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 <[EMAIL PROTECTED]>
Subject: Re: [Elecraft] K3 Harmonic Distortion
To: David Cutter <[EMAIL PROTECTED]>
Cc: [email protected], "Leigh L. Klotz, Jr WA5ZNU"
<[EMAIL PROTECTED]>, "Julian, G4ILO" <[EMAIL PROTECTED]>
Message-ID: <[EMAIL PROTECTED]>
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
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