Audio reduction doesn't work either but for a different reason. The audio
waveform only has a slight resemblance of the RF waveform in an SSB
transmitter. Audio limiting works well on AM with both side bands but does
not work well on SSB.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Rob Atkinson
> Sent: Sunday, January 31, 2010 10:16 AM
> To: amps@contesting.com
> Subject: Re: [Amps] Good amp to buy (or something like that)
>
> I'm afraid I must disagree in some cases. I know nothing about the
> Icom ALC design but I'll be willing to bet it is typical of modern ham
> SSB transceiver design attempts at a cheap way of preventing power
> output excursions above some front panel set limit.
>
> With modern SSB transmitters what is driving all the stages up to and
> including the PA is the AF current from the audio source coming into
> the rig. Designers usually put some kind of high vswr protection on
> the PA output so the transistors are not damaged by high vswr. This
> is often done with a reflected power coupler on the PA output that
> produces some control voltage that is fed back to a low level voltage
> controlled amplifier at some IF stage, 455 KHz perhaps, that performs
> a gain reduction to fold back drive to the PA and reduce output to
> protect the final transistors. This design works pretty well. It
> doesn't cost much to add a forward power coupler engineered to produce
> a certain voltage level such that with a certain setting of the power
> out pot (PA collector current pot) that voltage fed back to the VCA at
> a previous IF stage, performs the needed gain reduction to hold the PA
> to some set forward power limit.
>
> Obviously, while this is great for VSWR protection, it is a really bad
> way to limit forward power output peaks and spikes for the simple
> reason that once that horse is out the barnyard gate, it's too late to
> close it. Most ALC circuits employ some delay time constants so once
> a power spike is sensed and a gain reduction voltage fed back to the
> VCA it holds for a few seconds and continues to be effective if there
> is high average audio driving the RF but don't count on placing your
> SSB rig in transmit, turning your PA power pot to 100 w. and expect
> that not very well thought out design to trap every initial breath
> blast or sudden impulsive sound force at 100 w. Once that spike is
> out, it's on the way to the amp and everyone in radio land and your
> ALC can say "oopsie" and send a gain reduction v. back to the VCA and
> deal with any other pops and blasts for a few seconds but that first
> one has left the station. And far too many get out the gate in
> typical operation to make ALC something to depend on for power output
> limiting.
>
> So, since ALC is a bad scheme to rely on for peak power limiting.
> What to do? A far better solution is to employ a device that
> completely controls audio level peaks being fed into the ham rig as is
> done in broadcasting, so ALC is not even necessary and does not have
> to intervene. Why isn't this done? Because a zero attack time
> extremely fast gain reduction limiter on the audio input (i.e. mic
> jack) isn't cheap. And hams who do not understand how ALC works and
> realize that it is inferior as far as what it is supposed to do is
> concerned, won't want to pay for something that gets the job done.
> So, we're left with a sort of half assed method and too many hams
> overdriving their amps (those cheap s.s. ones are the worst)
> unfortunately.
>
> 73
>
> Rob
> K5UJ
>
>
> <<<<There are two kinds of ALC and I think Roger is talking about the
> kind
> of ALC that is fed back from the amplifier to the transceiver. With
> that kind, I fully agree with the above.
>
> But there is also ALC that is generated entirely within the
> transceiver (such as my IC-756Pro3), and is completely independent of
> the amplifier. With that kind you can safely use ALC to limit the
> drive and it does an excellent job, I might add.
>
> And if you use the internal type of ALC, you don't need the fed-back
> kind at all.
>
> 73, Bill W6WRT>>>>
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