Bad audio coming out of TRC-449, President Jackson, and President AR144 using 203P amp in SSB.

[doffo]

Been wondering what is the cause of the headaches. I have caught onto the fact that when I use either of the 3 radios I listed on the title of the thread, the audio becomes really bad sounding. TRC-449 or my Cobra 138 XLR, the audio sounds fantastic without the amp, but the moment you turn on the amp, the peaks sound crunchy and overall makes the quality bad. Using the President Jackson and the President AR144, the audio just massively distorts near its peaks with the amp on. Otherwise with the help of a power mic, the audio sounds greats. Without the power mic both you basically need to yell into the microphone of those two president radios.

Long story short, the only radio so far that sounds fantastic and great is when I have my Cobra 146 GTL radio connected to that amp. I used the same cables and microphones on each radio. I kept the SSB power on the other radios at about 10 watts and made sure the bias on the driver/final were good. Otherwise, I am stumped. I will need to try out a Uniden Grant XL radio to see if its doing the same as the rest of them. I am listening in on a 2nd radio when I hear the audio with the amp and amp off. The RF Gain is backed down as so it doesn't overdrive the receive.

Any help or at least some guidance would be appreciated. I am using a dipole antenna and a 30 amp power supply. Coax up to the dipole is some LMR-400.

 

[doffo]

To rule out the possibility of the amp going bad, I tried a Cobra 150 (Just a rebadged Palomar 250 amp), and it too behaves the same way on the audio sounding bad.

 

[BC Coyote]

The amp is running class C, which works fine for AM or FM but makes SSB sound like crap. This is due to the fact that AM and FM modes have a constant carrier which keeps the inputs of the transistors in the amp turned on. In SSB there is no carrier, and therefore no steady current to keep the transistor turned on.

The majority (but not all) cheap CB amps are built class C because it uses less parts and is therefore cheaper. Same reason they don't bother with a LP filter.
Don't be fooled by the "SSB" switch. All that does is switch in a little electrolytic cap that stops the amp from chattering.

There are two possible solutions.... You can buy a proper amp that's biased class A/B or some other class that is suitable for SSB operation.
Or you can do what I did and add a bias circuit to the amp so it is operating in class A/B. This was fairly easy to do with the old style transistor amps such as Palomar and all the others. I haven't done it with an RM Italy amp but the process should be more or less than same.

The information on how to do it is out there on the internet. I would strongly recommend reading up on RF amp theory first.........good luck !

 

[nomadradio]

Quick way to tell if you have a bias issue with an amplifier. The radio needs to have a mike gain control. You'll need a second radio to monitor the audio quality. Turn the mike gain down until the amplifier won't key, then just crack it up so the amplifier stays keyed. Compare the audio quality to how it sounds with the mike gain turned back up to a normal level. If it sounds less nasty turned up and raspier turned down you have a bias problem. And if the amplifier was built without that feature, the lower you set the mike gain, the rougher it will sounds.

73

 

[sp5it]

The amp is running class C

No. It is biased amp.
Mike

 

[doffo]

I do appreciate the information so far. I just found it strange that on the one radio the Cobra 146 GTL, the audio comes out great out of the amp on SSB. Now that I think about it, remember that day BC Coyote when you said "That last radio you were talking on sounded much better." the other week we were talking? That was me using that same exact 146 GTL on the amps in question.

I will give a try what nomad said though about adjusting the mic gain and back it off just enough, and see how it sounds on what its doing to the audio.

 

[LarryShaZam]

High Larry here. In most cases when the transmit audio sounds low and r distorted. This means you have too much dead key. (carrier) Some amps have to be driven with no more that 1 watt carrier. It depends on the amp some are High drive. Which can take a lot more drive. Hope this helps.

 

[doffo]

High Larry here. In most cases when the transmit audio sounds low and r distorted. This means you have too much dead key. (carrier) Some amps have to be driven with no more that 1 watt carrier. It depends on the amp some are High drive. Which can take a lot more drive. Hope this helps.

For sure. I do keep it around 1.5-1.75 watts on AM for a carrier, and try not to do more than 10 watts on SSB. Need to still find that grant XL and try it again on SSB to see how it sounds.