siliconchip.com.au Australia’s electronics magazine May 2019 37
high, it will wait for the audio signal to cross through 0V
before making any volume changes. This avoids clicks
which would otherwise be caused by a sudden signal level
step change when the volume is adjusted.
Unsurprisingly, pulling pin 8 low mutes the output,
and this function is not used, hence the pull-up resistor.
Mute can instead be controlled using the SPI serial con-
trol interface.
Power supply and signal routing board
Let’s turn now to the power supply and signal routing
circuit, shown in Fig.6. The cable from CON1 on the ADC
board connects to CON16, while two separate but identical
DAC boards are connected to CON14 and CON15.
10-way headers CON17 and CON18 connect to the mi-
crocontroller board. The signals to and from the ADC and
DAC boards are routed to the microcontroller pins via these
headers. At the same time, five power rails are distributed
to all those boards as required.
Except for the master clock, all the signals from CON18
are connected through to CON19, which the front panel
control board plugs into. This routes the control board sig-
nals back to the microcontroller.
Some things to note about the signals passing between
the micro and ADC/DAC boards: CON14 (DAC1) and
CON16 (ADC) share the same digital audio bus, while
CON15 has a separate bus. One DAC and one ADC mod-
ule can share the same bus since there is one pair of data
in/out lines and they only use one each (into the DAC,
out from the ADC).
The same master clock signal is distributed to all three
connectors, and the reset line is also shared between all
three, so the three chips will be reset simultaneously if
this line is pulled low.
None of the SPI control buses are wired up to anything,
as this is not required as long as you leave the volume con-
trol chips off the DAC boards.
The ADC and DAC boards are fed with +9V, -9V, +5V
(VA, not used by the ADC board) and +3.3V (to power the
digital interfaces of the ADC and DACs). A separate 5V rail
passes through ferrite bead FB15 and is then fed to the mi-
crocontroller board, to power the micro. Using a separate
rail avoids the possibility of the micro board ‘polluting’
the 5V rail used by the DAC boards.
All the digital audio signals connect to the micro via
CON17 (along with its 5V supply), except for the master
clock, which is on pin 8 of CON18. The other pins on CON18
are wired to general purpose I/Os on the microcontroller.
The power supply section is pretty straightforward: a
centre-tapped 18-24V AC (eg, 12 + 12V AC) transformer is
wired to CON13 and then connects to diode bridge recti-
fier D17-D20 via fuses F1 and F2.
The DC outputs of this bridge are filtered by a pair of
470 μF capacitors and then regulated by adjustable regula-
tors REG6 and REG7 to produce the +9V and -9V rails re-
spectively.
LM317/337 adjustable regulators are used because of
their excellent ripple rejection capability, especially with
10 μF capacitors from their ADJ terminals to ground. The
220 and 1.5kresistors set their nominal output volt-
ages to (1.5k/220+1) x 1.2V = 9.38V. The extra diodes
protect the regulators by preventing current from flowing
backwards through them at switch-off.
These regulators are fitted with small flag heatsinks to
keep their temperatures reasonable.
The positive output of the bridge rectifier is also fed
through ferrite beads FB13 and FB14 through to two extra
47 μF capacitors which power regulators REG4 and REG5
respectively, to produce the +5V and +3.3V rails. Differ-
ent feedback resistor values are used to change the LM317
output voltages.
The extra ripple-rejection capacitors are not used here
since these supplies do not need to be as ‘quiet’.
Another LM317, REG8, is fed from the main 470μF posi-
tive filter capacitor and is also set up for a 5V output. This
provides the 5V “VA” rail for both DAC boards.Coming up...
This is a monster project, so we can’t fit all the details
into a single article.
Over the next two issues, we plan to have details on the
microcontroller and front panel circuits, along with the parts
list plus construction and operation of the of the SC