Kurzweil K2000 - K2000 Audio;Power Board Theory - Part 6; D;A Converters, Filters, and Output Amplifiers Theory

..
SECTION 5
AIJDPWR3.IlOC 12/22/
5.1.3 K2000 AUDIO/POWER BOARD THEORY
SCHEMATIC PAGE 3
S.1.3.l
AUDIO SECTION
The audio section, less effects, is on schematic page 3. This covers D/A conversion, filtering,
switching, mixing, and amplifying circuitry that handles the "dry" audio Signals.
S.1.3.2 D/A CONVERTERS
U37 and U39 (B-1 to B-4) are each dual l8-bit D/A converters with serial data inputs. There is
thus one D/A converter for each audio channel. Although the Hobbes chips on the Engine Board are
capable of multiplexing one DAC among 2 or 4 channels, the one output per DAC mode was used on
the K2000 to minimize noise and distortion.
The AD1864 is a dual DAC with separate serial inputs, separate Latch Enable pins, and a common
clock pin. The Hobbes chip on the Engine Board was designed so that the sequence and timing of
the DAC signals would be directly compatible with the AD1864 and all other serial input DACs
claiming "PCMS6 compatibility". The serial bit input rate is 9.968MHz and the word input rate is
actually 96.000KHz. This is not the same as "2X oversampling" because each pair of very slightly
different words is derived from the same sample but it does significantly reduce distortion of
very low level signals. Either current or voltage output connections can be used. Here the DACs
are wired for voltage output which is fixed by the chip design at +/-3 volts full-scale. Note
that only the Engine Fake board or specially programmed sounds ever approach this high amplitude
level; normal keyboard playing produces amplitudes in the 0.1 to .OS volt range.
For minimum digital noise feedthru, the power and ground wiring of the AD1864 is very critical.
The digital section of the chip requires + and -S volts and has its own ground as does the analog
section. +Sana and -Sana power the analog section directly and the analog section ground is tied
to analog ground. +Sana and -Sana also go through RF chokes Ll and L2 (B-S) and filter
capacitors C128, C169, C130, C172 (A-2 to A-4) and power the digital section. The chokes and
capacitors prevent digital activity in the chip from feeding back into the analog power supplies.
Ground for the digital section is connected to digital ground. In the layout, the digital
groundplane is under the chip's digital pins while the analog groundplane is under the remainder
of the chip.
U38, U40 and surrounding components (E-l to E-4) amplify the +/-3 volt full-scale signals from
the D/A converters to just over +/- 6 volts to optimize the signal-to-noise ratio through the
following filters. They also provide a small boost at very high frequencies between lSKHz and
20KHz to counteract a slight droop in the downstream filters. Sampling theory is valid only for
signals reconstructed as impulses. When reconstructed as stair-steps, the high frequencies near
1/2 the sampling rate are rolled off by as much as 4dB. At 20KHz in a 48KS/s system, the loss is
about 2.8dB. Although the U38 and U40 circuits are low-pass filters with a cutoff of about
2SKHz, their "Q" is high which gives a boost to the frequencies just below cutoff. Note that
incorrect values for the resistors and capacitors could easily cause a very distorted frequency
response curve or even oscillation.
S.1.3.3 LOW- PASS FILTERS
Fll-F14 (F-l to F-4) are passive, L-C type, low-pass filters that remove high alias frequencies
from the D/A converted signals. With the source and load properly matched, the loss through the
filters is slightly more than 1/2 (6dB) which brings the +/-6 volt boosted signal from the DACs
back to just under +/-3 volts.
Although the filters are efficient and have a very sharp cutoff, they are vulnerable to AC
magnetic fields because of the coils. In order to minimize audible hum, they must be kept away
from the power transformer and the low-voltage wiring between the transformer and J20 which also
produces a magnetic field because of the large currents flowing. In the K2000 case, the power
transformer must also be oriented such that the filters are in a "null point" of the
transformer's flux field. In addition, they could even pick up hum from another piece of
equipment, such as a power amplifier or another instrument, placed immediately below or above the
K2000 case. TAKE CARE TO PREVENT THIS.
S.1.3.4
SEPARATE OUTPUT AMPLIFIERS
U34 and U3S (H-l to H-4) amplify the +/-3 volt signals coming from the low-pass filters to +/-6
volts. They also provide sufficient power to drive the 4 separate output jacks and loads as low
as 600 ohms. R163 and R168-l70 provide a source impedance to the jacks of 600 ohms, isolate the
amplifiers from capacitive loads (like long cables), and are also part of the RFI suppression
network along with C167, C170, C17l, and C177.
7