A GAME CHANGING EURORACK FILTER

The Matrix VCF in depth

After two years of development we have finally released what we consider to be the most versatile Eurorack VCF & VCA ever. Like many other AJHSynth modules it uses the very best vintage analogue circuitry as the starting point for the design, and in this case we chose the filter from the rare and very desirable Oberheim Matrix 12 synthesiser. We expanded this from the original fifteen filter compliment to sixteen distinct filter types by adding in a really useful four pole phaser!

So, behind the compact 14hp front panel lies sixteen distinct filter types, including all of the favourites: Low Pass filters with a choice of one, two, three or four poles, Band Pass filtering with two or four poles, High Pass filtering with one, two or three pole options, along with Notch filtering and four complex, combination filter types. Finally, a four stage phaser with two types of emphasis that can be forced into tuneful self-oscillation!

However, this is far from the end of the story. We decided to use our design expertise to add in some crazy resonance distortion circuitry, that could be switched on from the front panel. This was inspired by the filter from the Soviet Polivoks synthesiser, but again the implementation we have used here is considerably changed, improved, and best of all this can be applied to all filter types! This completely changes the filter character when engaged, and it forces all of the filters into self-oscillation at higher resonance settings. Better still, the oscillation waveforms are more harmonically rich than the usual sine waves - everything from gloriously fat to screaming and angry! Best of all, this takes the number of different filters up to a massive thirty two variations!

We put a lot of thought into the ergonomics and how to select the individual filter type. Filter type can be changed either with the front panel Mode knob, or by using a control voltage patched to the MODE CV input. The selected filter mode is displayed by the LED next to the filter in use. This is also very helpful when using a step sequencer to change filter mode as it gives a real time display of the filter chosen as you take the step CV voltage up or down, while setting up each individual step.

Lastly, we have also included a VCA hard-wired to the output - simply patch an envelope or an LFO to the VCA input, connect a VCO to the audio input and you have a synth voice (with a choice from 32 different filters!) using only three modules!

Thirty two Eurorack VCF's and a VCA would most certainly take up a lot of rack space - we've got all of this into 14hp and managed to have a really finger-friendly user interface, without it feeling cramped.

 

***SCROLL TO THE BOTTOM OF THE PAGE FOR DEMO VIDEOS***

 

FILTER MODES

Sixteen distinct filter modes to choose from

Here's a list of the different filter modes that are available:

1 - Four Pole Band Pass

2 - Three Pole High Pass combined with One Pole Low Pass

3 - Three Pole All Pass combined with One Pole Low Pass

4 - Two Pole Notch combined with One Pole Low Pass

5 - Two Pole Low Pass

6 - Four Pole All Pass (Phaser)

7 - Four Pole Low Pass

8 - Two Pole Band Pass

9 - Two Pole High Pass combined with One Pole Low Pass

10 - Three Pole High Pass

11 - Three Pole All Pass

12 - Two Pole Notch

13 - One Pole Low Pass

14 - Two Pole High Pass

15 - Three Pole Low Pass

16 - One Pole High Pass

All of the above filter types can be further altered by switching in P-Vox mode, this doubles the list up to thirty two different filter variants!

 

The Matrix VCF is available with either Silver or Dark Edition (Black) fascia panels

 

P-VOX MODE

Transforming the resonant character of every filter type

This magical switch completely transforms the resonance of the filter core by overdriving it in a similar fashion to the filter from the Soviet Polivoks synthesiser, which was renowned for it's powerful, aggressive character.

In common with the Oberheim Matrix 12 VCF, some of the sixteen filter types will not normally go into self-oscillation, however when we switch on P-Vox mode all sixteen filter types can be forced into self-oscillation at higher resonance settings. But that isn't the only change, the self-oscillation waveform is not the usual sine waveform, it is a rather more interesting and harmonically rich waveform that varies according to the filter type chosen - generally Low Pass modes produce the nearest to regular sine waves, whereas other filter types can produce very much more complex waveforms as can be seen in the diagrams below. It does not require any audio input for this, it can be a stand alone oscillator. Most of the filter types will track 1V/Oct reasonably well over a three to five octave range, so it can be used as a VCO with a few interesting waveshapes!

Of course, P-Vox mode isn't just useful for self-oscillation, it adds lots of character to any audio signal as P-Vox resonance is switched in.

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Examples of P-Vox self oscillation waveforms 1

 

Examples of P-Vox self oscillation waveforms 2

 

USER INTERFACE

Keeping it logical and easy to use

As Eurorack modules increase in complexity, it can sometimes take quite a while to get to grips with all of the features and work out how to integrate them into your music making. We strongly believe in designs that are intuitive and easy to understand, without having to read through pages and pages of text just to find the particular feature that doesn't seem immediately obvious.

With this in mind here is a breakdown of the front panel interface and controls:

 

 

1) Cutoff Frequency control - Rotate this control to change the filter passband.

2) Select Filter type - Manually rotating the MODE control knob selects filter type, from 1 to 16. Fully anti-clockwise selects Mode 1, which is 4P BP, and fully clockwise choses Mode 16, which is the 1P HP filter.

3) Filter chosen LED - As the MODE control is rotated, or an external MODE CV is applied, a single LED will be illuminated to show the selected filter type

4) Audio Input Level - Attenuates the signal fed into the Audio In jack. This can be useful to balance up signal levels, particularly when the filter is in self-oscillation, as it can be reduced down so that only the self-oscillation can be heard. With some filter types it may not attenuate fully off at low frequency settings.

5) Mode CV Input - An external CV (control voltage) can be patched in to this input to automatically select filter type. Please note that the MODE control knob acts as an attenuator when a patch cable is inserted into this jack. With the MODE control fully clockwise the control voltage range is from 0 to +5V and negative CV voltages should not be used as they can cause erratic behaviour. If higher control voltages are used then they can be attenuated to give the full range, so for example if 0 to +10V is used then reducing the MODE control down to position 5 will attenuate this to still allow the full range of control.

6) Audio Input - Patch in an audio signal here - modular signal levels of 10V p/p (+/- 5V) are the expected.

7) 1V/Oct CV Input - Patch a control voltage here to control the cutoff frequency so that it tracks at one volt per octave. When the filter is in self-oscillation this allows the filter to be used as an oscillator, typically over a three to five octave range.

8) 1/3 Freq CV Input - Patch a control voltage here to control the cutoff frequency so that it tracks at approximately three volts per octave, or 1V per 0.33 Octaves. This can also be used as an additional modulation input where the CV level can be controlled externally, for example when used with the Tap Tempo LFO the Output level control is very useful here. Or alternatively amplify, attenuate or bias external CV levels with our CV Mixer module before patching them in to this input.

9) Resonance Level - Rotate clockwise to increase the resonance level. At higher resonance levels some (or all) of the filter types can be forced into self oscillation. When an external Resonance CV is patched into the RES CV input the Res Level control acts as an attenuator on the incoming CV voltage, so normally it would be left fully clockwise to get the full range with external CV control.

10) Freq CV Level - This acts as an attenuator for the external control voltage (CV) patched into the Freq CV Input.

11) Peak Level LED - This will flash or stay solid red when the output level is above 10V p/p. Running with this LED constantly illuminated will not cause any damage or overload the Matrix filter itself, it is included to give an idea of the output level and make it easier to balance signal levels that are fed into other Eurorack modules further down the chain.

12) Audio Output Level - This control is used to set the filter audio output level. This is useful because different filter types can produce very different signal levels depending on the frequency and resonance settings, so this can be used in conjunction with the Peak LED to get the best signal to noise ratio, without causing distortion to other Eurorack modules that may be patched to the Matrix VCF output.

13) Freq CV Input - Patch an external control voltage (CV) here to allow voltage control of the filter cutoff frequency. The Freq CV Level control acts as an attenuator on these FCV voltages.

14) Res O/Drive switch - With the switch in the up (P-Vox off) position the normal resonance circuitry of the Oberheim Matrix 12 filter core is used, and some of the filter types will not self oscillate even with the resonance control fully clockwise. With the switch down (P-Vox on) the crazy Polivoks style resonance circuitry is selected instead - this changes the character of all filter types and makes it more aggressive and frequency rich. It is also able to force all sixteen filter types into self-oscillation. When self-oscillation is used in P-Vox mode, then instead of the usual sine waveform it produces more harmonically rich, distorted waveforms, and most of these will still track 1V/Oct so the Matrix VCF can be used as a VCO with an external keyboard or sequencer. Obviously it will not track as well as a dedicated VCO and works over a more limited range, but can still be musically useful.

15) VCA CV Input - When no patch cable is inserted into this input the internal VCA is held fully open by normalisation, in exactly the same way as if a +5V external CV has been patched in. When an external CV is patched into this input the OUT LEV control acts as an attenuator. With the OUT LEV control fully clockwise the CV control range is 0 to +5V. Negative CV voltages are ignored and will be treated as zero volts.

16) Audio Output - The audio output from the filter, the level is determined by the setting of the OUT Level control and (if used) the external VCA CV signal.

17) RES CV Input - Patch a control voltage (CV) to this input to externally control the filter resonance with other Eurorack modules. The RES control acts as an attenuator to this control voltage, with the RES control fully clockwise the CV control range is 0 to +5V. Negative CV voltages are ignored and will be treated as zero volts.

18) 2/3 Freq CV Input - Patch a control voltage into this input to control the cutoff frequency so that it tracks at approximately two volts per octave, or 1 Volt per 0.66 Octaves. This can also be used as an extra modulation input where the CV level can be controlled externally, for example when used with the Tap Tempo LFO, which has an inbuilt Output level control.

Notes on filter mode switching:

Changing the passband of a filter does not just alter the frequency spectrum of the audio signal, it varies the phase of the signal too and the amount of phase change depends upon the filter type, number of poles and level of cuff off used. Because of this there will be a difference in phase when switching from one filter mode to another - this can often create a large and very sudden DC offset to the signal that may be heard as an audible click. We did work on a muting circuit that removed these clicks, however this prevented the filter mode selection from being modulated at audio frequencies to create (amongst other effects) the amazing "Matrix Sync" sounds that Divkid demonstrated in his video, so on balance we decided that the tradeoff was worth it. If this is an issue then there is a work around - when using a sequencer to change filter modes it may be useful to include a triggered envelope generator with it's output connected to the VCA. By setting a carefully timed decay or release curve into the patch should allow smooth changes between modes by muting the signal just before the filter mode switch takes place. The DH-ADSR envelope could be very useful for this as it includes a Delay function too, so in this case an inverted envelope could dip the level of the VCA just before mode switching to mute the audio signal.

 

 

Built in VCA

Saving precious rack space

The MiniMod VCA is arguably the best sounding VCA available in Eurorack, as it captures the beautiful second harmonic distortion signature of the early Model D synthesiser, so we would always use this in a patch if we have unlimited modules available - for instance if using our RadioPhonic monster modular system. However, sometimes we just need a VCA that does what it says on the tin, without adding it's own sound signature to the signal path, and the inbuilt VCA on the Matrix VCF serves this purpose admirably. It is hard wired to the output of the Matrix VCF and is very simple to use.

If we don't patch anything into the VCA jack then the Output level control functions as a manual volume control, because the VCA jack’s internal switch is normalled internally to +5V. However, if we patch a CV (for example from an Envelope generator or an LFO) to the VCA jack then this external control voltage is used to control the level of the inbuilt VCA, and now the Output Level control will act as an attenuator for the incoming CV signal. This is very useful as it still allows the overall output level to be controlled - we can adjust the output level so that the Peak LED indicator is just starting to flash, knowing that we have set the output at optimum signal level of 10V p/p so that we don't push the output stage into clipping distortion, or overload any Eurorack modules that may be connected to the Matrix VCF Output.

 

Videos

An introduction to the Matrix VCF

Wayne Taylor (Dreams of Wires) gives a walkthrough of the Matrix VCFfeatures along with some interesting demo patches and short performances.

 

 

Divkid's in depth review of the Matrix VCF

Here is a review along with some killer Matrix VCF patches from Eurorack legend Divkid Ben.

The original Divkid YouTube video with timing Index can be found here

 

 

Ian Boddy demonstrates his Matrix VCF

Electronic Musician Ian Boddy has many years of experience with Eurorack modules, he gives his thoughts on his new Matrix VCF and demonstrates some interesting patches that he has created with it.

Ian's original YouTube from his YouTube channel can be found here

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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