µChorus 6

Compact version of the classic Juno style BDD chorus

The SoundForce uChorus is 4HP compact version of the classic Juno style BDD chorus. 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. This module is a stripped down version of the bigger Chorus 6 module which includes CV control.

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 BDD 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.

Features:
The SoundForce uChorus 6 features 3 simple tactile switches on the front panel. Press OFF, I or II to switch between the standard modes. 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

Stereo inputs:
The uChorus 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.

Input levels:
Both Eurorack and Line level signals can be used as input for the uChorus. 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:
– 4HP
– Depth: 38mm (including power connector)
– Power: +12V 46mA, -12V 20mA

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