From: Jim Thomson
Sent: Monday, February 01, 2010 1:40 AM
To: amps@contesting.com
Subject: ALC/peak limiting/ etc.
From: Rob Atkinson <ranchorobbo@gmail.com>
Subject: Re: [Amps] Good amp to buy (or something like that)
To: garyschafer@comcast.net
Cc: amps@contesting.com
Message-ID:
<d2bb1cb81001311117k21a87e4ema589fc76fed27b22@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@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
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