Motherboard/Backplane Card
Main Features
The Motherboard card
provides a backplane connection for up to 12 standard Eurocards in a subrack
frame - distributing DC supplies, carrying audio buss signals and marshalling
data/control line signals. When used with Dual Input Cards and Dual Output
Cards, it forms a complete high quality audio pre-amp. This Motherboard can
also be used as a general purpose passive backplane card for other designs and
is especially suited to audio applications.
Care has been taken to
separate audio signals from data/control line signals and to use separate
ground plane areas to minimise interference/crosstalk.
The key features of this
card are:
- Passive Motherboard card for up to 12
Eurocards (eg. Dual Input Cards or Dual Output Cards) - using DIN41612 64 pin
gold plated connectors.
- Four audio busses (i.e. two stereo
busses) - all covered by earth planes and surrounded by earth guard
tracks
- DC/Data control lines carried from
each card slot to four 40 pin IDC connectors on the rear of the Motherboard (8
data/control lines each for card slots 1 to 8; 16 data/control lines each for
card slots 9 to 12)
- Distribution of ±Vaudio, Audio
Ground and Chassis to each Eurocard slot - 48mm wide tracks for ±Vaudio.
(Main connections on spade terminals.)
- Distribution of separate auxiliary
power ("+Vdata" and ground) to each card, which can be used for mute relay
control (audio preamp design), logic supply, phantom power (microphone
amplifiers), etc. (Main connections on spade terminals.)
- Separate ground planes for audio/data
areas of the PCB, with a Data Ground plane covering the data/control lines
area
- 4 layer PCB, 426.7mm x 128.7mm (16.8"
x 5.067") size, fits a standard 3U high Eurorack, such as a Vero KM6-II
subrack
- DC supply decoupling capacitors by
each connector slot to maintain low noise DC supplies.
Possible
Uses
Stereo Audio Preamp
Backplane
This card can be used as the
backplane for a stereo preamp offering 2 stereo busses (i.e. 2 different stereo
sources can be used simultaneously) with up to 16 stereo inputs (8 Dual Input
Cards) and 8 stereo outputs (4 Dual Output Cards). However, if more inputs are
required, the configuration can be changed to a maximum of 22 stereo inputs (11
Dual Input Cards) and 2 stereo outputs (1 Dual Output Card).
The multiple outputs can be
used to feed different amplifier/loudspeaker systems, recording inputs and
house distributions.
Card slot 9 allows the audio
busses to be split on the backplane, with card slot 9 having connections to
both split sides of all four busses. This allows, for example, a Processing
Card to be used (eg. for response equalisation, etc) in connector slot 9, which
can intercept the audio path between the Dual Input Cards (left hand side of
the subrack) and the Dual Output Cards (right hand side of the subrack). If
such a card is used in the "16 in to 8 out" configuration then the maximum
number of stereo outputs will be reduced to 6 (3 Dual Output Cards).
The two stereo buss
arrangement could be used to provide the "classic" configuration of a listening
buss plus an independently selectable record buss.
Multichannel "Surround
Sound" Busses
One of these cards can be
used as the backplane for 4 channels of a 6 or 8 channel surround sound system
(i.e. "5.1" or "7.1"). Two Motherboards will provide up to 8 busses for
surround sound working.
User's Own
Designs
This passive backplane card
can be used to integrate the user's own designs of card into an audio preamp
design - or for completely different uses such as a microphone amplifier rack,
2 buss audio mixer, etc as the Motherboard card can be configured in a number
of ways.
Circuit
Description
Overview
The photo above shows a
Motherboard in a standard 3U high subrack (a Vero KM6-II) with Dual Input Cards
plugged in. Card slot guides are fitted on a regular spacing to the subrack
frame, therefore allowing cards to align with the connecotrs on the
Motherboard.
The Motherbaord is designed
such that the cards with the audio in/out connectors are installed from the
front (i.e. using the depth of the subrack) with power/ground (spade terminals)
and data connections (40 pin IDC connectors) made from the rear of the PCB.
This allows any data signals to be kept away from audio - and any
microprocessor/control logic can be housed in a separate screened zone at the
rear of the subrack case, etc (if required).
A circuit diagram for the
Motherboard printed circuit board is available on the Photos/Downloads page.
Card Slot
Connectors
The Motherboard has 12
DIN41612 2-row 64 pin socket connectors for Eurocards (such as the Dual Input
Card and Dual Output Card). The pins are allocated as follows (the list is in
the same vertical order as the pins appear on the PCB):
- 8/16 pins for data/control (top of the
connector)
- 3 guard ground pins -
unused
- 4 audio busses - each separated by 2
guard ground pins
- Auxiliary power "+Vdata" and ground
pins (used on the preamp design for mute relay)
- ±Vaudio, Audio Ground and
Chassis - 2 pins each (bottom of the connector)
The allocation of pins has
been made to minimise crosstalk and interference.
For connector slots 1 to 8,
both rows of socket pins are connected in parallel - effectively making a total
of 32 pins - with 8 data/control line pins. For connector slots 9 to 12, again
most rows are connected in parallel, except the data/control line pins which
are connected individually to provide 16 data/control lines.
Audio
Busses
The Motherboard has four
mono, unbalanced audio busses which are connected across all 12 card slot
connectors. As it is assumed that Eurocards are used to send audio signals in
and out of the Motherboard, there are no direct audio buss connectors on the
rear of the pcb (although, if required, wires could be soldered directly to the
connector pin pads).
On connector slot 9, the
connector pin rows for the four audio busses are linked with a track on the
rear side of the pcb - which can be cut if it is required to use a Processing
Card (one which can intercept the audio buss path between the Dual Input Cards
and the Dual Output Cards).
Resistors are provided to
terminate each audio buss with a constant load, regardless of the number of
Dual Input Cards or Dual Output Cards installed. Note that the buss loading
resistor value is chosen as a compromise between lowest distortion (high
impedance load) and lowest "off" isolation (low impedance load) and is a
function of the Analog Devices
SSM2402
audio switch ICs.
Data/Control
Connectors
Four latching 40 pin IDC
header plugs (on the rear of the PCB) are used to marshal the data/control
signals from each of the card slot connectors and connect them to control
logic/microprocessors etc located on other PCBs. The connector type chosen can
use the same plug cable which is used for IDE disk drive connections in PCs
(low cost and easily available!). 32 active pins are used in each connector,
with the remaining 8 pins earthed to Digital Ground.
These connectors are
allocated as follows:
- Connector X13 - 8 control/data pins
each for card slots 1/2/3/4
- Connector X14 - 8 control/data pins
each for card slots 5/6/7/8
- Connector X15 - 16 control/data pins
each for card slots 9 and 10
- Connector X16 - 16 control/data pins
each for card slots 11 and 12
Full details of the
connector pin assignments are available on the Photos/Downloads page.
DC Power Supplies and
Earthing
Spade terminals are provided
on the rear of the PCB for connection to the various DC supplies and
earths.
A Chassis earth is sent to
each of the card slots - in the Dual Input Card and Dual Output Card designs,
this is used to connect to the screen pin of the XLR audio connectors. Pads for
a resistor and a parallel capcitor are provided to allow Audio Ground to be
linked to Chassis. Note that this is isolated from the actual chassis metalwork
of the subrack, as the mounting holes/pads on the PCB are isolated.
It is recommended that a
standard isolating washer strip (available from the subrack frame
manufacturers) is fitted between the Motherboard PCB and the rack frame to
guarantee that no PCB tracks are inadvertently shorted to the subrack frame
metalwork.
There are two separate
earth/ground plane areas on the PCB to minimise interference/crosstalk. One is
connected to Audio Ground and covers the area of the audio busses, auxiliary
power "+Vdata" and its ground, and the ±Vaudio supplies. A second plane
is connected to Data Ground ("D-GND") and covers the area of the control/data
lines. These areas are arranged such that these planes do not overlap on any
layers. Note that it is not expected that the auxiliary power "+Vdata" and its
ground (a single track each) are used for noisy signals, as they are within the
Audio Ground plane area.
Decoupling
Capacitors
The PCB includes pads for
ceramic decoupling capacitors next to each card slot connector for the three DC
supplies: +Vaudio and -Vaudio (both connected to audio ground, referred to as
"A-GND"); auxiliary power "+Vdata" (connected to its own ground, referred to as
"D-GND2").
The three DC supplies also
have pads for electrolytic capcitors close to the spade connectors where each
supply is connected to the PCB.
Printed Circuit Board
A bare PCB is available for
this design (top view shown below). Details are on the Ordering page.
Component schedule (parts
list), connector assignment and a full circuit diagram are available on the
Photos/Downloads page.
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