VCF / VCA 6

The SoundForce VCF/VCF 6 is a Eurorack interpretation of the Juno 60/106 VCF, VCA and HPF sections. It was built based on the original service manuals documentation using modern components. This module is available in both black and grey background colors.

Tech-talk

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 and VCA 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.

Signal flow

The SoundForce VCF / VCA 6 features a simple 2-input audio mixer, a voltage controlled low pass filter (VCF), a VCA and a high-pass filter with 3 fixed cutoff frequencies plus a bass boost position. The signal flow is: mixer -> VCF -> VCA -> HPF.

VCF

The VCF section 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 slider has been calibrated to give you the best experience in the original 4 poles mode. FM1 and FM2 CV inputs can be used to modulate the cutoff frequency of the filter. Those CV signals are added to manual position of the FREQ slider. Note that you can invert the polarity of FM1 using a toggle switch. 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 slider is around position 9 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. Please note all CV inputs have attenuators expect the VOCT input.

LDC

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.

VCA

This section is a “vintage-style” single OTA VCA. There is a manual slider control as well a CV input for the VCA. Both are summed together.

HPF

The high-pass filter section is a combination of both the Juno 60 and 106 HPFs. Position 0 will give you the bass boost from the 106. Position 1 will not filter at all. From position 2 to 4 will cut progressively more bass and the cut off frequencies are 225 Hz, 339Hz and 720Hz.