[Amps] Fw: ALC/peak limiting/ etc.

[Jim Thomson]

From: Jim Thomson 
Sent: Monday, February 01, 2010 1:40 AM
To: amps at contesting.com 
Subject: ALC/peak limiting/ etc. 


From: Rob Atkinson <ranchorobbo at gmail.com>
Subject: Re: [Amps] Good amp to buy (or something like that)
To: garyschafer at comcast.net
Cc: amps at contesting.com
Message-ID:
<d2bb1cb81001311117k21a87e4ema589fc76fed27b22 at mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

I have performed tight peak limiting on audio operating SSB and find
it works well and the ALC is not needed. 

## agreed,  I can EASILY  get  1.5 kw out... with NO ALC action showing
on the MK-V at all.   I use a  [A] leveler  [B] compressor  [C] split band
limiter  [D]  distortion cancelled clipper.    You can also get loads of
clean talk power to boot.     If  I want 1485 watts.. and no more, I can
achieve that as well.  Or any other power level, and in no instance, does 
the ALC meter ever lift off the bottom.   

##  When I look at  TV audio.. on an audio spectrum analyzer... before,
and during TV commercials... here is the results.  During commercials,
PEAK output [ 20 hz - 20k, both channels]  DROPS .5 db  
AVERAGE power INCREASES  by exactly 6.0 db     

##  when they mean peak... they mean  2 x RMS, when dealing with audio.
100 w light bulb = 200 w peak... that kind of peak.... [ and not pep]  
A  1000 hz  tone  will result in the peak being 3db more than the average
on any audio spectrum analyzer. 


73 rob K5UJ

On Sun, Jan 31, 2010 at 11:19 AM, Gary Schafer <garyschafer at comcast.net> wrote:
> 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.

## what?   RF  is just HF AC.   The  audio baseband has been translated to say.... 20m. 


 Audio limiting works well on AM with both side bands but does
> not work well on SSB.

##  Audio limiting works extremely well on ssb.  The typ peak limiter
will respond < 1 micosecond.  The typ clipper is brickwall. 

##  a 2nd  method to solve the ALC  problem is to inject  a LOW value dc voltage onto 
the ALC buss, on the back of the xcvr.   That will totally eliminate the.. 'closing the barn door
after the horse has bolted'  problems.  Apply  9 vdc  to the ends of a 50 k pot. Pick off
the voltage between  one end and  wiper.. and apply to alc jack on xcvr.  Note, the xcvr
wants to see  neg Vdc on the center pin of the rca  alc connector. 

##  At this point, just adjust the 50 k pot  till you get the desired power output on a pep
meter. [ this scheme will also work on xcvr's, like the TS-430, that DON'T have a power
output control on ssb mode.] 

##  tweak the pot for say... 58 w pep... and scream all u want.   U get no more than 58 w pep. 

##  now here is the only ..'drawback'.   Whisper into the mic, or lower ur voice.. and u will only get
0-40 w pep !   ALC is just RF compression.   In this case, we have fixed the alc  at a set value. 

## The workaround fix for the above is to use an audio  compressor.   The compressor
will ensure a constant audio drive level into the xcvr at all times.   To stop the compressor from
picking up room noise, fans, blower's etc.. the compressor / limiter is preceded with a 
noise gate [ high ratio downward expander]  or a downward expander.  

##  If u want a cheap and dirty audio clipper, a pair of RVS connected diodes will suffice,  and insert
right after the normal compressor /limiter set up.   Just don't drive it into clipping too much. 
Quad transistor's  make for better audio clipper's.  2 x for  pos... and 2 x  for neg. 

##  BTW... I checked a bunch of schematics  for several xcvr's and older stuff.. like T4XC's etc. 
There is only ONE ALC circuit in the xcvr.   The ALC  voltage coming from any linear amp is simply
fed into the alc buss.. via the  alc jack on rear panel of xcvr. IF the  alc voltage from the amp is greater
than what's being developed in the xcvr, the linear's alc voltage  will over ride the xcvr's. ....
and throttle the xcvr back. 

later........ Jim  VE7RF 




>
> 73
> Gary ?K4FMX