XAOC Zagrzeb Stereo Four Pole VC Filter User Manual
- June 2, 2024
- Xaoc
Table of Contents
XAOC Zagrzeb Stereo Four Pole VC Filter 
SALUT
Thank you for purchasing this Xaoc Devices product. Zagrzeb is a stereo multimode 4-pole (24dB/oct) state variable voltage-controlled fil-ter (SV VCF). It offers five distinct frequency re-sponses, three of which are available simultane-ously. Zagrzeb is designed to sound smooth and clean unless it is overdriven with a hot signal—however, it does not lack character! We believe it will be appreciated by fans of old Japanese synthesizers but bear in mind that Zagrzeb is not a clone of any existing design. The four-pole state variable structure is a new development and offers unique sonic characteristics. While Zagrzeb is designed for filtering stereo signals (e.g. samples or stereo oscillators like Xaoc Odessa), it can easily handle mono signals and is capable of creating a faux stereo pair thanks to a special phase shifting network at the input.
Front Panel Overview
INSTALLATION
The module requires 8hp worth of free space in the Eurorack cabinet. Always turn the power off before plugging the module to the bus board using the supplied ribbon cable paying close attention to power cable pinout and orientation. The red stripe indicates the negative rail and should match the dot or –12V mark on the bus board as well as the unit. Zagrzeb is internally secured against reversed power connection, however flipping the 16-pin header may cause serious damage to other components of your system, because it will short circuit the +12V and +5V power lines. Always pay particularly close attention to the proper orientation of your rib-bon cable on both sides! The module should be fastened by mounting the supplied screws before powering up. To better understand the device, we strongly advise the user to read through the entire manual before use.
MODULE OVERVIEW
Zagrzeb’s front panel is shown in fig. 1. The stereo pair of left/mono and right signal inputs 1 is AC-coupled and accepts all Eu-rorack signal levels (up to 20Vpp), however, a conservative 10Vpp level is recommended as filtering certain signals may occasionally yield increased amplitude thus causing distortion. With a mono signal patched into the left/mono jack the module automatically creates a stereo signal. The three pairs of jacks below the inputs are the high pass 24 2 , band pass 3 , and low pass 24 4 outputs of the filter. The bp config switch 5 near the band pass outputs offers three variants of this middle type of response. The big cutoff freq knob below 6 controls the cutoff frequency of the filter (10Hz to 20kHz) while the vertical resonance slider 7 controls the degree of resonance. The illuminated slider shaft indi-cates how much the peaks of the output signal exceed the peaks of the input signal (mostly due to the resonant ringing around the cut-off frequency). The fm 1 8 and fm 2 9 CV inputs at the bottom of the panel control the cutoff frequency. The sensitivity of the fm 1 input is controlled by the small attenuverter knob 10 from –2 oct/V through 0 to +2 oct/V. The fm 2 input is fixed at 1V/oct (uncalibrat-ed). The spread CV input 11 allows the user to animate the stereo image by moving the left and right filter cutoff frequencies in opposite directions.
FREQUENCY RESPONSE
Zagrzeb is a state variable filter, hence it of-fers multiple outputs. Common
2-pole filters offer –12dB/oct low pass, +6/–6dB band pass, and +12dB/oct high
pass responses. Zagrzeb’s 4-pole structure gives the user as many as five
different characteristics: from –24dB/oct low pass up to +24dB/oct high pass
with all three possible combinations of 6dB/oct slopes that sum up to 24 in
between. These interme-diate combinations are different variants of band pass
(an asymmetric +6/–18dB, a sym-metric +12/–12dB, and another asymmetric
+18/–6dB) covering a wide spectrum between dark, mellow, boxy, honky, and
harsh—and all that in stereo!
Some example frequency responses offered by Zagrzeb at the high pass 24 and
low pass 24 outputs are shown in fig. 2. Please note that low frequencies are
not attenuated when the resonance is high. Note also that the high pass
response is particularly steep at 24dB/oct. This results in quite a radical
filtering effect.
Fig. 3 shows the frequency responses at the band pass pair of outputs,
comparing three set-tings of the bp config switch: h18l6, h12l12, and h6 l18.
Different slopes result in less or more radical filtering of components below
and above the center cutoff frequency yielding sig-nificantly diverse timbres.
Please note that the middle symmetric response features two 12dB/oct slopes
that are twice as steep (hence more selective) as a classic 2-pole SVF.
GAIN VS RESONANCE
In every resonant circuit, increasing the resonance introduces higher gain for signals close to the resonant frequency. In filters, this may yield increased amplitude of the filtered signal and some distortion at the output.
Designers of synthesizer filters handle this problem either by compensating
for the gain increase by mixing some inverted input into the variable feedback
that controls the resonance (which results in a weaker response at bass
frequencies), by introducing nonlinear saturation into the variable feedback
(it causes specific distortion that is quite often attributed to “fatness”),
or by providing advanced dynamics processing.
The feedback in Zagreb (which is necessary for satisfying the state variable
equation) is fixed. The resonance is not controlled by changing the feedback,
but by shifting the individual poles of the transfer function.
We have decided not to compensate for the increased gain, instead, aiming at a
smooth and natural response up to the clipping point. The range of resonance
is trimmed, though, so that the clipping point is hardly reached and is
therefore non-intrusive. It is also one of the reasons Zagrzeb is by design
incapable of self-oscillation.
VOLTAGE CONTROL
The two CV inputs at the bottom of the panel control the cutoff frequency
throughout the entire audio range.
The variable fm 1 input is equipped with an attenuverter featuring a center
detent. Note that the range of this knob is up to ±0.5V/oct (in other words:
±2oct/V), which is twice as wide as the fixed input fm 2 thereby
facilitating extreme sweeps while retaining shallow modulation near the middle
position thanks to the attenuation curve of the potentiometer. The fixed input
fm 2 is not calibrated, but it provides fairly accurate tracking at
1V/oct.
STEREO PROCESSING AND EFFECT
Besides being able to filter two channels of a stereo signal with the same
characteristics, Zagrzeb offers additional animation of the stereo image by
introducing subtle changes to the cutoff frequency. The spread input is
conveniently scaled at 1/10th the sensitivity of the fm 2 input. 0V at the
spread input has no effect, while positive and negative CV results in the two
stereo filters swinging slightly in opposite directions.
Patching a mono signal to the left input creates a pseudo-stereo effect. A
special multi-stage phase shifting network introduces a slight delay between
the left and right out-puts, thus making a stereo output signal with a wider
image. Please note, the effect is most noticeable with rich sounds that change
over time and may be less spectacular with a static VCO waveform.
PATCH IDEAS
It is quite easy to achieve a notch filter response, even though Zagreb does
not offer it by default. Take the filtered signals from the high pass 24 outs
and invert and mix them with the low pass 24 outs (in other words, subtract
them). Note: The effect is most pronounced with resonance set to minimum.
When one is willing to use the filter in mono, an interesting formant filter
response can be achieved by mixing two detuned band pass filters. Patch a
5-10V CV offset into the spread jack, mix the outputs of the two band pass
outputs, and set the resonance high.
It is possible to achieve a steep 48dB/oct low pass or high pass filter
response by stacking two of Zagrzeb’s channels. For a low pass response, plug
the audio signal into the input of the left channel, then patch from the left
low pass 24 output to the input of the right channel, and use the right low
pass 24 as the final output.
ACCESSORY
Our Coal Mine black panels are available for all of Xaoc Devices modules. Sold separately. Ask your favorite retailer. •
Main features
- Stereo, four-pole state variable filter
- Five frequency responses, three available simultaneously
- 24dB/oct low pass and
- 24dB/oct high pass filtering
- No LF drop at high resonance
- Mono to stereo image effect
- Voltage controlled stereo spread
Technical Details
Eurorack synth compatible 8hp, skiff friendly
Current draw:
+70mA/–60mA Reverse power protection
WARRANTY TERMS
XAOC DEVICES WARRANT THIS PRODUCT TO BE FREE OF DEFECTS IN MATERIALS OR WORKMANSHIP AND TO CONFORM WITH THE SPECIFICATIONS AT THE TIME OF SHIPMENT FOR A PERIOD OF 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.
LEGACY SUPPORT
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RETURN POLICY
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GENERAL INQUIRIES
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written by M. bArtKOwiAK. prOOfreAding And editing by b. nOll. deSigned by M. ŁOjeK. All RIGHTS RESERVED. CONTENT COPYRIGHT © 2021 XAOC DEVICES. COPYING, DISTRIBUTION, OR ANY COMMERCIAL USE IN ANY WAY IS STRICTLY PROHIBITED AND REQUIRES WRITTEN PERMISSION BY XAOC deViCeS. SpeCifiCAtiOnS Are SUbjeCt tO CHAnge witHOUt priOr nOtiCe.