Roland TR-808 Service Notes page 6

TR-808 JUN.
15,1981
Snare
Drum
This sound generator has
two
bridged
T-networks
for fundamental
waveforms and harmonic waveforms. The output
ratio
of the
two
can
be changed by
VR8
to tailor
sound
characteristic.
The amplitude of
snappy envelope can be controlled by
VR9.
LT/LC (MT/MC, HT/HC)
These three sound generators
are
composed of the
circuits
based on
the
same
principle.
LT/LC
is
described
below
as an
example.
This
sound
generator
is
composed
of
a
multi-feedback, bridged T-
network including IC5
as
an
active
element. Voices
are
switched by
SW8
(C77
—
frequency,
R224
—
level).
While the oscillation
is
large
in
amplitude immediately
after triggering,
it is
on
a
higher frequency
due to conductions of
D80
and D81, which reduce time constant of
the filter.
As
the
resonance
is
damped,
its
frequency
is
lowered by
the effect of increasing diodes' internal resistance. Timbre variations
corresponding
to
time
elapse will appreciably be heard as in the case
of Bass
Drum.
Pink noise with a slightly longer decay time
is
mixed
for
Low
Tom
Tom
to provide
artificial
reveberation.
RS/CL
CL
Output from multifeedback bridged T-network incorporated
with IC20
is
routed to IC19.
Output from
IC21 (for RS), also routed
via
R320, can be ignored because of
its
minimized
level
due
to
im-
pedance
imbalance at pin 7 of IC20b.
RS
Disconnected
R313 makes IC20b
just as a buffer for
CI20a
output. The output of IC20b
is
applied to
062
together with the
output
of IC21.
The envelope
applied to
062
is
formed by R107
and C24. As described
in
the beginning of
this
section,
VCA
of
this
type
is
intended to provide
many
high
harmonics
in the
output
signals.
Normally-conducting 074
remains off only while trigger pulse
is
transferred from 061 to allow IC19
to pass signals.
This switching
is
provided to eliminate noise leaking
from
IC20, especially for
CL —
relatively large
amount,
being
wired
for high O.
CP/MA
White
noise passed through the
band
pass
filter
(IC21)
is
applied
to two
VCAs
in parallel to
have
different envelopes.
These envelopes
are
combined
tp obtain sound source for the
CP
sound generator.
Since an envelope with
a relatively long decay time
is
applied to the
VCA
Q70, output from
this
VCA
constitutes reverberation of
CP
sound.
The
output envelope
at
the
VCA
(IC22,
Q71
and Q72)
is
a
unique
sawtooth
shape,
and
is
a
main component
of this
sound
generator.
The
sawtooth envelope generator
circuit
is
mainly
described
below
to explain
its
rather complicated operation.
When
trigger pulses are
applied to pin 8 of the quad comparator IC23, the output
is
inte-
grated by
R350
and Cl
40,
and converted into pulses of
30ms
wide
as
shown
in Fig. 13-2.
At
the falling edge of the pulse, pin 13 of
IC23 becomes H
(Fig. 13-3).
The
output from pin
1
of IC23
is
also
applied to pin
4 of IC23, pin 2 of IC23 becomes from
—15V
to OV,
IC23
OV
pin 14(7)
-15V
OV
IC23
pin l(lO)
IC23
pin 13
0144
-15V
-15V
OV
-15V
IC23
pin
2
OV
n
5
-15V
-
j
IC23
pin
5
IC22 pin
8
7
FIGURE
13
HAND CLAP GENERATING CYCLE
Q73
turns on, pin 5 of
IC23 becomes —15V,
pin 2 of IC23 returns
to
—15V, and
Q73
returns to off
state.
Accordingly, the output
waveform
at pin 2 of
IC23 becomes narrow
pulses
as
shown
in
Fig.
13-5,
The
moment Q73
is
turned on, C144
is
abruptly charged
to
—15V.
However, immediately after charging,
Q73
turns off and the charges
are
discharged through R365 and D71. When
the level of pin 5 of
IC23
becomes
higher than the
level
of pin 4 due to discharging, pin
2 of
IC23
reverses again
and C144
is
recharged to
—15V.
After this
process
is
repeated and advanced to the middle
of the third time,
pin
1
of IC23
rises
to OV. This signal
is
differenciated by
R357
and
C141, and
the
generated
pulse turns
on Q73. At
this time, although
the terminal voltage of
C144
rises
gradually from
—15V
due
to
discharging, pin 2 does
not
reverse since pin 4 of IC23 has reached OV.
The
output
(Fig. 13-4)
of
this
envelope generator
is
applied to the
base of
Q72
and converted exponentially by
Q72
together with the
signals
applied to the base of Q71
(offset adj. signal
from
TM3
and
accent signal via D68,
C143 and R362. The
converted signal
is
applied
from the
collector of
Q72
to pin
1
of IC22
to
change the amplitude
of noise
from
the
filter
IC21.
Note:
IC23 (AN6912)
is
constructed with open
collector
NPN
transis-
tors for output and operates on
single
(negative) power only.
MA
White noise
is
gated by
Q65
and supplied to the same
buffer
IC19
as for the
CP sound
generator
through
the
filter
Q68.
Envelope
for
MA
sound
generator
is
generated by
Q66
and Q67.
CB
This
sound generator
uses the
outputs
of
two square waveform
oscillators with different frequencies (by Schmitt triggers). Each
oscillation
output
passes the
corresponding
exclusive gate
(VCA,
Q14, Q15) and mixed by
the
filter
IC2.
A
series
of
R82
and
C34
connected
in parallel
with
C9
forms an
envelope having abrupt
level
decay
at the initial trailing
edge
to
emphasize attack
effect.
CY
The combined square wave outputs of
six
Schumitt
triggers
including
two
for
CB
generator
is
separated into high
and low
range
components
by
two
filters
composed of IC3. The high range
component
from pin
7 of IC3
is
further separated into
two frequency
ranges.
The
output
of the gate
Q16
has the highest frequency component of
this
sound
generator.
Its
decay time
is
short.
The
output of
Q17
is in a
frequency
range
slightly
lower than the above output, and
its
decay time
is
controllable.
These
three signals with different
frequency
ranges are
outputted
with
their level ratio
controlled by VR4.
OH
The
high
frequency
range
component
signal obtained
by
the
above
1/2 IC3
is
gated by
Q27
and supplied to the buffer IC7 through the
filter
Q26. When
the
CLOSED
HI-HAT
(CH)
is
triggered while the
OH
circuit
is
activated,
023
turns
on by the voltage applied through
R173. At this
moment,
the decay time of the
OH
circuit terminates.
CH
This shares the same sound source with the
OH. The
signal
is
gated
by
030
and
supplied to the filter
031 and
the buffer IC7
(1/2).
6