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Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback

Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
From: "" <>
Date: Sun, 18 Dec 2005 15:55:35 GMT
List-post: <>
Hi Tom,
   First, let me reiterate that to end up with -55dB you'll need to budget your 
distortion carefully.

   Consider your project has 3 stages:  The FT-1K,  the preamp (10mW to a watt 
or so), & the main amp.

   IMD does not add linearly so, for best results each "stage" should exhibit 
perhaps 10dB better numbers than the following one.

   I would verify your FT-1K numbers (looking for -75db) with a very low 
distortion two-tone source -- perhaps from a PC sound card?  Or..., not that 
you need another project at this point, I describe a stand alone unit, with 
enough info to duplicate, at:

  If you can't run down a good hybrid brick, then maybe an IC-tube combo would 
be an innovative approach to the preamp.  The TI op-amp I suggested could drive 
a tube grid directly with a (10dB?) negative feedback loop around these two 
components.  As for the choice of a tube, I'd search around a few audiophile 
websites and see if you can find the lowest distortion 50W tube ever designed.  
There just has to be something better than the 1950's vintage 6146 out there!  
You'll want this stage to output better than -65dB distortion.  When were you 
going to acquire a spectrum analyzer?  :-)

  The 4-400A is not a good substitute for the '350FJ.  It probably starts with 
10 dB or so worse numbers and even though it will be run even lighter than the 
'350, it probably won't catch up.  This is not the place to toss away hard to 
come by distortion dB.  Note: Also the 4-400A is directly heated so, you would 
have to include a big filament choke to provide a spot to couple in the 

  I don't know the numbers on the 4CX3000A, but, assuming it lives somewhere 
around -30dB (3rd Order) at full power, running it at 1/3 power (1500 watts) 
should get it down to perhaps -45 dB  (Note: A pair of 4CX1500B's will do this 
for sure.  Time to buy an old Harris RF-110A?).   Figure in the 10-12dB 
negative feedback around the "main amp" and you make your target spec.  

  I want to hear this contraption on the air when it's done!

73 & Good morning,
  Marv WC6W

P.S. -- Season's greetings to the folk at the NSA who have to read all these 


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