Western electric 25 b schematic

Page 2

Western Electric Company
No.25-B SPEECH INPUT EQUIPMENT

SECTION 'l-GENERAI. DESCRIPTION

The 25B Speech Input Equipment is a com-
plete AC operated console type, program
production unit for the amplification, control,
and monitoring of programs originated by
microphones, transcriptions, remote inputs, or
equivalent sources. It has two main program
channels, capable of simultaneous operation on
separate programs without interference. In
addition, it has an independent monitoring
channel for loudspeaker listening to programs
being transmitted through either of the two
main channels, or direct from incoming lines or
cue circuits. The monitor channel may also be
used to feed cue program back to the remote
line circuits, or talkback to one or the other of
two studio loudspeakers.

Other facilities provided are an audition, or
sound reinforcement output with volume con-
trol, two VI meters, headset monitoring jacks,
and studio light and signalling control circuits;
jack termination for 4 other lines in addition to
those mentioned above. In addition to pro-
vision for use of an external talkback micro-
phone, a mounting is also provided in the
console for such a microphone.

The equipment has a 7 channel parallel mixer.
Four of these mixer volume controls are associ-
ated with four preliminary amplifiers provided
in the equipment for operation from a maxi-
mum of eight connected microphones (4 simul-
taneously), or equivalent low level sources.
The other three mixers are associated with
higher level inputs which may be incoming
program lines. By the use of three externally
mounted preamplifiers, three additional micro-
phones or other low level inputs may be
operated into these mixers. Any combination
of the seven simultaneous inputs may be con-

nected to either one or the other of the two
main amplifier channels.

A functional schematic diagram showing these
facilities and indicating the operation of the
equipment, is given on Figure 1. This diagram
also shOWs levels at various points in the system
based on a +8 VU level into a 600 ohm out-
going line. It should be noted, however, that
the input levels shown are based on minimum
less in the volume controls and that normal
input levels will be higher than those shown.
A maximum net gain of about 100 db is pro-
vided from the low level input terminals to the
output line terminals, about 38 db from the
high level lines, and about 58 db from the
utility inputs to the outgoing line. The moni-
tor amplifier has a gain of about 50 db.

The 2513 Speech Input Equipment consists of
four principal units. The main unit is a desk
style Control Console mounted on a table; this
unit contains all the amplifiers and the con-
trols. The table top stands 27% inches from the
floor and is about 55 inches long by about 28
inches deep. The console occupies about 13%
inches in depth at the rear of the table top, and
the amplifiers are housed in hinged tray type
enclosures below the table top. The over-all
height of the console on the table is 36 inches.
The control and amplifier enclosures are hinged
so that complete and easy access is obtained to
all internal wiring and components. The second
unit is a wall mounted cabinet containing the
filament transformers and plate supply recti-
fiers. This unit is about 28 inches wide by 10
inches deep by 16% inches high.

Two flush type wall mounting connection or

junction boxes also form part of the equip-
ment. These are furnished with terminal strips

Instruction Bulletin H30, Issue 1

-184-