YuSynth ARP4072 VCF
A bare double sided PCB (no need for the wire jumper links shown on the PCB layout) for the Yusynth ARP4072 VCF synth module. These PCBs are manufactured by Soundtronics with a percentage of the sale going to the creator of the YuSynth - Yves Usson. These are early days for the YuSynth Modular Synth at Soundtronics, our plan is to stock PCBs for all of the projects as well as components, kits and front panels. This is going to take time but will eventually be as comprehensive as our MFOS range.
No components are included with the PCB but check out our Synth Components section where you should find what you need. We do suggest visiting the YuSynth ARP VCF project page for detailed information including Yves panel layouts but a summary is shown below.
Sorry but there is a small error on the PCB relating to the MOTM connector.
The PCB is designed to use either BC557 or 2N3906 matched transistor pairs. The silk screen shows the outline for the 2N3906, the BC557 transistors are inserted the opposite way around. See the layout images above for confirmation.
The ARP4072 filter equipped mostly the famous ARP2600 semi-modular synthesizer.
Yves Usson designed his own version of this filter. He included in the design an input buffer stage and an ouptut buffer stage.
The output buffer stage is also used to compensate the loss of low frequencies that is generally observed at high resonance in 24db/octave filters such as the famous Moog ladder.
Curiously enough, neither Bob Moog nor Alan Richard Pearlman cared to compensate for this loss. This is strange knowing that it is quite a simple modification. Such compensation was introduced by Roland for the filter of their nowadays very sought after TB303 !
On the two boards Yves built of this filter, he obtained pure sinewave auto-oscillation at high resonance within the range 20Hz-20kHz, for the first board, and within the range 60Hz-20kHz for the second board.
The PCB can accept a choice of three types of power connector, it can be either a Synthesizers.com 6 pins MTA connector, a MOTM 4 pins MTA96 connector or a 10-pin Eurorack connector.
Click on the schematic thumbnail above for the circuit diagram.
The heart of this circuit is based on the EFM-VCF7b (which is an adaptation of the original ARP-4072 VCF). Yves added two inverter buffer stages, one for the input signal (U2a) and one for the output signal (U2b). The second stage (U2b) plays also another part? In fact it is used as a adder inverter which adds the signal taken after the resonance potentiometer (through R43) to the direct output signal (through R40). This trick is used to increase the gain of the final stage and to compensate the drop of the signal when the resonance is cranked up.
The schematic shows 2SA798 dual transistors but our PCB is designed to use simple PNP transistors such as BC557 or 2N3906 instead. Improved results can be obtained by matching these transistors in pairs down to 2mV VBE.
The wiring schematic image in the above thumbnails show the panel wiring to the pots and sockets etc.
Setting and Trimming
V/Octave tracking :
- Turn the frequency knob to 0, turn all the control knobs to 0 (fully counter clockwise)
- Apply 0V to the V/Oct input
- Measure the voltage at common node between R46, R30 to R34, adjust T1 in order to obtain 0V at this very node.
- Apply 1V to the V/Oct input
- Adjust T2 in order to measure 18.7mV at the previous node.
- Apply 0V to the V/Oct input
- Adjust T1 in order to measure -187mV at this very node.
Frequency range setting :
- Apply a 40Hz squarewave to one of the audio input, connect the output to an amplifier.
- Set the FREQUENCY potentiometer to 0 (fully counter clockwise)
- Adjust T1 until no signal can be heard at the output, that's it !
- If you wish to change the range of the resonance potentiometer change the value of R44, increasing its resistance will reduce the resonance range, reducing the resistance will increase the auto-oscillating behaviour at maximum resonance.
- If you wish to keep the original ARP resonance behaviour suppress R43
The parts list below is direct from the YuSynth website.
The parts list excludes knobs although we have standardised on the Cliff KM20B but it does include 1/4" jack sockets. All parts are available individually (use the part number in the search box above) or as a components kit that excludes sockets and knobs.
|Q1 to Q12||2N3906 matched by pairs||7210-101||6|
|R1,R2||10 ohms 5%||7163-007||2|
|R8,R10*,R11*,R15*,R16*,R20*,R21*,R25*,R26*||220 ohms * matched to 1% 5% otherwise||
|R7,R9*,R14*,R19*,R24*,R42,R46||1k * matched to 1% 5% otherwise||7163-051||7|
|10k * matched to 1%
|C3,C4,C5,C6||470p matched to 1%||7212-701||4|
|T2||2k2 10 turns trimmer||7212-855||1|
|T1||47k 10 turns trimmer||7212-859||1|
|P1,P2||47k log potentiometer||7300-025||2|
|P13,P4,P5,P6||47k lin potentiomter||7300-250||4|
|Jk1,Jk2,Jk3,Jk4,Jk5,Jk6||female jack sockets||7212-206||6|
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