BA660.doc
01/01/97
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cathodes. When the cathodes are shorted together the tube gain is
almost the idealized Mu converted to db!
When the unit runs out of range using degeneration, ( it is a somewhat
noisy gain setting method at the lower ranges), pads are inserted and
gain stages are eliminated to maximize signal to noise ratio and
minimize total harmonic distortion.
System Gain Structure Control
Because of the humongous gain available, and the response to near
DC, the gain switching, stage switching and transformer removal
are accomplished via gas filled gold plated fast relays. These relays,
along with special timing circuits, enables the unit to delay the
change, mute the output, make the change, allow the output to
settle, and then remove the mute. This logic and timing is all
accomplished with Q46, Q17, Q10 Q29, Q56 and Q23 on sheet
two. The left side of the drawing delays the switch action for
approximately 10 ms while the right side enables the mute for
approximately 500 ms.
Because of this muting action, the
input selector nor the preset switch should be used
after initial setup when employing this unit for live
broadcast
!
Insert Sends and Returns
The signal level appearing at the insert send is nominally -10 when the
meter reads "0". This is done because the 2nd stage output is
buffered to this point with 2 op-amps. The maximum level at
these devices is +26, the maximum level out of the 2nd stage is
+42. Therefore, in order to not make the insert send buffer the
limiting element of the headroom of the box, the level is lowered
at the send, so that +40dB can be accommodated internally. The
insert return is nom4 (this gain differential must be made
up via the insert loop) and goes directly into the gain cell through
2 small isolation resistors, and 2 100k dividing resistors. Any
unbalance that occurs will result in signal degradation, so care
must be exercised in this regard. The silicon is bypassed when
there is no insert connection. This is accomplished via relay K7.
The output of this relay is also the point that the side chain signal
is developed. The nominal signal for the side chain is also therefor
+4.
