872 Chapter 25
really quiet!). The impedances around the second stage
largely determine the noise performance of the ampli-
fier, and this is such that it need not be considered in
relation to input noise at any sensible gain setting. Head
room is satisfactory because no attenuation after the
first gain stage is needed for any gain setting. The two
gain stages operate nicely complimentarily.
An operational advantage can be gleaned from Fig.
25-54C. This is the combined gain/rotation curve for the
entire two op-amp circuit. Note that for a very large
percentage of rotation around the middle of the gain
swing (where it’s most often used) the dB gain change
per rotation is as good as linear. It gets a bit cramped at
the top and bottom, but you can’t win them all. For
reference a little later on, it may be noted that there are
two available resistors (R 2 and R 3 ) that may be used to
modify the gain structure independently of the
potentiometer.
25.10.7.2 Input Coupling
As a microphone amplifier, the fairly high optimum
source impedance of the op-amp used in Fig. 25-55 (a
Signetics NE5534, or AD797) needs to be matched to
the likely real source impedance of some 150–200:.
No apologies are offered for the use of transformer
input coupling, as grossly unfashionable as this may
currently seem. Transformers still offer outstanding
advantages—especially simplicity, impedance step-up,
protection, and filtering—over electronic inputs in this
application.
Many circuit values (marked with an asterisk in Fig.
25-55, with some in quite unexpected places) are depen-
dent on the specific transformer type in use. Several
differing transformers can be very successfully used
provided their differing ratios are taken into account in
level calculations; a ratio of 1:7 is optimum to match the
OSI of the input device employed. Phase and response
trimming values will vary significantly. For example,
with the Jensen JE-115-K, it is simpler than with the
Sowter 3195 around which this circuit was originally
developed. Despite the apparent simplicity of the
circuit, a lot of effort has gone into defining the
front-end bandwidth and straightening out the phase
response at audible extremities. Taming the
high-frequency transformer resonance in particular is
quite tiresome.
On the front of the transformer hang the usual
components to make the microphone amplifier useful in
this world of capacitor microphones: a 20 dB input
attenuator and 48 V phantom power via matched 6.8 k:
resistors per leg carried common mode along the micro-
phone line. Further to earlier discussions, the component
values in the pad are chosen such that the microphone
still sees the same general impedance whether or not the
pad is inserted, while the mic-amp still sees about a
200 : source to keep all the transformer-based filtering
in trim. It is essential the 6.8 k: phantom resistors areFigure 25-54. Gain versus pot rotation for two op-amp
input stage.dB gaindB gaindB gain% Rotation% Rotation% RotationA. Noninverting amplifier gain.B. Inverting amplifier gain.C. Combined noninverting and inverting gains.