Obie-Wan – Episode 2

Modulation Panel

Introduction In January 2021 I set out to recreate the Oberheim OB-1 in a Waldorf KB-37 keyboard and Eurorack case. The OB-1 was one of the first analog mono synths with presets and luckily the schematics are available, unlike the PPG 1003 Sonic Carrier. It also has a full set of hardware controls, unlike the later Moog Source with its membrane control pad. The OB-1 also sounds fantastic with a SEM VCO, a Roland VCF and a set of CEM based ADSR’s.

The Obie-Wan project is part of an AMSynths plan to diversify from analog modules, to complete mono analog synths, ending up with replicating analog poly synths (like a rack munt SCI 600).

Progress Update By late March I had the analog PCB’s (VCO, VCF, VCA & ADSR’s) at prototype testing stage, and set them aside whilst I focused on getting the rest of the synthesizer boards designed and manufactured. I also got the left hand MODULATION panel manufactured, as a test of grey and white printed on black anodised aluminum. The quality is very high but the grey is a bit too light compared with the white lettering. I may go for a darker grey.

CPU and TOUCH Test

First CPU Test I populated the Tauntek CPU PCB, which replaces a lot of the Oberheim processor board with a PIC18F2525 processor and a smaller number of CMOS chips. This solution was my project breakthrough, as it reduced the number of chips and the size of the CPU board.

Before getting the large PANEL PCB manufactured, which holds all the pots and switches, I wanted to check that my substitution of momentary switches for the original capacitance switches was going to work.  I designed a small TOUCH PCB that contains the 10 momentary switches with indicator LED’s, and the CMOS logic that deals with the pressing of switches and the lighting of LED’s. The switches I am using are the same as used in the Prophet 5 (E-Switch E5501), whilst wider versions were used on the Oberheim OBX.

Preset Switches

I made a wiring loom to connect the TOUCH PCB to the CPU board, and after checking the voltages were ok, I powered the two boards on. Major Success! The new CPU sequenced the LED’s at power on and then rests at Preset 1. I was able to change to any of the Presets, and enter and exit WRITE mode successfully, after correcting three errors I had made in the schematics. This proved the TOUCH circuit and the CPU board, and it also avoided my mistakes being made on a costly PANEL PCB.

The next set of PCB’s Originally I planned to have a single large PANEL PCB that would contain all the scanning and the residual analog circuits that don’t fit on the individua PCB’s. However this was too much circuitry for a double sided PCB and I didn’t want the expense of a 4-layer board.

I have separated out the scanning, modulation and analog circuits with a LFO PCB set behind a 16HP MODUATION panel to the left, and an ANALOG PCB, that sits behind the main PANEL PCB, and contains the additional analog circuits. My PANEL PCB hold the pots and switches, the 4051 multiplexers and the TOUCH circuit. I have to be careful not to have more than 2 PCBs in depth behind the right hand side of the design to avoid interference with the KB37 power supply.

The MODULATION section provides connectivity to the Aftertouch, Velocity, Bender and Wheel CV’s from the KB37 using short patch cables. The PITCH and GATE CV’s are derived from the Doepfer bus with the KB37, so only 4 cables are needed. The main OBIE-WAN panel is 84HP wide, which leaves 7 HP spare at the right (I may add a BBD Chorus).

I also designed a SYNTH PCB which holds the DAC and Sample & Holds, and a CPU PCB that replicates the Tauntek processor board. Both these PCB’s are 90mm in height and can fit on the base of the KB37 with ribbon cables going to the PANEL and ANALOG boards, and power coming across from the Doepfer 16-pin bus.

I checked the PCB’s for accuracy and placed an order on 02 April and ordered a set of E5501 switches from Mouser. Work continues on designing the 4 remaining PCB’s.

Power Rails The original Oberheim OB-1 has rather a large number of power rails which I need to replicate:

• +18V for the VCO cores
• +/-15V for the Op Amps and analog circuits
• +10V for CMOS logic
• +/-7V for CMOS logic
• Additional +5V for the PIC

There are also two reference voltages; AREF for the PIC microprocessor at +4.5V and VREF for the DAC at +10.69V.

For one of the 2019 Behringer projects I tried out the Oberheim SEM VCO with +/-12V analog power and +15V for the VCO JFETs. Whilst I didn’t proceeded with the  Behringer project, I did get the VCO to work successfully. This is going to be my approach on this project, as the KB37 has a 15V switched power supply that is then regulated down to +/-12V and +5V for the Doepfer power bus. The OB-1 has a central +10V power supply but I am devolving this and the +/-7V rails to local linear regulators on each PCB that needs it.

Next Steps The plan for April is to bench test the VCO and VCF, complete the design of the remaining PCB’s, test my CPU and SYNTH PCB’s with the Tauntek PIC microprocessor. I am aiming to complete the Obie-Wan project by the end of May! More in Episode 3!