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Introduction
This paper discusses the pitfalls (often subtle) of our industry's
failure to define and standardize what "unity gain" means,
and the conditions necessary to measure it. It further discusses
how people improperly use one piece of misinformation (impedance
matching) to correct for this lack of standardization. All done,
without knowing discrepancies exist between different pieces of
equipment, and without knowing impedance matching is unnecessary,
signal degrading, and wasteful.
For me, it began with a phone call. The caller said he wanted to
know our output impedance so he could add the proper load impedance.
"Why would you want to do such a thing?" I asked.
"Because I want to maintain unity gain through each piece of
signal processing gear," he replied.
That gave me pause. Then I laughed and realized what he was doing
right, and what he was doing wrong.
The problem stems from another case of our industry working without
proper guidelines and standards. This one involves the conditions
used to establish unity gain. Lately, the popular trend of including
unity gain detent points and reference marks only aggravates things.
This Note identifies and explains the problem. Once understood,
the solution becomes easy -- and it doesn't involve impedance matching.
Unity Does Not Always Equal One
It begins with an understanding of unity gain. Simple enough. Ask
anyone and they will tell you unity gain means that if I put, say,
1 volt in, I get 1 volt out, i.e., a gain of one, or unity. Nothing
could be easier. That is until that same someone asks the question,
"Is that unity gain balanced , or unbalanced ?"
Herein lies the problem. Today we find that many (most?) pro audio
signal processors have a gain difference of 6 dB between unbalanced
and balanced out (exceptions to this are units with output transformers,
or cross-coupled output stages -- see Appendix ). This x2 difference
results from differentially driving the line. Figure 1 shows how
an input signal drives one side of the line positively and the other
side negatively (each line driving amplifier has a gain of one,
but together they yield a gain a two). For example, a +1 volt peak
AC input signal drives one side to +1 volt while simultaneously
driving the other side to -1 volt. This gives a balanced output
level of +2 volts peak (the difference between +1 volt and -1 volt).
Alternatively, that same input signal drives an unbalanced line
to +1 volt peak. Thus, there is a 6 dB disparity between an unbalanced
and a balanced output -- a gain difference factor of two.
Here, unity equals two.
 .
No Standards
This brings us to the part about no standards. Without a standard
defining the specified conditions for unity gain, manufacturers
make their own decision as to what "unity gain" means.
For one, it means 1 volt in gives 1 volt out unbalanced, and 2 volts
out balanced. For another, it means 1 volt in gives 1 volt out balanced,
and 1/2 volt out unbalanced. For yet others, it means 1 volt in
gives 1 volt out (using transformers), or 2 volts out (using cross-coupled
stages), either balanced or unbalanced. Very confusing.
Figure 2 shows how this creates problems. Here different definitions
result in a gain of 12 dB, with all controls seemingly set for unity
.
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