[Elecraft] K3 Harmonic Distortion

[Larry Phipps]

This discussion reminds me of some of the design choices I had to make when designing LP-PAN last year. I wound up using the SP-70 MILSPEC xfmrs, each driven by a pair of op-amps in a push pull configuration with "zero ohm" source. My original goal was to reduce 2nd harmonic distortion and improve response at frequencies approaching 0 Hz without the need for large capacitors in the output. The xfmrs are quite expensive, even in large quantities, but I have been very happy with the results.

Larry N8LP



> Date: Thu, 04 Sep 2008 14:33:19 -0400
> From: Jack Smith <jack.smith at cliftonlaboratories.com>
> Subject: Re: [Elecraft] K3 Harmonic Distortion
> To: David Cutter <d.cutter at ntlworld.com>
> Cc: elecraft at mailman.qth.net, "Leigh L. Klotz, Jr WA5ZNU"
> 	<Leigh at WA5ZNU.org>,	"Julian, G4ILO" <julian.g4ilo at gmail.com>
> Message-ID: <48C029EF.20009 at cliftonlaboratories.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> David:
>
> Even a rather poor transformer improves significantly when driven by a 
> "zero ohm" source such as an op amp. The typical modem line transformers 
> that are spec'd at 0.5% THD, for example, come in one to two orders of 
> magnitude better when driven by a low impedance source. In fact, it's at 
> the point where my lowest distortion audio generator (HP 200CD) isn't  
> good enough to measure a $4 transformer when driven by an op amp. I'm 
> generator limited at -63 dB at the moment.  I hope to improve that 
> measurement floor in the next week or so.
>
> Driving the same transformer with a 600 ohm source Z brings you back to 
> the 0.5% THD range.
>
> It's all quite interesting and reminds me again that "it's not what you 
> don't know that's the problem; rather it's what you think you know that 
> bites you in the backside."
>
> Jack