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TD-000487-00
K Series Service Manual
Class D amplifier
Each of the BOP amplifiers is a type of class D amplifier called ‘self-oscillating’, as opposed to a class D amplifier that
requires a clock to switch. Self-oscillating class D amplifiers create a PWM signal solely by using feedback; although many
different variations have been invented. Unique to the BOP’s type of self-oscillating amplifier, PWM is created by taking
feedback only after the output filter and bringing it to a comparator input through a lead filter network. The lead filter in the
feedback network, combined with the output filter phase shift and the delay between the comparator and FETs, creates the
required phase shift with gain to allow oscillation at a specific frequency. Idle frequency is about 400kHz, and the switching
frequency dips to about 100kHz at full power since this is a self-oscillating amplifier and switching frequency varies with
modulation index.
The modulated switching voltage must be filtered to recover the audio signal before being connected to a transducer. The
filter uses a toroid inductor and film capacitor for each channel. When idle, a capacitor between the amplifier outputs cou-
ples high frequencies between the channels, thus forcing a common switching frequency to eliminate sirens and lower hiss
from HF transducers. Because this isn’t enough to get acceptably lower noise floor, a high pass filter is used to further lower
both the amplifier power and the noise from only the amp channel driving the HF transducer. Although not required with
the type of self-oscillating amplifier used in the BOP, small Zobel filters are present on each channel to aid in stability and
help reduce cross-channel interference during over-current (OC) conditions.
A terminal block, sticking through the back of the bucket into the loudspeaker cabinet, brings both amplifier outputs to the
transducers via a detachable wire harness. Another pair of wires goes to the front LED which is mounted in the loudspeaker
cabinet
Input/DSP board
The BOP has two separate input sections, A and B, each with a balanced combination female XLR/TRS jack with a pass-
through male XLR jack. On 2-way versions of BOP, each input also has unique features. Input A has a switch which increas-
es the gain by 25.4dB to boost low-amplitude signals, specifically microphones. When in MIC mode, a yellow LED is lit to
indicate this mode setting to the user. Input B has an unbalanced stereo phono jack, with 6dB more gain, that can mix its
signal with the XLR/TRS input. Each input has a separate gain knob, allowing adjustment from OFF to +10dB, with 0dB right
in the middle. The gain knob markings also indicate the level
of another female XLR, labeled LINE OUT, which carries the
signal of the two inputs after being mixed together. With the
gain knob at 0dB, the LINE OUT XLR is the same amplitude as
a signal at either input; with the gain knob at +10dB the LINE
OUT XLR level is 10dB greater than a signal at either input.
The subwoofer version of BOP has only the balanced XLR/
TRS inputs, with pass-through female XLRs, that are summing
to a single gain knob. No MIC/LINE or phono connections are
present.
Green LEDs for each input indicate the presence of an audio
signal before the gain controls, meaning even with the BOP
output level at zero the LEDs will still light up and indicate if
a signal above about 3.5mVp is presented to the inputs. The
subwoofer version, naturally, only has one signal presence
LED as it has only one input. The DSP controls the other 3
LED on the faceplate, labeled LIMIT, STBY, and PWR. STBY
and PWR are mutually exclusive and indicate the operation
of the amplifiers; if the DSP turns off the amplifiers the green
STBY LED is on, while the blue PWR LED is on when the DSP
allows the amplifiers to run. A condition in which the DSP
performs long-term amplifier limiting or muting triggers the
red LIMIT LED. OC conditions are handled by the amplifiers
without the DSP’s knowledge so the LIMIT LED is not turned
on when OC is detected.
Besides the MIC/LINE switch on input A, 3 other switches are
present on the BOP faceplate. The FRONT LED switch selects
between 3 different modes of operating for the blue LED
on the front of the loudspeaker cabinet: PWR, LIMIT, or OFF.
When switched to PWR the front LED is always bright and
Summary of Contents for K8
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