[Elecraft] Boost power supplies for RF amps

[Bill Frantz]

The short answer is they can. It is only a matter of engineering 
time, cost, resulting price, number of customers and competing 
projects. In other words, the usual business considerations.

Lets work the idea through for several of the Elecraft RF amplifiers:

The 10-15W K3(s) amp: This amp can have significant IMD at low 
voltages as we have discovered with our QRP, all Elecraft, field 
day operation. The KX3 seems to do better, so reengineering the 
amp may be a better solution. For field day, we try to have 
fresh batteries available.

The 100W K3(s) internal amp: This amp also has low voltage IMD 
as we found during our 100W California QSO party expeditions. 
But, consider the K3(s)'s construction. The front of the radio 
is the digital portion, including the DSPs and the front panel 
computer. All of this digital noise is isolated from the analog 
circuits in the back of the radio. If Elecraft were to include a 
boost power supply with the internal amp, that high-current 
switcher would be in the analog section, and probably impossible 
to quiet. Solution: Get an external boost power supply.

The KXPA100 external amp: This form factor would be an ideal 
place to include a boost power supply. The use profile includes 
portable battery operation and designing isolation for the power 
supply into the package seems straight forward.

The KPA500 external amp: The amp setup procedure sets the 
voltage to to 65v to 85v and the specifications call for about 
1000 VA of power or about 13A at 75v. That would be nearly 75A 
at 13.5 volts assuming 100% efficiency, so it would be a good 
idea to put the boost power supply next to the battery and run 
75v to the amp. It would make a nice high power mobil rig.

73 Bill AE6JV

In thread "Re: [Elecraft] Chinese ebay amps for KX3", on 2/20/17 
at 5:47 AM, pincon at erols.com (Charlie T, K3ICH) wrote:

>So now, tell me why a mid-sized radio can't have a built in 12 V to say, 50
>V converter to drive some HV FETs for cleaner output specs like the Yuge
>radios.
-----------------------------------------------------------------------
Bill Frantz        |The nice thing about standards| Periwinkle
(408)356-8506      |is there are so many to choose| 16345 
Englewood Ave
www.pwpconsult.com |from.   - Andrew Tanenbaum    | Los Gatos, 
CA 95032