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Maximum convenience is afforded if two separate 7/9 boards are used
.
One is inserted in slot 7 as a reproduce preamplifier and one in slot 9 as an overdub
preamplifier.
It
is then only necessary to employ the record select switch to change
from normal record to overdub operation.
Economy but less convenience is
obtained with a single overdub preamplifier, changing its location from slot 7 to
slot 9 when changing the switch from normal record to overdub.
Great care must
be exercised to turn off the recorder completely before moving the 7/9 card from
slot 9 to slot 7 since the playback head may be magnetized from a charge residual
on Cl unless sufficient time is permitted for it to discharge completely.
Line Amplifier (6) (See Figure
34)
This is a full-spectrum flat response amplifier which provides sufficient gain
and output power to adequately drive an outgoing line at up to peak level of +28 dbm
(600 ohms) or to drive a loudspeaker with up to one watt peak excitation (distortion
1% THO).
01 is an emitter follower accepting an input impedance of 10. 000 ohms or
lower, and providing low impedance excitation for 02.
02 drives 03 and 04
(complementary symmetry types) to result in push-pull excitation of 05 and
06.
A
required static potential difference between the bases of 03 and 04 is established
by the contact potential drop across CRI. CR2, and CR3.
CR4. R20. and C12; CR5, R19. and Cll are drift compensation networks
to stabilize the operating points of 05 and 06.
DC operating point for these two
transistors is set
'
by R 13, R 14, and R 15; and 02. 03, and 04.
A
feedback path
through C13, R21, and R22 assures minimum distortion for all signal frequencies.
C 15 and R23 provide a stabilization network to reduce the pos sibility of high fre
quency oscillations (parasitics) when certain types of loudspeakers are connected
through Sl to terminal 20, and ground.
In the alternate switch position the amplifier
feeds out on either terminal 21 or 22 to the matching output transformer mounted in
the main module, whose purpose is to match the amplifier to a 150 or 600 ohm line.
R24 provides a small impedance in series with certain type transformers which
would otherwise approach a short circuit at extremely low frequencies and thereby
upset the stability of the amplifier at frequencies in the order of one cycl.e or less.
R 1 and R3 are fixed resistors which may be readily changed to alternate
values to change the working line level as indicated by the VU meter. The amplifier
is normally equipped with the
pr~p_er
values for reading a plus four dbm 600 ohm
line. To operate from a +6, +8, or +14 dbm, 600 ohm line, select ±5% calibrating
resistors according to the following table 9.
M64 PAR 2.-71
_
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_
_
_
_
_
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_
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-_ _ _
-
_......-
-
-
-
....
.......
.
- -
.
._~
.-c~
~
93
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