which comes to part two. It is so sensitive, that it can't take the stray RF swirling around. I don't run a lot of power, and my feedline is well and properly choked, but that mic will squeal and yell and cause all sorts of oscillations in the preamp that my MD421 doesn't do. It's perfectly clean until I key up.

Another item most people don't consider when seeking to wideband an AM transmitter is all the additional sideband energy that is created. That means we've now created a whole new brand of splatter and "bleedover". Now I realize any concern for that went out the window decades ago, but if we're going to pursue better audio, we have to consider that also.

Using a mic that has better than communications grade response with some compression/limiting and a good impedance match will go miles toward improving an AM transmitter. And if the limiting can keep the negative modulation peaks from pinching off the carrier, it keeps the distortion and splatter down significantly and that will do wonders toward improving the sound. It just kills me to watch YouTube videos of some radio tuner screaming "auuuuuuuuuuuuudio" into the mic and looking at his scope and seeing the carrier be off more than it's on. It might "swing", but it sounds like crap.

 

[Shadetree Mechanic]

Bump

 

[Shadetree Mechanic]

Gotta bump this one too.