Dual Input Card
Main Features
The Dual Input Card provides
two stereo input channels and, when used with the Dual Output Cards and the
Motherboard, forms a complete high quality audio pre-amp.
The card has four high
impedance balanced (differential) inputs, each with RF rejection filters,
adjustable gain/attenuation over a 27dB range and electronically controlled
audio switches to connect sources to the audio busses on the
Motherboard.
The key features of this
card are:
- Two independent stereo inputs on XLR
sockets, each individually selectable to two stereo buss outputs (to the
Motherboard) using Analog Devices
SSM2402
audio switch ICs
- Differential (balanced) inputs, which
are source impedance independent (works equally with balanced and unbalanced
sources)
- RF (Radio Frequency) rejection
filters on the inputs, allowing a flat audio band frequency
response
- Adjustable input gain range: 0dB,
+3dB, +6dB, +9dB (with a jumper plug)
- Adjustable input attenuation range:
0dB, -6dB, -12dB, -18dB (with a pair of jumper plugs)
- Earth (Chassis) link to the "cold"
side of the input source, if required (with a jumper plug)
- 4 layer PCB, Eurocard 160mm x 100mm
(6.3" x 3.94") size
- On-board ±15V regulators for
guaranteed low noise DC supplies on the card
Possible
Uses
Two Stereo Sources -
to listening and record busses
Two stereo sources (e.g. CD
player and tuner) can be connected to this card. Each source (and actually each
channel) can be remotely selected to feed one of the two stereo busses on the
Motherboard. The gain/attenuation of each source can be set using jumper plugs
to compensate for different signal levels from the source equipment.
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" Sources
One of these cards can be
used to connect 4 channels of a 6 or 8 channel surround sound source (i.e.
"5.1" or "7.1"). Two Motherboards can be used to provide up to 8 busses for
surround sound working.
Domestic/Professional
Signal Levels
As each input has its own
selectable gain/attenuation setting, this card be used in systems which use
domestic equipment (i.e. mainly unbalanced sources at low signal levels) or
which are a mix of domestic and professional equipment. The operating signal
level (on the busses) can be chosen by the user, according to the sources and
the gain/attenuation jumper plug settings.
Circuit
Description
Overview
The diagram above shows one
of the four channels on the printed circuit board (a higher resolution drawing
is available on the Photos/Downloads page). The
component numbering shown on the drawing is generic - 1/2/3/4 is inserted to
denote the channel number (e.g. IC201 is the IC01 used in channel 2; the
generic component numbering is described as "ICx01", etc
below).
A classic instrumentation
amplifier configuration is used to make the input stage common mode rejection
ratio, independent of the source impedance as, for example, a CD player with an
unbalanced output will have an impedance of a few hundred ohms in the driving
output and zero ohms in the "earthy" output lead.
Jumper plugs are used to
adjust the gain over a 27dB range (+9dB to -18dB in 3dB steps). Correcting the
source signal level difference on this input card helps to achieve good
crosstalk performance between channels and maximises the isolation from the
audio busses on the Motherboard when the channel is switched off.
All resistors are low
values to keep the noise floor low and to minimise the distortion with the
chosen Burr
Brown OPA134 opamp. If standard 1% tolerance resistors are used, the gain
matching between channels will be better than 0.2dB for all jumper plug gain
settings (theoretically 0.09dB for each of the two gain/attenuation stages)
with no correction adjustments needed.
Input
Stage
An XLR socket is used for
the input connection, with conventional pin assignment (Pin 1 - chassis, Pin 2
- Hot, Pin 3 - Cold). The hot and cold signals are passed through DC blocking
capacitors, and then through a resistor/capacitor/bifilar coil network to
prevent unwanted RF signals being passed into the Input Buffer stage. (A brief
description of the bifilar coil rejection technique is given in the
Design Philosophy page.) The values of the
resistors/capacitors can be adjusted, if required, to change the audio band
roll-off performance and RF rejection performance of the input stage (e.g. if
the design is to be used in an area with very high power, low frequency
transmitters).
The cold side of the input
can be connected to Chassis via a resistor (which can be shorted out if
required) using a jumper plug. This may be required for sources which are fully
"floating", such as a battery operated CD player, which are likely to have very
high common mode (unwanted) signals present and have no earth
reference.
Input
Buffers
Two opamps (ICx01 and ICx02)
buffer the hot and cold signals from the input stage, and are the first part of
the instrumentation amplifier.
With no jumper plug
inserted, each buffer is unity gain (0dB). Gain can be added (+3dB, +6dB, +9dB)
by inserting the jumper plug into one of three positions - arranged in the
layout of a rotary switch. Note that this technique for increasing the gain
also improves the common mode rejection ratio (a "quiet" source is likely to
need better common mode rejection to maintain a good signal/noise performance
for the listener).
Differential
Amplifier
A single opamp (ICx03) is
the second part of the instrumentation amplifier which performs the
differential function.
Two jumper plugs (again
arranged in the layout of a rotary switch) are used in pairs to adjust the
attenuation of this stage. With no plugs inserted, this stage will attenuate
the input signal by 18dB. Three plug positions are available allowing the
attenuation to be set to 0dB, -6dB, -12dB.
The common mode rejection
performance of this stage in the "audio" band is controlled by the tolerance
(matching) of the resistors (Rx10 to Rx19 inclusive, plus Rx05 and Rx09). If
standard 1% resistors are used, then the rejection should be of the order of
40dB (0dB Input Buffer gain) to 50dB (+9dB Input Buffer gain), which should be
sufficient for most applications. However, if better performance is
really needed, then closer tolerance resistors can be used or resistors
can be hand selected/matched (a time consuming procedure!). If "hum" is audible
on sources then there is probably another problem which needs resolving first
(e.g. earthing arrangements for source equipment)...
This opamp is also the low
impedance buffer to feed the audio switches/busses.
Buss Outputs -
Switches
The opamp output is
connected to the inputs to two audio switches in the same Analog Devices
SSM2402
dual audio switch IC, to allow the user to select which buss is fed (both can
be fed simultaneously) to the Motherboard. Each switch DC control is wired to
the backplane DIN41612 connector, allowing the user to select each left/right
channel independently. (As a DC control is used, a microprocessor is not needed
to operate the switch.)
Each input channel can feed
two busses. The printed circuit board card is designed so that input channels 1
and 3 feed the two "left" busses on the Motherboard and channels 2 and 4 feed
the two "right" busses.
Low value resistors are used
to connect the opamp to the audio switch - these prevent very high
level/clipped audio signals at the opamp output possibly causing the switches
to turn on when they should be off!
Power
Supply
Two low noise voltage
regulators (LM317/LM337) are used on each printed circuit board card. The
design as shown sets the card voltage to ±15V, although this can be
changed if required (e.g. increased, if higher audio headroom is needed, or
reduced, if the supply rail on the Motherboard will be at a lower voltage). The
maximum which can be used is ±18V (less than a 2dB increase in
headroom), which is limited by the supply voltage ranges of the ICs.
One layer of the PCB is a
Ground plane - another layer mainly carries the DC supplies to the ICs with
wide copper tracks.
Ceramic "decoupling"
capacitors are used next to each IC. Although ceramic capacitors do change
their state slightly with acoustic/physical vibration, tests have shown that
this will not cause any problems in normal use. However, this might be an issue
if the card was installed inside a large/loud speaker cabinet,
etc.
Input Stage - DC
Blocking Options
In a standard design with
the audio signal connected to an Input card, and then to an Output card via the
Motherboard, the input stage is the only point where the audio signal passes
through DC blocking capacitors (Cx01 and Cx03). The PCB design has four solder
pads for each capacitor allowing either a metallised polyester capacitor
(listed in the parts list) or a non-polarised electrolytic capacitor to be
used. As electrolytic capacitors are physically smaller, a larger capacitance
could be used, giving slightly improved low frequency audio performance -
although the current industry trend is not to use electrolytic capacitors in
the audio path.
Note that with a balanced
audio source, these capacitors are effectively in series with each other, so
halving the effective capacitance, which in turn raises the low frequency
roll-off point higher than might be expected at first sight. The capacitors are
needed at this point in the circuit to prevent any DC signals from passing
through the bifilar coil, which might saturate the ferrite ring and prevent it
from working correctly.
Technical Specifications
The measured performance of
this card is:
- Frequency Response: 10Hz to 20kHz
+0.01/-0.1dB
- Phase Response: 10Hz to 20kHz
±12°
- Maximum Input Level: +26dBu (balanced
source, 6dB attenuation setting), +22dBu (unbalanced source)
- Distortion (THD+Noise): better than
0.005%
- Noise (measured at the buss, 10Hz to
22kHz filter): -102dBu (ie. the absolute maximum signal to noise floor is
122dB)
- Common Mode Rejection Ratio: better
than 70dB (10Hz to 100Hz), better than 45dB (10Hz to 20kHz)(see graph)
- Inter-channel crosstalk (eg. left to
right): better than 105dBr at 1kHz, 90dBr at 10kHz.
- Input Impedance: 80kohm (balanced
source), 65kohm (unbalanced source).
Graphs of the card's
performance are available on the Dual Input Card -
Performance page, together with details of the measurement
methods.
Printed Circuit Board
A bare PCB is available for
this design (top view shown below) - Version 01. Details are on the
Ordering page.
Component schedule (parts
list), Motherboard multipin connector assignment and full circuit diagrams are
available on the Photos/Downloads page.
Version 02 is planned for
release in 2006. The known minor revisions will be:
- Chassis earth connection (XLR
connector Pin 1) - revised to meet new AES recommended practice (when
agreed)
- XLR connectors to be moved closer to
the front of the PCB (Version 01 requires a metal spacer to be used between the
XLR ring and a front panel plate, if an insulating spacer is used between the
Motherboard and the subrack chassis - the current position is correct if no
insulating space is used)
- XLR PCB pin hole positions to be
corrected (connector is stiff to insert into the PCB due to a few thou
error).
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