Xaoc Devices OSTRAWA User manual

OSTRAWA
full stereo
voltage
controlled
mixing console
Model of 1966
operator’s manual rev. 1966/1.0

SALUT
Thank you for purchasing this Xaoc Devices
product. Ostrawa  is a fully ste-
reo four-channel voltage-controlled mixer
featuring a stereo mixing bus, one stereo
auxiliary send per channel (switchable be-
tween pre and post-VCA) with stereo returns,
clickless muting, individual level meters, and
a super-clean and DC-coupled signal path
designed with high-quality VCA and opamp
chips. We have carefully crafted Ostrawa's
voltage control response to achieve what
we believe to be the optimal user experience
found in a Eurorack mixer. The design fea-
tures an elaborate control circuit that com-
bines the internal voltages generated by the
panel potentiometers with external CV over
volume and stereo balance. The result is a
natural attenuator response that constrains
VCA gain to a usable range while minimizing
distortion. Ostrawa, a sister module to Xaoc
Devices Praga, features similar response
characteristics and layout of controls with
some functional differences due to its stereo
inputs and limited panel space. Multiple Os-
trawa and Praga units may be chained for a

To better understand the device and avoid
common pitfalls, we strongly advise the user
to read through the entire manual before us-
ing the module.
INSTALLATION
The module requires 20hp worth of free space
in the Eurorack cabinet. The ribbon type pow-
er cable must be plugged into the bus board,
paying close attention to polarity orientation.
The red stripe indicates the negative 12V rail
and should align with the dot, –12V, or red
stripe marks on both the unit and the bus
board. The module itself is protected against
reversed power connection; however, reversing
the 16-pin header may cause serious dam-
age to other components of your system by
short-circuiting the +12V and +5V power rails.
Unless you are using Ostrawa with the Bo-
humin expander, make sure the factory-in-
stalled jumpers are present on the bohumin
expander 2 header as indicated on the PCB.
The module should be fastened by mounting
the supplied screws before powering up.
MODULE OVERVIEW
     
resembles a typical mixer with four identical
channels. The vol knobs 1allow for manual
control of each channel's respective level. The
bal knob 2adjusts the channel's position in
the stereo panorama. The aux send a knob
3adjusts the amount of stereo signal (pre or
post-VCA) sent to the auxiliary outputs. The il-
luminated active|aux pre button 4allows
the user to mute the channel and switch be-

 5displays each channel's
post-fader level of combined left+right sig-
nals. The lower section contains pairs of jacks
for signal inputs 6, and CV inputs for bal
7and vol control 8.
The bottom row of sockets is common to all
four channels and offers pairs of sum out-
puts 9, stereo aux send a outputs 10 ,
2
module
explained

3
front panel
overview
1
2
3
6
7
13 10 11

4
5
4
8
9
stereo aux return a inputs 11 , and direct
inputs jacks 13 , that allow you to stack other
sub-mix modules with Ostrawa.
STEREO VS MONO SOURCES
Ostrawa features four stereo pairs of inputs,
which enable mixing up to four stereo sourc-
es. However, monophonic sources may also be
mixed in, as each stereo pair of inputs is in-
ternally normalled so that connecting a mono
signal into the l input correctly places it in
the center of the stereo scene.
VOLUME CONTROL
The signal amplitude in Ostrawa is controlled
using high-quality VCA chips with an expo-
nential response we carefully bent to achieve
the natural fader curve found in professional
studio consoles. The control circuit combines

The vol knob generates appropriate offsets
to the incoming CV while retaining the dy-
namic response to CV regardless of the at-

the interface
gain response to knob position
0 1 2 3 4 5 6 7 8 9 10
10
0
-10
-20
-30
-40
-50
-60
-70
5V
10V
8V
Attenuator setting
Gain [dB]

5
tenuator position. For example, with attenu-
ators at max, a CV of 8V opens the channels
to +1dB, while closing the attenuators silenc-
es the channels to -85dB. Control voltages
above 8V are well-tolerated, although the
gain response is strongly compressed above
0dB, offering only up to +3dB to minimize
distortion. This behavior affects the sound
similar to dynamic compression; however,
lowering the attenuators' setting diminishes
the effect, bringing it down to a non-com-

Each vol input jack is internally normalled to

at full range without any CV patched in.
CHANNEL MUTING
A short press on the active|aux pre button
silences the corresponding channel. This state
       -
light. The clickless muting action is achieved
by introducing a few-millisecond fade-out to
near -90dB. Pressing the button again reac-
tivates the backlight and brings the channel
back through a similar clickless fade-in. The
operation of mutes can be controlled remotely
via the Bohumin expansion module that offers
four gate inputs.
PANORAMA CONTROL
Ostrawa offers both manual and CV control
over the position of each source in the output
stereo panorama. Please keep in mind that
Ostrawa operates similarly to normal studio
mixers, i.e., it only controls the attenuation of
the left vs. right signal and will not magical-
ly rotate the auditory image of your stereo
sound source around your head.
The bal knobs act as offsets to the bal input
control voltages, which are expected in the
range of -5V to +5V. Negative and positive
CV values shift the auditory image to the left
and right, respectively. The response of the
combined manual and CV controls provides
     
setup (3dB pan law); however, as channel
volume
& panorama
 gain response to cv  balance control response
0 1 2 3 4 5-5 -4 -3 -2 -1
0
-10
-20
-30
-40
-50
-60
-70
810
5
Balance response
Pan control voltage
0 1 2 3 4 5 6 7 8 9 10
10
0
-10
-20
-30
-40
-50
-60
-70
10
4
2
7
1
Gain [dB]
Volume control voltage

auxiliary
send & return
6
gain approaches 0dB, the response is slightly
compressed to prevent an increase in loud-

AUXILIARY OUTPUTS
The stereo pair of aux send a outputs and
the pair of stereo aux return a inputs facil-
itate adding external effects in parallel to the
main mixing bus. A dedicated aux a knob in
each channel sets the amount of signal add-
ed to the aux bus.
There are two modes of the send function:
pre-VCA and post-VCA, similar to pre-fader and
post-fader sends in studio consoles. Pressing
the active|aux pre button for one second
      
changing the backlight color from green to yel-
low. note: pre-VCA mode (yellow backlight)
uses the original signal before attenuation
and panorama balance are applied. The pre-
VCA mode is the only way to obtain a fully wet
processed signal (with the channel's vol at-
tenuator turned all the way down).
Ostrawa does not offer control of the return
level. Therefore, you must take care of prop-
er external attenuation of your effect signal.
However, such a control is available in the
Bohumin expander, which also features a
second effects loop (aux b).
LEVEL INDICATORS
AND MIXING BUS LEVELS
Each of Ostrawa’s channels is equipped with
an individual post-fader level indicator. The
        -
ume detector with thresholds at -40dB, -23dB,
1 2 3 4
ostrawa block diagram
main summing bus
aux summing bus
stereo
output
AUX
sends
AUX
return
chain outputchain input
vu
control
CV
volume
CV
balance
mute
input R
pan
aux
vca
vol
vu
control
CV
volume
CV
balance
mute
input R
pan
aux
vca
vol

7
expandability
& chaining
-15dB, -10dB, and 0dB referenced to a 10Vpp
signal. Bear in mind that Eurorack electronic
circuits cannot handle voltages greater than
10V (20Vpp), therefore it is impossible to mix
four signals of 10Vpp without serious distor-
tion. It is recommended to keep your attenu-
ators between 50% and 80% of the full range
(unless your sources are very quiet).
The mixing bus in Ostrawa features a soft
clipping circuit that offers a gentle overdrive
for signals exceeding 16Vpp. This solution
prevents the harsh-sounding distortion re-
sulting from hard clipping should the sum of
your signals exceed the dynamic range of the
output stage.
EXPANDABILITY
Multiple Ostrawa and Praga units may be
chained to provide a cascaded sub-mix setup
such that the content of the mix bus of all up-
stream units is injected 1:1 into the mix bus
of every subsequent unit appearing at its sum
outputs (i.e., the output of each chained
module is added to the output of the following
module in the chain). The chaining headers
at the back of the module (fully compatible
between Ostrawa and Praga) should be con-
nected with the supplied ribbon cable so that
the out header of the preceding unit goes into
the in
inout header
of the last unit should remain unconnected.
Ostrawa features a special set of expander
ports allowing connection to a dedicated Bo-
humin expander. Please refer to the Bohumin
manual for its functions and installation in-
structions. note: Praga's Hrad expander is
not compatible with Ostrawa!
An additional pin header at the back of Ostra-
wa connects to a simple passive breakout pan-
el called Ustka (available separately) offering
individual VCA outputs that may be used for
multitrack recording.
ACCESSORY
Our Coal Mine black panels are available for
all Xaoc Devices modules. Sold separately. Ask
your favorite retailer. •
 chaining multiple ostrawa and praga units
POWER
CONNECTOR
1967VOLTAGE CONTROLLED MIXING SURFACE
REV.03.2017

MAIN
FEATURES
Full stereo
architecture
Natural volume
response
Channel level
indicators
Stereo auxiliary
send and stereo
returns
Clickless channel
muting
Expandable by
chaining more
units and optional
expander modules
TECHNICAL
DETAILS
20hp, Eurorack
synth compatible
Depth: 46mm
Current draw:
+180mA/-150mA
Reverse power
protection
EASTERN BLOC TECHNOLOGIES MADE IN THE EUROPEAN UNION
ALL RIGHTS RESERVED. CONTENT COPYRIGHT ©2023 XAOC DEVICES. COPYING, DISTRIBUTION, OR COM-
MERCIAL USE IN ANY WAY IS STRICTLY PROHIBITED AND REQUIRES WRITTEN PERMISSION FROM XAOC
DEVICES. SPECIFICATIONS ARE SUBJECT TO CHANGE WITHOUT PRIOR NOTICE. EDITING BY BRYAN NOLL.
WARRANTY TERMS
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AND TO CONFORM WITH THE SPECIFICATIONS AT THE TIME OF SHIPMENT FOR ONE YEAR FROM THE
DATE OF PURCHASE. DURING THAT PERIOD, ANY MALFUNCTIONING OR DAMAGED UNITS WILL BE
REPAIRED, SERVICED, AND CALIBRATED ON A RETURN-TO-FACTORY BASIS. THIS WARRANTY DOES NOT
COVER ANY PROBLEMS RESULTING FROM DAMAGES DURING SHIPPING, INCORRECT INSTALLATION OR
POWER SUPPLY, IMPROPER WORKING ENVIRONMENT, ABUSIVE TREATMENT, OR ANY OTHER OBVIOUS
USER-INFLICTED FAULT.
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FOR LABOR, PARTS, AND TRANSIT EXPENSES WHERE APPLICABLE.
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