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The uVCF 6 is a Eurorack interpretation of the Juno 60/106 VCF section. It’s the little brother of the VCF/VCA 6, without the VCA and HPF in a compact(er) 6HP format.
This filter is a voltage-controlled low pass filter with a selection between 4 poles (24db/octave) or 2 poles (12db/octave). The travel of the FREQ pot has been calibrated to give you the best experience in the original 4 poles mode. The FM CV input can be used to modulate the cutoff frequency of the filter. This CV signal is added to the manual position of the FREQ pot. Note that the FM input has an attenuverter. The VOCT input can be used to close the filter more or less based on the pitch of the note played by your musical CV source. It can also be used to “play” the filter when it is self-resonating. The filter will be in tune across about 8 octaves. Self-resonance is occurring when the RES pot is around 4 o’clock or higher. The resonance can be also voltage-controlled using the Q CV input, this signal is added to the manual position of the RES slider. Q CV has also an attenuator.
LDC stands for “loudness drop compensation”, it is engaged when the switch is in the upper position. This feature avoid the usual input signal loudness drop with high level of resonance. Please note you will need to lower the input signal and/or VCA level to avoid distortion as LDC requires some extra headroom.
Most Juno users agree that the Juno 60 and 106 are sounding quite different. This is due to many factors: the different base clock frequency of the DCOs, the different SAW directions (falling vs rising), different packagings of the ICs, etc… Hideki Izuchi also mentioned using “a lot of premium parts” on the 60 and “With the JUNO-106, we just wanted to lower its price, whatever it took.”. That being said if you take a look at the schematics you will find that the topology of the VCF in both synthesizers is exactly the same. In the 60 the VCF was built around the IR3109, which was a custom IC including 4 OTAs. The VCA was built with a BA662, another custom OTA chip. The 106 was built using the 80017A, an IC/PCB module including both the IR3109 as well as the BA662. So basically the 60 chips but in a different package. Those were also mounted vertically to save space on the circuit board. The 80017A was tragically dipped in resin during production that caused most units to loose voices as time passed by. Some modern replacements are available and those have been tested during the making of this module.
As all the original custom ICs are now obsolete and unavailable for purchasing, the popular Texas Instruments LM13700 dual OTA IC was used to recreate the VCF using modern and widely available parts. All the passives needed have been taken over from the original schematics with some tweaks to adapt this design to the Eurorack format.