[Elecraft] What happened to ALC?

[Bob McGraw]

I'll show my age by doing this, but lets revert mentally back to the 
Collins S Line.   The transceiver, using a pair of 6146's in the PA, was 
most efficient and linear when properly tuned and loaded.  It produced 
about 120 to 140 watts of PEP output.  And there was NO Power control or 
method to reduce the power as we know it today.   Introduce the 30L1 
linear amplifier, being 4 x 811's, which required about 60 to 80 watts 
of drive to reach maximum rated output.  So the issue now is, how do we 
get the properly tuned and loaded transceiver down to the correct drive 
level for the 30L1?   ALC is the Collins answer.  Thus the voltage 
determined by the amplifier grid current was fed back to the inter-stage 
of the driving transmitter to effectively reduce the output while still 
retaining the correct tuning and loading of the transceiver PA.

Today it seems that the designers have monkeyed around with the time 
constants in the ALC algorithm to improve certain aspects of the signal, 
thus reducing key clicks and overshoot,  but seemingly at the sacrifice 
of certain other areas of the signal.  Thus we find that ALC is not to 
be used to control power or raise the average power but designed to 
prevent over drive of any stage. Correctly, Speech Processing is deemed 
much more effectively used and with less undesired effects on speech 
than ALC.   In fact, one amp manufacturer says "run the transmitter at 
full power and let the Smart ALC in the amplifier adjust the required 
drive accordingly via the ALC signal".  Oh wow!  Certainly asking for 
trouble in my thinking.

In my world of audio, ALC is much like limiting with an input to output 
ratio of 20:1.  This says a 20 dB increase on the input causes only a 1 
dB change in output.  Now enter compression, where the input to output 
ratio is anywhere from 1:3 to about 10:3. Again a 10 dB input change 
yields a 3 dB output. Of course there are different attack times and 
release times applied to both limiting and compression.  Newer 
technology systems have the ability to analyze the various components of 
the input signal and adjust the attack and release times quite 
effectively.  Many Speech Processing algorithms have the same ability.  
Thus they are better suited for raising average PEP output.

A second factor existing is the issue where hams like to see their power 
meter indicate the rated transceiver power.  With some of the electronic 
power measuring devices that may work OK. However, with mechanical power 
indicating devices, the ballistics of the meter movement, meaning the 
slow response and overshoot damping, will not allow a mechanical meter 
to correctly show the energy level of ones SSB voice signal.  The 
typical mechanical movement shows about -6 dB less or 25 watts for a 100 
watt PEP signal. Of course speech processing will raise the average PEP 
level, but again the true PEP remains at 100 watts.

And as Paul Harvey would say; "and now you know the rest of the story".

73

Bob, K4TAX


On 1/31/2022 9:01 PM, elecraft-request at mailman.qth.net wrote:
> On Jan 30, 2022, at 1:10 PM, Fred Jensen<k6dgwnv at gmail.com>  wrote:
>
> I can remember, shortly after SSB appeared on the ham bands that "ALC" showed up and was immediately inducted into that top category of things like canned beer and sliced bread.  It would reduce the dynamic range of your signal and fill out your RF envelope with DX inducing energy.  It didn't matter if you preferred the James Earl Jones sound by close talking your microphone, or the Storm Drain sound with the gain up and speaking from across the room.  Not to worry, ALC will take care of it for you and you will fully pack your RF envelope.
>
> Tomorrow is the first day of February 2022 and ALC occupies a less than coveted spot in the same category that also contains last night's broccoli casserole.  One wonders why manufacturers even put connectors and meters labeled "ALC" on their radios.  "Four solid bars, and the fifth barely flashing because the fifth bar is where ALC starts.  Just barely flashing ... you DO NOT want to engage the ALC!"
>
> I realize that while we believe that the rules of electricity don't change and none of us ever inspect our electricity very closely ... if we did we'd find the power company is selling us used electricity as if it was new ... what changed?  If ALC was slicker than snot back then, why isn't it now?
>
> 73,
>
> Fred ["Skip"] K6DGW
> Sparks NV DM09dn
> Washoe County