Shure A27M User manual

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A27M -- Stereo Microphone Adapter
General Description
The Shure model A27M is a stereo microphone
adapter designed to hold two microphones in se
lectable positions for stereo broadcasting,
recording or sound reinforcement. It allows two
microphones to share one stand without losing
independent positioning flexibility. The A27M per
mits coincident or closely spaced mounting with
a wide range of directional angles, and vertical
microphone separations of 1 1/4 inches (31.8
mm), 2 5/8 inches (66.7 mm) or 4 inches (102
mm).
The A27M consists of two vertically stacked sec
tions that rotate on their centers. Each contains a
5/8"-27 threaded stud and locking ring to accept
a variety of microphones. The bottom section
contains a 5/8"27 adapter for mounting on a mi
crophone stand. A cable clamp for holding micro
phone cables is included.
Installation
To install the A27M, follow these steps:

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1. Determine the vertical separation between microphones you desire. Minimum separation (coincident spacing)
is generally desirable, but it may be necessary to provide greater spacing to accommodate the size or style of
the microphones. By repositioning the sections of the A27M, the threaded studs may be 1 1/4 inches (31.8
mm), 2 5/8 inches (66.7 mm) or 4 inches (102 mm) apart.
2. To reposition the sections, unscrew the knurled knob at the top of the A27M and slide the two black sections off
the vertical rod. Reposition the sections with the studs positioned for the vertical spacing desired. Replace the
knurled knob.
3. Attach the microphones or swivel adapters (such as the Shure A57D or A57E) to the 5/8"-27 studs.
4. Attach the A27M to a microphone stand with a 5/8"-27 mounting thread.
5. Insert the microphones in the swivel adapters, loosen the knurled knob on the top of the A27M, and position
the microphones for the desired response angles. The top of the knurled knob contains reference marks every
45o; these may be used to position the microphones at the desired angle (see Figure 1).
6. Tighten the knurled knob, attach the microphone cables, and use the supplied cable clamp to fasten the micro-
phone cables to the A27M.
7. Raise the microphone stand to the desired height. Fig-ure 2 shows a pair of Shure SM81 microphones mount-
ed in an “ORTF” configuration on the A27M.
Stereo Recording Options
The A27M supports various popular pickup systems. Those described below result in (1) improved imaging over
widely spaced microphones, and (2) superior depth reproduction over individually miked instruments. The spaced
microphone technique creates a stereo effect mainly because of differences in the arrival time of the sound at
each microphone. Coincident systems produce minimal arrival time differences. The stereo effect they create is
due to in-terchannel amplitude differences. Closely spaced or near-coincident placement lends both time and am-
plitude differences to the stereo effect.

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The table below describes five popular stereo pickup sys-tems. It specifies microphone types, angles and spacing.
Those with coincident spacing have their diaphragms verti-cally in line. Closely spaced microphones have their
dia-phragms separated by the specified distance. These and other microphone systems are detailed in the book
“Stereo Microphone Techniques” by Bruce Bartlett (1991, Focal Press. ISBN 0-240-80076-1) and in “Microphone
Tech-niques for Music Studio Recording” (AL1265) available from Shure Incorporated.
Stereo Pickup Systems Microphone Types Microphone Configurations
X-Y 2 Cardioid Axes of maximum re-
sponse at 135°; spacing:
coincident
ORTF (French Broad-
casting Corporation)
2 Cardioid Axes of maximum re-
sponse at 110°; spacing:
17 cm. (7 in.)
NOS (Dutch Broadcast-
ing Foundation)
2 Cardioid Axes of maximum re-
sponse at 90°; spacing:
30 cm. (12 in.)
Blumlein Pair or stere-
osonic
2 Bidirectional Axes of maximum re-
sponse at 90°; spacing:
coincident
MS (Mid-Side) 1 Cardioid
1 Bidirectional
Cardioid and bidirectional
forward pointed; spacing:
coincident
Full Two Year Warranty
Shure warrants this product to be free in normal use of any defects in workmanship and materials for two (2) years
from the date of purchase. If your Shure product fails within this period, call 1-800-516-2525 in the United States
for details on repair, replacement, or refund. In Europe, contact Shure Incorporated Europe GMBH at +49 (7262)

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924911 4 or [phone] (service hotline) or contact your authorized Shure distributor. This warranty does
not cover abuse or misuse of the product, use contrary to Shure's instruction, ordinary wear and tear, an act of
God or unauthorized repair.