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Master Gain Control:
The f r o n t panel control designated "Master"
is the
master gain control and controls t h e overall o u t p u t
level o f all the mixed sources (including signals which
may b e applied t o the "Mix Bus" input).
VU METER
Range:
The V U meter is factory calibrated f o r use with a 600-
ohm terminated line. The " V U Range Switch" above
the master gain control selects either a
+ 4
o r
i-
10
d b m o u t ~ u t a t 0 V U meter indication. (This switch
changes \he meter indication, and does h o t change
the actual output level.) Microphone o u t p u t levels are
6 0
db
below line output.
The
t 4
range is recommended f o r normal use t o pro-
vide 14 d b margin f r o m operating level t o clipping
level.
The V U meter
i s
calibrated with no load on the head-
phone
output.
Low impedance (under 250 ohms)
phones may have some loading effect and alter t h e
V U calibrations slightly.
If
t h e use o f such phones
appears t o be normal, (or
if
other than a 600-ohm
terminated line is used)
it
may b e desirable t o recali-
brate the V U meter b y means o f the " V U Cal." control
(see Figure D.) (Range o f this control is approximately
'
2
db.)
Illumination:
The V U meter is illuminated b y t w o N o . 4 7 lamps
operating well under the normal ratings f o r a life
expectancy o f greater than 10,000 hours. The lamps
are used only with
AC
operation. Consequently, when
a b a t t e r y supply is used as a safety stand-by, the
illumination serves as a visual alarm
if
the
AC
is inter-
r u p t e d and the unit has switched t o battery.
For lamp replacement instructions see Figure
E.
D C Isolation:
The V U meter is connected on the primary side o f the
output transformer t o assure protection f r o m any D C
level on the telephone line. (See OUTPUTS, LINE)
TONE OSCILLATOR
A
built-in tone oscillator is provided f o r line test a n d
level checks. The tone oscillator is activated b y setting
t h e "Mic. I/Tone Osc." switch (rear panel) t o "Tone
Osc." and may then b e controlled in level b y control
"Mic.
l
/Tone
Osc."
(on the f r o n t panel). The fre-
quency is approximately 700
Hz.
and t h e signal will
appear on b o t h line and microphone outputs as well
as t h e headphone and mix bus connectors.
The oscillator should b e switched off ("Mic. I " position)
when n o t actually in use.
LO-CUT FILTERS
These filters are activated b y the switch above each
o f the individual channel level controls, and may b e
used individually with each control as needed. The
typical response function i s shown in Figure
B.
The
M i c . 4/LineW filter will operate with either micro-
phone o r line input.
FREQUENCY IN HERTZ
TYPICAL LOW-CUT FILTER ACTION
FIGURE B
HEADPHONES
The headphone output appears on the rear panel and
i s designated "Headphones."
A
two-circuit phone jack
is used t o provide a choice o f level f o r different head-
phone impedances. Normally, a single circuit plug
should b e used.
If
inserted only partially ( t o t h e first
detent), the available voltage is approximately . I 8 volts
into 1,000 ohms with
4-4
d b m o u t o f the "Line O u t "
terminals. W i t h the same operating conditions the
second position will provide approximately .38 volts.
These voltages are selected f o r normal operation with
600-ohm t o 2,000-ohm headphones. O t h e r headphone
imwedances mav b e used. b u t with a variation i n the
mdnitor level an'd t h e pos;ibility
o f a slight amount o f
V U meter loading with very low impedance phones.
(Maximum change is 0.5
d b with headphone
jack
shorted.)
If
stereo phones are used, the two-circuit plug may b e
inserted completely ( t o second position) and output
will appear i n b o t h phones.
The "Tip" connection o f t h e headphone plug will b e i n
phase with Pin 3 o f all input a n d o u t p u t connectors,
and with the
tip
o f the mix jack.
MIX BUS
D i r e c t access t o the mixing bus is p r o v i d e d through
t h e "Mix Bus" jack (phono type) o n t h e rear panel.
This provision is made primarily t o facilitate "stacking"
(or multing) o f M67's t o achieve additional i n p u t
capacity without losing any inputs.
W i t h t w o M67's, f o r example, t h e t w o mixing buses
are directly connected, providing t w o independent
masters and t w o isolated line amplifiers with e i g h t
individually controlled inputs. Since t h e buses are
directly paralleled, a
6
db
level d r o p will occur, b u t t h e
V U m e t e r indication will
s t i l l
b e correct. Noise speci-
fications are n o t adversely affected
by
this intercon-
nection.
The i n p u t clipping level t o t h e mixing bus is 0.2 volts
through 3.5
K
ohms. The gain f r o m "mix bus" i n p u t t o
line output is 7 0
db,
and f r o m 150 ohm microphone t o
"mix bus" is 2 0
db.
To facilitate stacking, a hardware and cable
k i t
is avail-
able as Shure Accessory M o d e l A68S.
If
other use o f
this
provision is contemplated,
it
should
b e remembered t h a t t h e "mix bus" terminal is con-
nected directly t o t h e mixing bus and operates a t
3.5
K
ohms impedance, hence connecting devices
should have a t least
3.5
K
ohms internal impedance.
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