Bose AmPlus 100, Supplement Manual, Page: 9 / 58

9
THEORY OF OPERATION
FreeSpace ® Model 1B Equalizer Card
Introduction
This card is intended for use in the AmPlus™ 50 and 100 amplifiers. It accepts a full range
(50Hz to 16kHz) signal, equalizes it, and passes a full range signal back to the main
amplifier signal path.
Refer to the Free Space Model 1B EQ Card schematic diagram, sheet 1 of 1 for the
following information.
Note: The designators inside the brackets “[ ]” are the schematic grid coordinates which
are provided in order to make it easier to locate components on the schematic sheet
indicated in the description.
The equalization is segmented, with one block (U2 and U1, all sections [D1-8])
dedicated to bass region equalization for the Model 1B Bass Module. Following the output
of this section, U2A [D1], the signal is presented to two parallel equalization paths, one
comprised of U3 [B4-7],U4B [C3], and dedicated to EQ for surface mounted Bose
® Free
Space System satellite speakers (Models 25, and 32). The other path, comprised of U5
[B4-7], U4A [B3] provides equalization for Bose Model 8 flush-mount satellite speakers.
The operation of all three equalizers is similar. Considering the bass equalizer, we see
U2B's [D7] output passed through R8 [D7] and into the non-inverting input of U2A [D2].
This input is extended as a bus to which EQ sections Bass 1 [D6] and Bass 2 [D5] are
connected. These will introduce dips into the audio response. Section Bass 3 [D4] is
connected to a potentiometer straddling U2A non-inverting and inverting inputs. When the
wiper is moved toward the non-inverting input, the section introduces a dip; when moved
toward the inverting input, a peak.
Each of the EQ sections utilizes a gyrator-connected op amp in conjunction with a series
capacitor to simulate an LCR series tuned circuit. The values of the capacitors and
resistors are chosen to implement EQ dips and/or peaks at the appropriate frequency,
amplitude, and Q to realize the desired EQ curves. Each EQ section has optional steering
resistors that could allow other curves to be implemented in the future with the same card.
There are also some spare sections not currently used.
S1 [B2] selects either the flush or the surface mount EQ curve, while potentiometer R33
[D1] provides a boost/cut range of adjustment of the 90Hz bass region. Pin 12 of J1 [B2]
is grounded and is sensed by the dynamic EQ circuit on Sheet 2 of AmPlus 50 and 100.