[Elecraft] K-Line DC Supply

[Howard Hoyt]

>From: "Ron D'Eau Claire"<ron at cobi.biz>  said:
>I'm not sure what you mean about the regulator
>not being "quiet". The LM723 is a simple linear
>I.C.; no switching noise and no acoustical noise.

Hi Ron,

I was referring to the broadband thermal and shot noise present at the Vout pass transistor drive output of the regulator itself.  Yes it is a linear regulator, but at least with the parts I have used, the 723 internal reference voltage and error amp are fairly noisy by modern IC standards. I have tried to use these regs in audio equipment and I find a quieter and wider BW regulator can be made inexpensively using modern bandgap references and low-noise opamps.  I am sure that Elecraft designed their circuitry to have as good internal sub-regulators and as high PSRR as possible, but having a really quiet supply can't hurt.

Of course, maybe anything better than a 723-based supply is just unnecessary overkill:  I do not have an Astron or other 723 supply here, so a good test may be to compare a K3 or KX3 powered by a battery, with the same setup powered by a supply.  In order to provide the maximum sensitivity, the antenna should be disconnected and the RF and AF gains at max to allow hearing farther into the rig's own noise floor.  I just performed this test on my KX3 using a Xantrex supply (@13.8V, <400uV noise RMS 10Hz-300KHz as measured on an HP3468) I hear zero noise level difference between the supply or internal batteries, so if this is the case with an Astron, and it's regulation is good enough, then the 723 based circuit is adequate for this particular application.

I agree with whoever stated that a power supply should be out of the way and out of sight; having no controls or displays that we need to see every day there is no need for it to take up desk space.  I would like to get a small 13.8V dedicated supply that could handle a KX3 and the pending KXPA100, but others have very different requirements, such as power for multiple hundred watt amps, etc.  My feeling based on my experience with the KX3 is that if Elecraft made such a supply it would be no compromise for those of us who have such a need.  One of the main reasons I would like to see this is the same reason I like the Astron: it is made in the US.

Just my $0.02 worth,

Howie - WA4PSC