[RFI] Is a low pass filter necessary?

Roger (K8RI) k8ri at rogerhalstead.com
Fri May 11 13:21:51 PDT 2012

On 5/11/2012 2:18 PM, Roger (K8RI) wrote:
> On 5/11/2012 12:20 PM, Jim Brown wrote:
>> On 5/11/2012 7:02 AM, John G. wrote:
>>> I am using one of the Drake TV-1000LP right now.
>> Remember that the original Drake filters were designed and built when
>> FCC Rules restricted us to 1,000 watts PLATE INPUT power, which, for all
>> practical purposes, limited us to 600-700W output power, depending on
>> the efficiency of the output stage.  I don't remember when that Rule was
>> changed to allow 1.5kW OUTPUT, and I don't know if the Drake filters
>> were later redesigned to handle 1.5kW.
>> Following up on Dale's suggestion that tube amps are more likely to need
>> an HPF than a solid state amp -- I would disagree to the extent that it
>> depends on the topology of the output network.  My 30 year old Ten Tec
>> Titans use a manually tuned Pi-L output network, and their harmonic
>> suppression seems to be a bit better than my new Elecraft KPA500.  IM
>> seems to also be quite low when I tune them carefully.
>> It is also well known that using ALC between a power amp and the driving
>> rig to set TX power level is a recipe for massive IMD. I suspect that it
>> would also increase harmonics.  It's OK to use ALC to prevent damage to
>> the amp when something breaks in the antenna system, but NEVER to set
>> the power level.  Power output should ALWAYS be set by controlling the
>> drive level.
>> HOWEVER -- W8JI, in  an excellent applications note on his website, has
>> observed that MANY rigs, especially low cost rigs,
> Low cost?  The real culprits I've seen, seem to be some very expensive,
> top end rigs when run at partial power.

Another thing I've noticed with the newer rigs is they give the user far 
more, if not total control over the audio, both coming and going. 
Because of this, the set up is far less tolerant of mistakes. You have 
to follow the adjustment steps to the letter or they can end up sounding 
terrible.  Set up is not difficult, but it seems there are many hams who 
just have to tweak the audio, ALC, gain, compression, and power levels.  
Then to top it off most rigs now have audio equalization so you can 
tailor the gain to increase or degrease response across the spectrum 
often increasing the gain at either end of the audio pass band for that 
"Natural" sound, or a harshness that supposedly makes them stand out. It 
appears to me,  If the gain is increased at the low end and the response 
widened beyond normal, it can result in the compression badly distorting 
the signal.  Compression will typically compress the strongest portion 
of the signal. If those lows are too strong the compression will end up 
clipping them so those who add lows and gain on the low end  will end up 
compressing those lows back to the level or close to the level of the 
other frequencies. Also any intermod from the lows will be within the 
audio pass band and on to the amp.  Sending a distorted signal to an amp 
and particularly a high gain Tetrode can produce all sorts of artifacts 
up and down the band even with Triodes.
A while back I heard a group running some very nice SDRs that normally 
sound good.  They seemed fixated on the accuracy of their readouts and 
were complementing each other on the signals which to me sounded really 
bad. They were of respectable signal strength, but not super strong and 
they sounded really bad.  I have heard other SDRs of the same make and 
model that sounded excellent that were much stronger.

Now whether I've come to the proper conclusion, I'll leave up to those 
who are up-to-date on this portion of the technology.  I do know I've 
received more unasked for compliments on the current top end rig driving 
a tetrode although I did get quite  a few with my old 756 Pro driving 
the same amp and I did take a shortcut on the audio set up and created a 
horrible sounding signal,   so it must be possible to get a good signal 
out of some of these rigs.

This "full control" is showing up in less expensive rigs as well which 
means we are likely to be hearing a lot more "tweaked" signals in the 


Roger (K8RI)
> 73
> Roger (K8RI)
>>    put out an initial
>> spike of full power at the beginning of each transmission when adjusted
>> for less than full output. This spike WILL cause distortion -- both
>> harmonics and splatter. Tom recommends using a simple passive attenuator
>> (resistive voltage divider) between the rig and the power amp.
>> 73, Jim Brown K9YC
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