Working through the K3's schematic this morning, it seems that both the
headphone output port and the LIN OUT port are driven by similar
devices, a Texas Instruments DAC with integrated headphone amplifier.
(Two DACs, one for LIN OUT and one for headphones and speaker output.)
As near as I can see from tracing from one sheet to another, U29, a
TLV320ACC23 DAC with integrated headphone amplifier drives the TTC-108
LIN OUT transformers directly. Headphone and speaker audio, in contrast,
are derived from a second TLV320 DAC driving a LM4950 amplifier for
speaker output and an LM4811 for headphone output.
I've ordered a couple of TTC-108 transformers today and will run some
experiments with them over the next week or so. I've looked at similar
transformers developed for modem and telephone line coupling, but not
the TTC-108 specifically, when trying to find a cheaper output
transformer to use with the Softrock Lite receivers. There, the emphasis
was more on high frequency response, up to 100 KHz, but distortion was
also a major issue. The solution of choice in my opinion for the
Softrock receivers turns out to be a MILSPEC Triad transformer, SP-70,
with excellent frequency response and quite low distortion. In single
lot prices, an SD-70 costs about four times as much as a TTC-108.
Jack
Leigh L. Klotz, Jr WA5ZNU wrote:
It would be nice to see this problem with the the K3IO board addressed
at some point. It may be, as Jim K9YC has hinted, related to core
saturation from low frequency components that could attenuated before
reaching the board. Or it may be that the transformers that fit in
the space are as big as they can be, and a third-party outboard audio
amplifier and isolation board is necessary, bypassing the transformers
in the K3.
But whatever the solution is, I'm looking forward to it, as using PSK
with a waterfall display on the K3 requires me to ride the AGC, LIN
OUT, and filters in a way that I didn't on the K2.
Leigh/WA5ZNU
I've found that the LIN OUT port has typical third harmonic levels
around -45 dB from the fundamental, with the LIN OUT level set at 10,
the factory default. The headphone port, under the same conditions,
will show the third harmonic down about 70 dB or so at levels
appropriate for my Sony high sensitivity headphones. Cranking the
audio level up will cause higher distortion on the headphone port,
but only at levels that are impossible to listen to as they are so
loud as to be unbearable.
I've also seen some non-harmonic artifacts, particularly when very
strong signals are pumped into the K3's antenna port.
Jack K8ZOA
Julian, G4ILO wrote:
I think it was me who used the term "ghost" signals when referring
to the
harmonically related CW traces I could see on the CW Skimmer display. I
probably would have been completely unaware of it if I had never
tried this
program. It was only obvious as I was listening to CW with a narrow
filter
and the "ghost" traces were appearing in the part of the waterfall
that you
would be expect to be dark because it was outside of the passband. I
would
agree with the suggestion that the harmonics are at a low level
relative to
the main signal because they are masked by band noise when you open the
filter up. I don't think there is much risk of calling someone on
the third
harmonic by mistake. I don't know what the voltage level of the line
output
is but the traces disappear if you turn the level down to 2, which
unfortunately is too low for most sound card programs.
I don't know if those same harmonics are present on the speaker or
headphone
outputs, as distinct from the computer audio feed. I can only say
that I
find the K3 audio exceptionally pleasant to listen to. I much prefer
it to
the K2, or the FT-817, which are the only other radios I have. The only
other radio I have had that sounded as clean and low-distortion as
the K3
was an IC-756PRO that I had many years ago.
If the harmonics are down by the kind of amounts that others have
measured
then the distortion figure is still within the specification for
what after
all is not meant to be an audiophile quality hi-fi output stage.
-----
Julian, G4ILO. K2 #392 K3 #222.
http://www.g4ilo.com/ G4ILO's Shack http://www.ham-directory.com/ Ham
Directory http://www.g4ilo.com/kcomm.html KComm for Elecraft K2
and K3
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