Symetrix 601, Руководство пользователя

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3-1
Rev 2.2, 10/31/94
3. Technical Tutorial
This section discusses a multitude of things, all related to getting signals in and out of the 601.
3.1 Matching Levels vs Matching Impedances
In any audio equipment application, the question of "matching" inevitably comes up. Without
digging a hole any deeper than absolutely necessary, we offer the following discussion to
(hopefully) clarify your understanding of the subject.
Over the years, we have all had impedance matching pounded into our heads. This is important
only for ancient audio systems, power amplifiers, and RF. Technically speaking, the reason is
power transfer, which reaches a maximum when source and load are matched. Modern audio
systems are voltage transmission systems and source and load matching is not only
unnecessary, but undesirable as well.
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Ancient audio systems operate at 600 ohms (or some other impedance value), and must be
matched, both at their inputs and at their outputs. Generally speaking, if you are dealing
with equipment that uses vacuum tubes, or was designed prior to 1970, you should be
concerned about matching. These units were designed when audio systems were based on
maximum power transfer, hence the need for input/output matching.
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Power amplifiers are fussy because an abnormally low load impedance generally means a
visit to the amp hospital. Thus, it's important to know what the total impedance of the pile
of speakers connected to the amplifier really is.
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RF systems are matched because we really are concerned with maximum power transfer
and with matching the impedance of the transmission line (keeps nasty things from
happening). Video signals (composite, baseband, or otherwise) should be treated like RF.
Some folks seem to believe that balanced/unbalanced lines and impedances are related; or
even worse that they are associated with a particular type of connector. Not so. Unbalanced
signals are not necessarily high-impedance and balanced signals/lines are not necessarily low-
impedance. Similarly, although 1/4 inch jacks are typically used for things like guitars (which
are high-impedance and unbalanced), this does not predispose them to only this usage. After
all, 1/4 inch jacks are sometimes used for loudspeakers, which are anything but high-
impedance. Therefore, the presence of 3-pin XLR connectors should not be construed to mean
that the input or output is low-impedance (or high-impedance). The same applies to 1/4 inch
jacks.
So, what is really important? Signal level, and (to a much lesser degree), the impedance relation
between an output (signal source) and the input that it connects to (signal receiver).
Signal level is very important. Mismatch causes either loss of headroom or loss of signal-to-
noise ratio. Thus, microphone inputs should only see signals originating from a microphone, a
direct (DI) box, or an output designated microphone-level output. Electrically, this is in the
range of approximately -70 to -20 dBm. Line inputs should only see signals in the -10 to +24
dBm/dBu range. Guitars, high-impedance microphones, and many electronic keyboards do not
qualify as line-level sources.
The impedance relation between outputs and inputs needs to be considered, but only in the
following way:
Always make sure that a device's input impedance is higher than the output
source impedance of the device that drives it.
Some manufacturers state a relatively high-impedance figure as the output impedance of their
equipment. What they really mean is that this is the minimum load impedance that they would
like their gear to see. In most cases, seeing a output impedance figure of 10,000 (10K) ohms or
higher from modern equipment that requires power (batteries or AC) is an instance of this type