Chorus 6

JUNO-style BBD chorus

The SoundForce Chorus 6 is 16HP Juno style BBD chorus with plenty of CV inputs. It is using the Xvive MN3009 and the same audio path as the original circuit. It can do the classic Juno I, II and I+II modes. I+II modes are available from both the Juno 6 and Juno 60, each in mono (as the original) or stereo, giving you 4 I+II options. But the manual and external modes can take the module away from sweet and pretty sound territories right into weird and noisy soundscapes.

Some tech-talk:
A chorus effect is created by mixing a dry signal with a delayed signal. The delay time is modulated by an LFO to create more movement. To create some stereo width, the left and right modulation are 180 degrees out of phase. Exactly like the chorus in the Juno 6/60/106, the delay is made with MN3009 BBD chips. Also as in the original design, filters are added both before and after the BBD chips in an attempt to hide some of the artefacts of this very crude IC.

To get the audio “moving” through the chain of MOSFETs in the BBDs, driver signals are needed. This used to be done with the MN3101 BBD driver chip. Unlike the MN3009, there is no clone available of this ancient IC and this was a chance to add some modernity to the design. The drivers were replaced by a modern STM32 micro-controller along some extra ICs to convert the 3.3V logic to the negative voltage needed by the MN3009.

In order to recreate a genuine Juno-type modulation, the BBD driver signals of my Juno-106 were sampled over a few periods. Using the timer peripherals of the STM32, a replica of the BBD driver signals can be sent out to the MN3009’s. Although the driver signals only follow a very basic triangle LFO shape and could have been generated using direct synthesis in the firmware, this method gives an exact copy of the original driver signals including all the potential imperfections.

Implementing a digital programmable chip also allows a digital-style interface with tactile switches and LEDs as in the original Juno interface. The last used mode is saved and loaded when the module powers on. The CV inputs (with the exception of the MIX CV) are acquired by the STM32 ADC and can modulate the BBD driver signals based on parameters in the firmware.

Features:
The SoundForce Chorus 6 has 4 main modes: I, II, I+II, MAN for manual and EXT for external. I and II are the classic Juno modes available in the Juno 6/60/106. Press both I and II to enter the I+II mode. While holding down I and II, press the OFF button to cycle through the 4 I+II options:

– Juno 60 mono – 1hz – 8% depth
– Juno 60 stereo – 1hz – 8% depth
– Juno 6 mono – 8hz – 25% depth
– Juno 6 stereo – 8hz – 25% depth

In I, II and I+II, only the mix controls can influence the sound, all the other parameters are fixed and CV inputs are not active.

Mix control:
Using the slide pot (fader) or the CV input, the mix between the dry input signal and the BBD delays output can be set. The orignal Juno balance is reached when the slide pot is in the middle (without CV connected). The mix control is a VCA-based crossfader and is 100% analog which means it can withstand really high audio rate frequency modulation.

MAN mode:
The manual mode gives you the opportunity to control every parameter of the chorus and gives you acces to the LFO that modulates the BBD delay times. Here is what you can tweak:
– time slide pot: sets the start delay time
– rate slide pot and CV input : sets the frequency of the delay time modulation
– depth slide pot and CV input: sets the depth of modulation around the start time
– mix slide pot and CV input: see above
– LFO wave: selection of different waveforms to modulate the delay time
– range: selection of 3 different frequency ranges for the LFO (S: 30Hz to 500Hz | M: 1Hz to 100Hz | L: 0.016Hz/1min to 10Hz)

In manual mode, the EXT IN CV input can be used to change the LFO wave, range or add additional modulation of the delay start time.

EXT mode:
In external mode, the BBD delay times can be modulated using an external signal through the EXT IN cv input. Plug in a CV source from your eurorack case into the EXT IN, set manually the starting delay time using the time slider and use the EXT ATT slide pot to attenuated the incoming CV signal (equivalent to setting the depth). In external mode, the depth/rate sliders and CV inputs are not active.

Stereo inputs:
The Chorus 6 can be used with either mono or stereo input signals. When the right channel is unconnected, the left channel is sent to the right jack so it can be used with mono signals. When using both inputs, each input signal is going through its own BBD delay line.

BBD delay time range:
The Chorus 6 gives you in the MAN and EXT modes the full delay range of the MN3009 from ∿0.64ms to ∿12.8ms. The original Juno chorus was using only a very limited range of the BBD chip and really stayed in the sweet spot. When you apply high frequency modulation with great amount of depth, the delay time will be pushed in the extremities of its time range and quite nasty artifacts will appear in the form of squeaks and whines. Please note this is a characteristic of the BBD. If you want to reduce those artifacts, reduce the depth of modulation. On the other hand, if you are looking to add characteristic and vibe to the sound, audio rate modulation and plenty of depth will help you to get there.

Input levels:
Both Eurorack and Line level signals can be used as input for the Chorus. A small slide switch between the 2 PCBs can switch between Euro and Line. In Euro levels, the input signal is attenuated to an healthy level that is well below the BBD distortion point. Some headroom is also needed as the chorus is a sum of the input signal and the modulated delayed signals. For that reason, when the chorus is turned off the output attenuation is about 50% of the input level. When the chorus is on, the resulting output amplitude is similar to the input level. Line levels are also accepted by the module when the switch is on Line. Please note that Line levels input will be amplified to the Eurorack levels range at the output of the module. So the output levels are Eurorack in both Euro or Line input ranges.

Specifications:
– 16HP
– Depth: 38mm (including power connector)
– Power: +12V 76mA, -12V 35mA

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