[TenTec] LONG: Audio improvements on Argonaut VI

[Paul Christensen]

>"Also, TenTec had only installed C4 a .22uF mylar, when national called for
the output stabilizer pair of a 0.2uF and a 1-ohm resistor in series. I
added the resistor."

Good decision.  That's a Zobel network on the output to stabilize the
LM-383.  The 383 often oscillates if the strict National Semiconductor
design rules are not followed.  The 383 will oscillate if the feedback phase
shift goes outside of the maximum datasheet parameter.

>"My suggestion is to examine the Argonaut schematics for the audio chain,
and look to the manufactures recommendations. Did TenTec follow these
recommendations, or are there compromises that may come into play?"

Even if a manufacture follows the device datasheets, there's still a
variable that comes into play with circuit board layout and power supply
decoupling.  For example, Elecraft used a modern AF chip from Texas
Instruments.  Users started complaining about high IMD - well, not that they
knew it was IMD but that it was grunge caused by IMD.  Sherwood analyzed the
output which showed excessive IMD, even though THD was reasonably low.  The
culprit?  Inadequate power supply decoupling.  A power supply choke was
added as a fix in a service bulletin.  Elecraft probably paid better
attention with the K3s design so I would be surprised if the same issues are
present today.

Paul, W9AC