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Ssb audio distorted when using amplifier

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  #9  
Old 02-13-2009, 11:18 AM
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Quote:
Originally Posted by TonyV225 View Post
Could be RF feeding back into audio section dirty amplifer but more info would help class of amp A/B biased?
Ok. If the amp has a SSB switch it is probably biased for that mode. I'm kinda with Tony up to this point. Sounds like an RF feedback issue which could be remedied with some proper grounding and a check on how good a ground you have on the antenna install. I had the same issue once in a previous mobile install and the extra grounding took care of the issue.
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  #10  
Old 02-13-2009, 11:27 AM
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Quote:
Originally Posted by EDUK8TR View Post
Ok. If the amp has a SSB switch it is probably biased for that mode. I'm kinda with Tony up to this point. Sounds like an RF feedback issue which could be remedied with some proper grounding and a check on how good a ground you have on the antenna install. I had the same issue once in a previous mobile install and the extra grounding took care of the issue.
the audio is perfect on fm and am with the amp, and it's also good on ssb without the amp switched on. I use a 1/2 wave basic vertical. Suggestions as to grounding my system would be appreciated. Would joining a wire from the bracket of my antenna into the garden help do you think? I'm willing to give anything a try to eliminate this problem. Cheers again.

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Old 02-13-2009, 11:57 AM
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First, let me preface this by saying that I'm not that great at reading schematics...

...but I just looked at the schematic for that amp and it looks like a class C amp to me. It's using FETs and I'm not very familiar with amplifier design with them. Perhaps someone else can look at the schematic who is better than me. The SSB switch on the schematic is clearly just switching a capacitor inline with the relay to keep the relay from chattering in SSB mode.

If it's a class C amp, you're going to have this problem on ssb. You still didn't say what radio you're using.

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Old 02-13-2009, 02:20 PM
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Quote:
Originally Posted by moleculo View Post
First, let me preface this by saying that I'm not that great at reading schematics...

...but I just looked at the schematic for that amp and it looks like a class C amp to me. It's using FETs and I'm not very familiar with amplifier design with them. Perhaps someone else can look at the schematic who is better than me. The SSB switch on the schematic is clearly just switching a capacitor inline with the relay to keep the relay from chattering in SSB mode.

If it's a class C amp, you're going to have this problem on ssb. You still didn't say what radio you're using.
cheers for the reply. I also noticed that all the ssb switch does is switch in a cap. I'm using a Superstar 3900 turned down to 4 watts on fm/4 watts pep on ssb. Can anybody else confirm that this amp is unsuitable by viewing the schematic please? Cheers.

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  #13  
Old 02-13-2009, 02:27 PM
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Is this a base or mobile? How are both units being powered? Running both on too small a power supply or the amp on small feed wire and a week battery can cause what you are seeing.

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  #14  
Old 02-13-2009, 03:22 PM
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Quote:
Originally Posted by 9C1Driver View Post
Is this a base or mobile? How are both units being powered? Running both on too small a power supply or the amp on small feed wire and a week battery can cause what you are seeing.
I'm base. I'm running the radio and burner from the same battery. And the power cable is rated at 10 amps. I'm beginning to think that the amp circuitry isn't designed for ssb, even though a ssb switch is on the unit. Have you had a look at the schematic?

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Old 02-13-2009, 05:10 PM
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I think you are drawing too much from the battery when the amp is in-line and turned on. If you starve an amplifier it will sound like crap. Buy a decent power supply and go that route. My money says thats your problem. I did a google search on your zetig amplifier, it has a SSB delay so it will work on SSB.

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  #16  
Old 02-14-2009, 12:07 AM
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Quote:
I did a google search on your zetig amplifier, it has a SSB delay so it will work on SSB.
Common CB myth: "my amp has a SSB delay, so it will work on SSB". No. The SSB delay on most of these amps is simply a capacitor that is switched in with the relay that prevents the relay from chattering because there is no carrier to keep the relay engaged between voice peaks. That alone does not make it suitable for SSB. For SSB, it's important that linearity is maintained which is nearly impossible with a class c amplfier inline.

I compared the schematic with one I found on the web for class c mosfet rf amplifier and the relevant sections look pretty much the same. I also looked at a schematic of an AB biased mosfet amp. Please understand that I'm not very versed with MOSFET PA design so I could be very wrong, but I can't see how that amp you have is much different than the class C design I found. It is possible that you have other issues like drawing too more current from a battery than it is capable of, but I doubt it. FM and AM modes typically draw more current than SSB does and you're not having problems with anything other than SSB. Everything you're saying points to a non-linear amplifier which should not be used on SSB. Sorry to be the bearer of bad news.

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