A question about basic audio processing

[cjruger]

I'm reviving this thread 1 year later, i have another thread going about my progress with using a shure sm58 mic into the mic jack on my 2950 base station. I am runnig it thru a guitar preamp on my peavey 112 acoustic amp, specs for the preamp are here:
http://assets.peavey.com/literature/manuals/80300298.pdf

So now I have the set up going, the 10 11 mete band has been dead for a few days now and have 1 report form a guy on base 60 miles from me and he says its sounds fantastic. Ill take that for what its worth.
Now that i have my mic and preamp set up and going i want to start looking at what can be done to the radio to pass wider bandwidth to get all that sound thru the radio. Is it a matter of changing some caps and be done with it or im guessing the radio will need to be aligned and tuned after any of the mods are done. Is this somehting i can send out and have done?
im running a rci 2950dx also have a older cobra 29 gtl to play with. I always get comments on how great the audio is on the 2950 running just a stock mic from my galaxy dx949.

side question, is the Ranger 2950dx 's bandwidth any wider stock than other cb's or comparable radios. or are they all about the same? I got the radio used and the guy said there was no mods to it but I 'm not sure if i believe him.

 

[GnG8d]

The 2950 is surface mount, correct? If so I wouldn't mess with it, the 29 might be the better option.

 

[Robb]

Making the audio sound better requires changes in the mic as well as some changes in the radio's circuitry. It involves changing out capacitors in the mic audio circuitry. The other approach is to directly inject the mic audio into the balanced modulator, Each radio chassis has different group of caps than the next radio chassis. Just depends how far you want to go with it, I have done receive and transmit audio mods on 148 GTL, Grant, and Galaxy 959 with outstanding results in all of them.

 

[unit248]

For the record, for 11 meter rigs, a balanced modulator is only used for SSB modes. The Cobra 148GTL in particular uses a different modulation scheme for AM. (Several ham radios do use a balanced modulator for both SSB and AM, but not CB or export rigs.)

Improving audio response boils down to two things: more bass and more treble. The limits on them come from different places.

Bass is usually limited by audio circuitry, like the microphone preamp. As has been mentioned previously, it's not unusual to see the bass roll off anywhere from 100Hz to 300Hz. This can often be overcome by replacing some capacitors. Small capacitors in series with the audio path will limit the bass response. If you see a .1uF cap, you should probably replace it with a 1uF or 4.7uF one instead. But it takes time to go through the audio chain and identify the weak spots.

Treble is usually limited at the RF level, not the audio level. Depending on the transmitter design, your signal may be passed through a filter to limit its width, to ensure that you stay within your allotted bandwidth and don't splatter someone on an adjacent channel. The filters often have a 3KHz bandwidth for AM, and that places the upper limit on your treble range. Ideally you should replace the filter with a wider one, but that can be hard to do since you need designed for the same center frequency as the existing one. Some people just bypass the filters completely, but that has two caveats: the same filter is typically used for the receiver as well, which means if you bypass it completely you could be opening yourself up to hearing more splatter yourself, and without the filter you could widen your bandwidth so much that you'll start splattering others.

Some of the direct injection techniques involve applying your audio to the RF driver stage in the transmitter, which is a quick way to avoid both of these problems. You can get some very wideband audio this way. There are documented mods for the Cobra 148GTL and 2000GTL for this. Note however that these are effective for AM only, not SSB. (There are some additional considerations for sideband.)

At the end of the day though, you'll only sound as good as the receiver on the other end will let you. You can set yourself up with beautiful broadcast quality audio with lots of nice resonant lows and crisp highs, but if you're talking to someone who's on an old Kraco rig they bought off eBay for $2, they won't be able to appreciate it. I exaggerate a bit, but my point is that most 11 meter radios are going to be limited to about 3KHz of bandwidth on receive anyway, so if you go beyond that, most people won't be able to appreciate it. There are some Hifi aficionados out there with wideband receivers and if you talk to them, then great: you can pull out all the stops. But you may need to maintain a separate audio profile for everyone else.

In general, resist the urge lean too heavily on the bass. Without enough treble to balance it out, it may end up sounding muddy or "woofy." A lot of this also depends on your voice; in my case I find I need to add a cut right around 80Hz with my EQ otherwise it the bass is a little too fat.

-Bill, unit248

 

[Robb]

I've replaced the stock 455khz filter for a hifi filter unit (get it at Mouser Electronics for $3-4/ea) with outstanding results/improvements for AM. Worth every penny! Done it to a couple of 148's (including my daily use 148 radio) and included the cap mods for RX and TX. The cap mods will affect the higher freq tones to a degree. But using the hifi filter with the cap mods will work pretty decently. If you want better TX SSB audio; then direct injection into the balanced modulator with ProTools, computer, and decent mic is always a solid option.

 

[cjruger]

For the record, for 11 meter rigs, a balanced modulator is only used for SSB modes. The Cobra 148GTL in particular uses a different modulation scheme for AM. (Several ham radios do use a balanced modulator for both SSB and AM, but not CB or export rigs.)

Improving audio response boils down to two things: more bass and more treble. The limits on them come from different places.

Bass is usually limited by audio circuitry, like the microphone preamp. As has been mentioned previously, it's not unusual to see the bass roll off anywhere from 100Hz to 300Hz. This can often be overcome by replacing some capacitors. Small capacitors in series with the audio path will limit the bass response. If you see a .1uF cap, you should probably replace it with a 1uF or 4.7uF one instead. But it takes time to go through the audio chain and identify the weak spots.

Treble is usually limited at the RF level, not the audio level. Depending on the transmitter design, your signal may be passed through a filter to limit its width, to ensure that you stay within your allotted bandwidth and don't splatter someone on an adjacent channel. The filters often have a 3KHz bandwidth for AM, and that places the upper limit on your treble range. Ideally you should replace the filter with a wider one, but that can be hard to do since you need designed for the same center frequency as the existing one. Some people just bypass the filters completely, but that has two caveats: the same filter is typically used for the receiver as well, which means if you bypass it completely you could be opening yourself up to hearing more splatter yourself, and without the filter you could widen your bandwidth so much that you'll start splattering others.

Some of the direct injection techniques involve applying your audio to the RF driver stage in the transmitter, which is a quick way to avoid both of these problems. You can get some very wideband audio this way. There are documented mods for the Cobra 148GTL and 2000GTL for this. Note however that these are effective for AM only, not SSB. (There are some additional considerations for sideband.)

At the end of the day though, you'll only sound as good as the receiver on the other end will let you. You can set yourself up with beautiful broadcast quality audio with lots of nice resonant lows and crisp highs, but if you're talking to someone who's on an old Kraco rig they bought off eBay for $2, they won't be able to appreciate it. I exaggerate a bit, but my point is that most 11 meter radios are going to be limited to about 3KHz of bandwidth on receive anyway, so if you go beyond that, most people won't be able to appreciate it. There are some Hifi aficionados out there with wideband receivers and if you talk to them, then great: you can pull out all the stops. But you may need to maintain a separate audio profile for everyone else.

In general, resist the urge lean too heavily on the bass. Without enough treble to balance it out, it may end up sounding muddy or "woofy." A lot of this also depends on your voice; in my case I find I need to add a cut right around 80Hz with my EQ otherwise it the bass is a little too fat.

-Bill, unit248

Thanks for the info, I'm not looking to get Crazy with this. Just some basic simple improvements for now, maybe more when I'm ready

 

[GnG8d]

Thanks for the info, I'm not looking to get Crazy with this. Just some basic simple improvements for now, maybe more when I'm ready

I've got a new 929 I'd sell ya. The TX/RX caps have been changed out and it's tuned up ready to go. It's not as fat as a direct injected 148, but wide enough to utilize some processing gear.

 

[cjruger]

no thanks,