Handbook for Sound Engineers

Consoles 873

matched; poor matching is a very easy way to wreck

carefully won common-mode performance.

25.10.7.3 Line Level Input Facility

A line-in option is brought in via the transformer also. It
features far stiffer input attenuation (about 36 dB) while
simultaneously disabling much of the gain swing of the
first amp. The resultant gain swing of 35 dB (between
25 dBu and +10 dBu input level) with a bridging-type
input impedance of some 13 k: should accommodate
most things that the microphone input or machine-return
input differential amp can’t or won’t. A small equaliza-
tion network is used in the attenuator to bolster the
extreme low-frequency phase response.

An alternative and in many ways preferable line-in

arrangement might be to use either a discrete or inte-

grated instrumentation amp-style stage, switched into

the second stage of the mic-amp. In Fig. 25-55 this

would come in where “from B-check Diff-Amp” is

marked. This would avoid the necessity of using the

transformer with attenuators, which of course does no

favors to the common-mode rejection ratio.

25.10.7.4 Common-Mode Rejection Ratio

Common-mode rejection ratio (CMRR) in the trans-

former is dependent mostly on the physical construction

of its windings. The Sowter, in common with many

other transformers, may be in need of compensation by

deliberately reactively unbalancing the primary winding

Figure 25-55. Channel input amplifier, high-pass filter, and limiter.