From: firstname.lastname@example.org (Tom Bruhns)
Subject: Re: Output Impedence
Date: 31 Oct 2002 12:36:01 -0800
References: <email@example.com> <firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: 31 Oct 2002 20:36:02 GMT
Winfield Hill wrote in message news:...
> Tom Bruhns* wrote...
> > I thought it would be interesting to remind myself of what a real
> > transformer's frequency response does as a function of output load, so
> > I dredged out an old 600:600 "bridging transformer". What I see is
> > that the low end isn't terribly affected by load (-0.1dB at 0.6Hz,
> > yeow!), but the high end is quite sensitive to load. ...
> Conversely, the low end is quite sensitive to source resistance,
> especially if it's higher than the "spec" resistance.
:-) Actually, you caught me in a slight indiscretion here, Win. When
I measured that, I was doing it from a 50 ohm source impedance, but in
a manner which tested only the transformer, not the system. That is,
I was measuring the transformer output voltage/input voltage. I
should (do) know better than to pass that off as a reasonable
characterization, ('specially since I'm in the midst of coming up with
a DC-6GHz three-way power splitter specifically so we can make just
such a measurement correctly!) and have re-tested with a 600 ohm
resistance between the reference channel and the transformer; now I
see a 14.0Hz -0.1dB point, still not too shabby. Also, putting in
that resistance damped the resonance with the unloaded secondary,
though only partially. The load for optimal 0.1dB flatness increased
slightly, but the high end results are materially the same; most
noticable is the lowering of the resonant peak and a corresponding
increase to about 19kHz of the 0.1dB flatness bandwidth.
For John: I'm interested in 0.1dB because I do instrumentation.
What's required for audio is very subjective; what's required for
instrumentation is to a degree also, but folk at least often have
valid reasons for wanting some particular level. And 0.1dB does get
it down to a level where not many audiophiles will balk. But the key
point is that load resistance does affect the bandwidth, whether
you're measuring to 0.1dB flatness, 1dB, or 3dB.
WRT the board line input impedance/what board question: you may be
right. I don't deal with boards very often these days, and the inputs
I'm specifically remembering may be only mic inputs. That board is
not handy to me here, so I can't answer specific quetions about it.