Hi Arny et alius,
Thank you all for your comments. They have been very helpful.
FYI, the reason for my question is that I am attempting to review the
design of the front-end of a heart rate monitoring device. This design
is based on the SSM2167 (e.g., from National). My thinking is that this
is the wrong part for the job since the sensor is fed to the preamp via
a run of cable perhaps 4 feet in length, and there could therefore be
lots of common-mode noise. Also, I believe the AGC action is, in this
stage, not appropriate.
This appears to be an electret type of microphone in that DC power is
fed to the device, so the poster who discussed how to feed phantom power
and simultaneously receive the input differentially was right on the
money.
Thanks again for your input and suggestions.
--Randy
"Arny Krueger" writes:
"David Nebenzahl" wrote in message
.com
I'd say that since a differential amp practically
eliminates common-mode noise, there's no reason *not* to
use one (which, after all, was the question the OP asked).
Transformers are an effective means of elimination common-mode noise. They
are as a rule more effective than differential amplifiers. If you have an
input transformer, you don't need a differential input and you pick up some
inherent protection against EMI and casualty losses.
If you've ever had a grounding problem that a standard active differential
input can't handle, you know what I mean.
Trnasformers also provide galvanic isolation, or isolation of signal
ground. We would probably be a little less critical of the configuration of
safety grounds in audio if we were still in the days of transformers.
Transformers would probably rule the inputs of professional grade gear were
it not for costs. While it is hard to make transformers have as flat of
frequency response and as have ultimately low distortion as differential
inputs, it is possible to make them perform well enough to be sonically
innocious.
Transformers are still the preferred means for obtaining the best possible
freedom from grounding problems in areas where the cost issues are not so
severe, such as analog and digital communications.
--
Randy Yates % "Maybe one day I'll feel her cold embrace,
Digital Signal Labs % and kiss her interface,
% til then, I'll leave her alone."
http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO