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
David Cutter wrote:
I haven't done the sort of testing that you must do for your
applications, but I've used the OEP8000 transformers in several
applications with good results for speech only. I've no idea of its
high frequency response. They are tiny in comparison.
http://www.oep.co.uk/admin/UploadDocuments/OEP8000.pdf
David
G3UNA
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
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