Kurzweil K2000 - K2000 Audio;Power Board Theory - Part 9; VCAs, Effects Mixer, and Input Filter Theory

SECTION 5
AIJDPWRLOOC 12/15/92
S.1.4 K2000 AUDIO/POWER BOARD THEORY
SCHEMATIC PAGE 4
5.1.4.1
VOLTAGE CONTROLLED AMPLIFIERS
Four voltage controlled amplifiers are also part of the audio switching matrix plus they allow
adjustment of the ratio of unchanged "dry" signal to altered "wet" signal from the Effect Unit.
The SSM-2l22 amplifiers used are duals but each half has independent input, output, and control
terminals while the bias current terminal is common.
U9-l4 (E-3), which controls the gain between SEPOUT 2 and OUT2 MOD, will be used for discussion.
The amplifier is a current-in/current-out device in-which the current gain is varied by a control
voltage. For low frequencies, R108 ((B-2) converts the SEPOUT 2 signal into a current which is
fed into U9-l0. Its value of 15K converts a maximum signal peak of 6 volts to a current peak of
400uA. When U9 is set for unity gain, its output current will be the same as its input current,
namely 400uA. This current, fed into the summing junction of op-amp U36 (F-3), will be converted
back to 6 volts by virtue of its 15K feedback resistor.
At frequencies greater than approximately 2KHz, R107 and Clll begin to boost the input current
for a given input voltage while R124 and C124 supply an exactly opposite cut in output voltage
for a given current. This boost/cut, which reaches a maximum of nearly l2dB at 20KHz, reduces
the apparent random background noise level of the SSM-2l22 by about 10dB. The SSM-2l22 will clip
at input/output currents greater than 500uA peak, which is twice the bias current through R68 (F-
4), so the maximum signal at high frequencies is limited. Under virtually all normal conditions
an audio signal carries substantially less energy at high frequencies than at low frequencies,
thus making this nearly impossible. R5l and C35 (E-3) prevent the SSM-2l22 from oscillating.
Because the SSM-2122's input impedance is very low, they have minimal effect on the high
frequency response.
For gain control, the SSM-2l22 has differential voltage inputs, +CTL and -CTL. When they are at
equal voltage levels, gain is unity. As the + input becomes more negative than the - input, gain
reduces on a logarithmic scale at the rate of .005V/dB. The 6.5:1 voltage divider formed by R106
and R103 make this .039V/dB which for a 5 volt control range translates into l28dB of control
range. The control voltage ultimately comes from an 8-bit D/A converter in the scanner
microprocessor, the control sensitivity becomes 1/2dB per bit. R57 (E-3) is needed to balance
the control voltage inputs.
For the UO-14 example, the control voltage is switched by an analog switch. This allows that
particular amplifier to be set to unity gain while the other amplifier (U9-2, E-4), driven from
the same control voltage, continues to be controlled. Unlike the audio path switches, this
control path switch is operated in the voltage switching mode which is OK because any non-
linearity won't cause waveform distortion.
Control voltage buffers, U32-l (B-6) and U32-7 (B-4) allow the relatively weak DIA converter
outputs from the M37450 to drive the qpproximately 600 ohm load presented by two SSM-2l22 control
voltage inputs. R86, R92, C106, and C107 (B-4) smooth sudden changes in the control voltage
level to provide smooth, clickless changes in gain.
5.1.4.2 EFFECTS MIXER
The Effects Mixer is shown on the top section of schematic page 4. U6-7 (E-l) is the actual
mixer stage which combines SEPOUT_l and optionally SEPOUT_2 into a mono signal for the effects
processor. Analog switch U2 (C-l) determines whether current from the SEPOUT 2 signal enters the
mixer stage or is diverted to ground. Because the reverb circuit is noisy and also because the
AlOIA conversion process does not have a very flat frequency response, two poles of high
frequency boost are added across the input resistors R3 (B-1) and R43 (E-l). The gain of this
stage is unity from wither input signal at low frequencies. The effects AID converter clips at
+1-8 volts leaving some headroom for both inputs to approach their 6 volt maximum.
5.1.4.3
EFFECTS INPUT LOW-PASS FILTER
The input anti-alias lo-pass filter is split between schematic pages 4 (F-l to G-2) and 5 (A-5 to
B-6). The complete filter consists of two cascaded 3-pole Butterworth sections with component
values again scaled from the DSP-256 values to account for the 32MHz clock. Although quite
different from the L-C filter described elsewhere, this circuit was selected to avoid altering
the desirable "DSP-256 sound".
10