Behringer 1004 VCO

Behringer 1004

Introduction One of the projects I worked with Behringer on during 2019/20 was the range of ARP 2500 clone modules, and the toughest modules to get right were the 1004 VCO and 1050 Mix Sequencer. I bought a Behringer 1004 module in September 2021 to see how well the VCO had turned out and to consider using it in a 5U replica of the ARP 1045 Voltage Controlled Voice.

The Behringer 1004 The Behringer VCO is a close replica of the original ARP schematics and the early prototypes proved impressive and worked first time! SMD components are a big advantage when it comes to VCO accuracy and stability, the B2600 VCO’s also proved precise (possibly too precise). High quality SMD Tempco resistors are used in the VCO cores.

The Behringer Manchester team completed the production version and added LDO regulators to the VCO to provide well regulated +/-11.34V voltage rails, which helps VCO stability. This is the same approach I use at AMSynths to create precision voltage rails for VCO circuits. There are five 4580 dual Op Amps, which are low noise and use BJT’s not JFETS at the input. They are a good replacement for the original 1339 and 301’s.

Front of the PCB

Timing Capacitors The original ARP 1004 used a set of seven timing capacitors in the VCO cores which are way too big to fit in a Eurorack module, and required manual selection. Behringer have chosen 4x polyester capacitors for the larger values (yellow and grey boxes in the photo) and SMD ceramics for the lower values. The low range capacitors are larger than the original; 1,150nF, compared with 710nF.

What is surprising is that the 3.45:1 ratio of capacitor value between the triangle and sawtooth cores in the original has not been retained, it is more like 10:1. This probably explains the extra frequency scale trimmer for the Triangle core (VR5).

VR9 HI END ADJUST

Trimmers The B1004 has a total of six trimmers, five of which are blue multiturn versions and not accessible, and one single turn which is accessible by the customer from the rear of the PCB (VR9). The Quick Start manual provides no details about the trimmers, so here is what they do:

  • VR10 OFFEST – adjust the overall frequency of the VCO
  • VR11 SCALE -adjusts the overall volts per octave scale of the VCO
  • VR12 SAW ADJUST -adjusts the shape of the sawtooth waveform
  • VR5 TRI SCALE – adjust the volts per octave of the Triangle core
  • VR6 TRI OFFSET – adjust the frequency of the Triangle core
  • VR9 HI END ADJUST – adjust the scaling at higher frequencies (4-8KHz)

The additional triangle frequency trimmer is probably used to compensate for using standard capacitor values in the two cores and avoids capacitor selection.

Incorrect Sawtooth

Waveform Accuracy All the waveforms were accurate and calibrated correctly by the factory, except for the sawtooth. The SAW ADJUST trimmer adjusts the shape of this waveform, which is generated by the sawtooth VCO core. The triangle VCO core is separate and is used for generating sine and square/pulse waveforms. The sawtooth waveform that has not been calibrated will show a step in the waveform, but you are unlikely to hear the difference in the audio range. You will notice it when using the B1004 as a LFO.

My B1004 VCO showed exactly this issue and I removed the front panel and adjusted VR12 to create a linear sawtooth with a 10v p-t-p amplitude. I used an expensive oscilloscope to adjust the VCO but a free software plugin can be used, in fact I detected the issue using MScope running in Cubase. The pictures show the before and after, note also the corrected waveform amplitude.

Corrected Sawtooth

The pulse waveform has a nice wide range from 5% to 95% (in fact 1.2ms of 25ms at 40Hz, which is spot on), and is exactly the same as the original ARP specification. The waveforms are all 10V peak-to-peak, again matching the original specification. The Triangle and Sine are bi-polar +/-5V whilst the Sawtooth , Square and Pulse are unipolar 0 to +10V.

The 1004 uses 100R Tempco resistors which are at R1, R2 and R19. These are also used in the Behringer 1047 filter but in series to give 200R.

Frequency Accuracy Once warmed up for 20-30 minutes the B1004 proved to be calibrated accurately at the factory, with a good set of results from C1 to C4;

  • C1 is -4 cents
  • C2 is 0 cents
  • C3 is +1 cent
  • C4 is +3 cents
  • C5 is +7 cents
  • C6 is +11 cents

The VCO was factory tuned to C1, with the Coarse control at minimum and the Fine control at midway. The VCO goes sharp above 1kHz and the calibration notes of the original ARP 1004  suggest only 1 cent inaccuracy at 1, 2 and 4kHz – this is a pretty demanding specification. There is a High Frequency trimmer (HI END ADJUST) that may help achieve better scaling above 1 kHz, in my own tests this does not do a lot.

Conclusion The Behringer 1004 VCO has worked out very well, good waveform shape accuracy and pitch stability over 5 octaves. I am building one into a 5U 1045 replica, more details in a future post.

 

PPG 340 & 380 Wave Computer

PPG 340 380 390

Overview The PPG 340 and 380 is an 8 voice wave table based synthesizer combined with a 8 track digital sequencer, which also has an additional 15 trigger event tracks. It consists of three 4U high 19″ rack modules, connected with a 50 wire bus cable and ribbon cables. There is also a VDU terminal and a 5 octave keyboard, and some times 2 VDU’s were used so that the sequencer and wave table synthesizer could be programmed at the same time.

In the late 1970’s it was a musicians dream to have a synthesizer with a wide range of sounds and a digital sequencer all in one instrument. This format became the standard in the 1980’s and increasingly accessible with workstation synthesizers like the Korg M1 launched in 1988. The original price of the 340 and 380 is unknown but probably between £5k and £10k.

It was a development from the earlier Wave Computer 360 (1978) and was one of the first, but rather large, rack synth modules ever manufactured. Very few were made, maybe 10, and they all look like hand crafted prototypes. Thomas Dolby (Windpower 1982) and Edgar Froese (Stuntman 1979) were early users who even took them on stage for playing back songs, prior to the use of personal computers and MIDI. The 340/380 were outdated by 1982 as new sampling and digital sequencers arrived (Emulator I and Fairlight CMI).

Innovation In 1979 the 340/380 was a significant technical innovation but at a high cost; a digital sequencer combined with a wave table synthesizer and VDU, too expensive and unreliable to be successful. The lack of analog filters made the digital wave sounds unfamiliar and harsh to many musicians, raw digital sounds well before the DX7 of 1983.

PPG went on to develop the Wave 2.x wavetable synthesizers with VCF’s and VCA’s as clear successors to the 340 & 380 and far more successful. with over a thousand made. The PPG Wave Term  reused the Event Generator as software programmed on the same Motorola microprocessor but using the Flex Operating System. The 340 and 380 stand out as amazing innovations for 1979 but they were really hard to use, as it meant being a software programmer!

The PPG 340A is called the Generator Unit, as it creates the sounds. The 4U rack contains six vertical PCB’s attached to a motherboard. A Wave Computer 360A processor board, four voice boards to create 8 independent voices together with an expanded TONR wave memory (8k byte) board and an IO board that has been simplified and the 6802 processor removed. The processing is done in its own rack module, so that it can be expanded in features from those offered in the 360. The expanded wave memory (which was doubled from the 360A) enables the 64 partial waves to be stored completely with 128 wave samples each, rather than half of the data stored and the other recreated by mirroring this first half.

PPG 340 Monitor

The 340A does not have a conventional synthesizer control panel with potentiometers, so the patch configuration is entered in digitally via the VDU and its keyboard. The VDU is 24 lines x 80 columns which is used to access pages of parameters for each voice. The commands available are:

  • P : Pitch Control
  • E:  Envelopes
  • W: Waves
  • C:  Compound
  • S:  Spectrum
  • Q:  Program
  • T:  Tape Handing
  • G:  Event Generator (takes the user to the 380)
  • M:  Monitor

The PPG 340B is the Processor Unit, which contains three vertical PCB’s attached to a motherboard. A new processor board called PER, featuring a Motorola 6802 CPU, some RAM, a UART  chip for connecting a serial terminal to the 340B, a timer chip, some I/O and circuitry to operate a mini cassette drive. 32 kBytes of RAM and 4 kBytes of ROM are installed in the 340B. The ROM contains a boot loader which then enables the full OS and wave tables to be booted from a mini cassette attached to the front panel.

Of course these mini cassettes have failed over time, which makes booting a 340/380 system tricky, however with modification a Waveterm A can be used instead. There was also a dual 8″ floppy diskette drive that could be used instead of the mini cassette drive.

The PPG 380 Event Generator is a highly programmable digital sequencer with 8 tracks for controlling the 8 wave table generators in the 340A, and 15 tracks for controlling external devices (Thomas Dolby used a LinnDrum). It also has a monochrome VDU with ASCII keyboard and a synthesizer style keyboard interface. It has the same CPU, ROM and RAM boards as the 340B but with slightly more (48k bytes) RAM. The 380 is also booted from the mini-cassette on the 340B. A dual port RAM board interfaces the Event Generator to the 50 wire bus allowing for high speed communication with the Wave Computer.

The 340B and 380 software design is implemented on an Operating System running on the Motorola 6802 (called Monitor), with a digital sequencer and cassette interface as the two main applications in the 380, both of which are command line driven using the VDU to display the commands and data. The 380 uses a similar approach to the 360A with sequences displayed in tables on the the VDU. Sequences can be recorded from the 5 octave keyboard and edited with the VDU keyboard. The Event Generator application has a lot more commends available than the patch configuration and you can use a command to switch between the Event Generator and the Patch Configuration.

Storage Constraints The capacity of RAM and ROM chips in the late 1970’s were small in size compared with today and the Wave 360 and 340 had to work within significant limitations. The theoretical size of the Wave ROM is 128 samples x 64 waves x 30 tables , which is 245 kBytes. This size of ROM was too expensive in 1978, so the sample size was reduced by 50% and the number of waves stored was reduced by x8, to keep the size of wave storage to 16 kBytes.

The clever part of the 360/340 design is that the microprocessor recreates the full sample and the wave table when its loaded into RAM for playback, by calculating the missing wave structures and interpolating the missing waves in the table. These calculations are done quickly when a wavetable is loaded from wave ROM into sound RAM when a  new preset is called up, and not during the actual playback of the wave table.

Digital Cassette

Digital Tape The 340 and 380 use an early digital cassette (DCR) designed for microprocessors to save and load program data. The drive is a Philips LDB4051 which uses a mini-cassette from a Dictaphone to record up to 64 kBytes of data per side. It takes about 2 minutes to play the tape end to end, which is probably the time gap between Thomas Dolby songs when he played live!

Large format diskette drives were available in the late 1970’s with more capacity and faster read times, but were not used by PPG until the Waveterm A, presumably due to cost or the need for a floppy drive controller chip. The LDB4051 was very easy to work with as it had a CMOS level interface that is microprocessor friendly.

Edgar Froese In the spring of 1980 Tangerine Dream recorded Tangram in Chris Franke’s new Berlin studio, and around the same time Edgar started using a new 340 and 380 system (dual VDU) in addition to his previously acquired PPG 360 and 350. The Tangram album marks a move into digital sounds and more elaborate sequencing, thanks in part to the 340 and 380. Edgar toured with the 340 and 380 until mid 1981 and then replaced it with a brand new PPG Wave 2. The next album Exit, had even more PPG sounds over it, but we don’t know when the 340 and 380 were retired. Edgar used the PPG Wave 2 series for another 5 years.

Henry in 1982

Thomas Dolby Sometime in 1981 Thomas acquired a PPG 340 and 380 system (with a single VDU) during the recording of his first album ” Golden Age of Wireless”. He used it extensively for the next year or so and as the core of his system, calling it Henry, before being replacing it with a Fairlight CMI. The 380 Event Generator triggered a set of Simmons drums and the PPG 340 sounds are unmistakable across the album. Henry fell down a lift shaft in the USA in 1984.

 

Bruel & Kjaer 1621 Band Pass Filter

1621 Front Panel

Introduction Inspired by Hainbachs use of test equipment I bought a 1621 Band Pass Filter in May 2021, to add a different sound into my analog synthesizers, especially the Big Moog. The 1621 was in good condition and was made in the 1980’s. It is a tunable single pole Butterworth filter with switchable bandwidths, with the addition of external digital control of the frequency.

You can download a very nice user manual here.

How it Works The filter is a more complex and precise device than a regular analog synthesizer band pass filter. It contains two Integrators which work as a tunable oscillator. The circuit starts oscillating when the input frequency is the same as the tuned-in frequency.  The feedback circuit controls the amplitude so the output is the same as the input amplitude. For an input frequency lower than resonance, the amplification will be very high feedback which increases the input to the first Inverter and thus stops the oscillating.

Inside the 1621

Frequency Range There are five frequency ranges selected by a front panel rotary control, with the large frequency control sweeping between the lower and upper points in each range. With a range of between 0.2Hz to 20kHz, the filter works perfectly in analog synthesis. Here are the ranges:

  • 0.2Hz  to 2Hz
  • 2Hz to 20Hz
  • 20Hz to 200Hz
  • 200Hz to 2,000Hz
  • 2,000Hz to 20,000Hz

The band pass bandwidth is switchable from 3% or 23% ( 1/3 octave), and the filter can be put in bypass mode (called Linear) whilst powered on. The filter has two voltage ranges and I have used the larger 1V RMS which is selected on the rear panel.

Nice filter capacitors

Power Supply The 1621 can be powered from a range of external power supplies (+/-14V or a +12V car battery). However I rather like the internal battery option as it makes the device truly portable, initially I powered it from 12VDC using a mains adapter and 7-pin DIN plug. A nice red LED comes on when the power is switched on with the front panel control knob. This knob also selects the bandwidth of the filter, either narrow at 3% or broad at 23%.

Connections The 1621 uses BNC connectors which I have added phono connectors to, then patched it into my 984 Quad Mixer in the Big Moog. The external power and the digital control both use DIN connectors.

Digital Control When placed in Auto mode the 1621 can be controlled by an external serial clock stream that sweeps the selected frequency range of the filter, through 1024 steps. The circuit is a discrete 10-bit DAC which transforms the serial pulses into a ramp generator that sweeps the frequency. There is also a start pulse which goes out from the 1621 when the START switch is momentarily depressed.

 

Oberheim DPX-1 Refurb 1

My DPX-1’s in 1998

Overview  I bought an Oberheim DPX-1 in July 2020 with a failed power supply, which is very typical as the original contains a weak spot – the Rifa X2 capacitor across the mains inlet power which burns out rather spectacularly. I originally had two of these sample players back in the late 1990’s along with a HDX-20 hard disk, which I never got to work (the hard disk has to be formatted at the Oberheim factory). I used it to play Emulator II samples when I ran the Emulator Archive, its a nice sound but not an exact replica.

Fast forward 20 years and the DPX-1 remains a very good way to replay Emulator II samples (and S900, Mirage, Prophet 2000), but with the added flexibility of a Gotek floppy emulator and 1000;s of sample banks. Hopefully this will be a quick repair and I can get this item into the studio.

Technology The Oberheim DPX-1 is an 8 voice polyphonic digital sample player, released in December 1986. It uses a Motorola 68000 microprocessor to reproduce 8 and 12 bit digital samples. The DPX-1 is entirely controlled by computer software which is loaded from internal EPROM at boot time (OS 2.2 is the latest version). The DPX-1 uses quite a large collection of TTL logic in addition to the SSM2045 VCF/VCA on the analog outputs. The analog channels also use fast settling JFET Op Amps, the LF400c which is now obsolete but a possible modern replacement is the OP42.

There are two neat and well laid out circuit boards, sitting on top of each other. The lower board has eight voice channels, and the upper board has the memory chips and microprocessor with 768 MB of sample memory (in 24 chips!). The DPX-1 can boot up without the analog board plugged in.

Refurb Plan I am hoping for a fairly simple refurbishment; a new power supply, a recap the analog and digital boards, and fitting a 3.5″ Gotek drive with access to lots of sample disks. However it did not go well, and I am documenting my long journey to repairing the DPX-1 to help other owners.

Power Supply First step is to replace the old PSU with a modern Meanwell RT65B or PT65B. Checking the old power supply it looks like all the electrolytics and the Rifa X2 capacitors need replacing, which adds up to £20. There are no obvious burn outs and the fuse is missing, but it does look well used and may need more parts replacing, so I decided to replace it.

The open frame PT65B has the right PCB width and length to slot into the old housing, but its a bit too high and the heat sink will touch the chassis and need isolating. The RT65B is closed frame and wider at 98 mm but will fit in the DPX-1 casing, and at £20 its a better investment than repairing the old one.

I have retained part of the original PSU housing for the power inlet socket and switch (which I upgraded to DPDT). The RT65B attaches to two new holes in the DPX-1 chassis with M3 screws.

The original Oberheim low voltage wiring is incorrectly colour coded. The floppy drives use the standard 4-pin Molex connectors and colour codes. Oberheim then added a brown wire for -12V into the analog board and decided to use a yellow wire as +5V into the digital board! I checked this before powering on, and moved the yellow wire to +5V on the RT65B. I will replace it with a new red wire during the refurb.

Gotek Drive Options The DPX-1 has two drives that could be replaced with a Gotek floppy diskette emulator. The lower 5.25″ drive can only read EII diskettes, whilst the upper 3.5″ drive can read S900, Mirage, Prophet 2000 and the DPX format backup disks. I have chosen to replace the 5.25″ drive as I have 1000’s of EII samples in .EII format, which I can load onto the USB stick, and that leaves the 3.5″ drive for using with the occasional S900 floppy disk.

To install the Gotek we need some parts:

  • 5.25″ to 3.5″ connection adapter
  • 5.25″ to 3.5″ plastic casing, buy a good one
  • 5.25″ to 3.5″ power cable adapter

Once all these parts arrived I fitted them into a case with an OLED pre-fitted, as it has the wider display hole machined out of the case. I then flashed the bar Gotek drive with HXC firmware and configured the drive for the 5.25″ Emulator II rather than the 3.5″ DPX-1. I also added a rotary encoder and a spare Boss knob.

OS Versions Oberheim actively developed the DPX-1 and released OS upgrades via ROM chips. This development was clearly well planned as the digital board has jumpers that set the ROM size . Version 2.x uses larger 512kb ROM chips and needs the jumper setting changed from Version 1.x.

  • 1.3 and 1. are bug releases
  • 1.5 Added diskette backup to the internal 3.5″ DS/DD drive, using a proprietary DPX1 format. MIDI echo on MIDI out
  • 2.0 Filter cutoff variable from 00 – 99
  • 2.1 Support for the Akai S900
  • 2.2 Support for the HDX-20 and CDS3 CDROM and individual voices

There also quite a few ECO changes to improve the quality of the sound (documented in the service manual) and the DPX-1 needs specific brands of chips in some locations.

Oberheim 3.5″ Diskette

Oberheim Factory Library Oberheim made a factory library of sample diskettes, some in 3.5″ and some in Emulator II 5.25″. Here is a list of the 5.25″ Emulator II diskettes:

  • Woodwinds 1,2, Horns 1,2,3, Plucked 1
  • Sound FX 1,2,3, Strings 1,2, Piano 1,2
  • Drums 1, Woodwinds 1, Vox 1, Cymbals 1

CDROM Upgrade With OS 2.2 came the support for the uber rare RS422 CDROM player, eight individual voices and SCSI for the HDX-20. Oberheim released an upgrade kit with the hardware and new chips. Unfortunately Oberheim did not put a hard disk format capability into the OS (I guess there wasn’t sufficient space), so only Oberheim HDX-20 hard disk units can be used and the drive was formatted at the factory.

USB Sample  Transfer OMI produced just a RS422 interface card which attached to the analog board, so that there CXROM could be used. More recently a RS422 to USB interface has been developed, the Emuser. It looks possible to combine these two interfaces, and attach the Teensey microprocessor of the Emuser straight into the serial interface chip at U106, bypassing the RS422 TX and RX chips.

A simple daughter board on the analog PCB using the same J4 connector as the OMI board, would provide USB sample transfer into/from the free EMXP software running on a Windows PC. The advantage over the Gotek drive is that the computer can be used to transfer sample banks to the DPX-1, without have to use the front panel.

Outcome The new power supply worked perfectly and the DPX-1 got to Load Disk sage with Ld on the display, however it could not find the diskette drives, possibly because they are broken. Any subsequent power on did not make it this far, no display at all. I tried various drives but no luck. An “Episode 2” post will detail the next steps in getting the DPX-1 back to life!

Lexicon PCM80 On Fire!

LPT45 Power Supply

Overview One of the downsides of owning vintage synths and FX’s from the 1980’s is that there is a risk of power supply failure, with capacitors emitting magic smoke or worse still bursting into flames. I bought an Emulator I many years ago for just £50 because when we switched it on it created a huge plume of smoke as various capacitors failed.

My approach over the last few years has been to refurbish the power supplies as I acquire the item, because they are usually 30 years old. In my new studio I also have a smoke alarm fitted and never leave items powered on overnight. I have fitted 1U Samson Power Strip PS10’s to supply all the gear, this enables me to turn off the power to all the gear quickly and simply.

Blown Capacitor

A Cautionary Tale  I recently installed my PCM80 into the main studio and powered it on, and it worked fine. However 30 minutes later magic smoke came out of the rack and I traced it to the PCM80 power supply and immediately switched it off. Looking at the top of the old switched mode power supply you can just see a capacitor top pushing up, which is a common failure mode for these old units.

But once I had taken out the power supply module you can see the failing capacitor has cause other components on the bottom of the PCB to burn out and worse still the plastic shield between the PSU and chassis has started to melt. Within a few minutes this could have become a major fire with plastic dripping onto the units below and smoke filling the room.

Make sure your studio has a Smoke Alarm!!

Fire Damage!

Lexicon PCM80 PSU Replacement With the PCM80 (1993) Lexicon moved from fully linear to switched mode power supplies. They also added subsequent linear regulators for the analog rails, to ensure low noise. They used a standard third party switching power module, which is easy to replace with a modern version with the same voltages, current supply and fixing holes.

The Artesyn LPT45 is the model to go for, and it can be bought for £50. This is a lot cheaper than sellers on eBay want for a PCM80 PSU, which is simply a LPT45. The service manual states the power rail voltages and current demand, so it was easy to check this was the correct part. PCM80 Max Ratings are:

  • +5V is 5A
  • +15V is 2A
  • -15V is 300mA

The LPT45 delivers 4A on the +5V rail but the consensus is that is sufficient to safely power the PCM80. Lexicon did a great job on the power supply design with carefully designed analog power conditioning, using linear regulators, inductors and electrolytic capacitors. This circuitry looks to be in good condition, as it is not stressed as much as the switched power supply. However I will replace the power capacitors to be safe.

Tufnol Protector

Fitting the LPT45 The new power module is an exact fit, same connections and mounting holes. The only problem I had was the plastic shield had been melted by the power failure. I replaced it with a 1mm piece of Tufnol which provides the correct electrical insulation and is not going to melt or catch on fire.

The LPT45 is grounded to the chassis ground by two of the fixing holes which have earth pads, I double checked the earth continuity from chassis to the LPT45 ground connector to be sure. I returned the PCM80 to its rack and powered up – everything works and no smoke!

Learning Lesson Whilst I have refurbished the linear power supplies of the vintage gear I own, I have left the switched mode power supplies alone. That strategy has now changed, and I am replacing them ASAP. This means replacing the Emax power supply, where I will design a mounting  PCB for the Meanwell RPT-60B, and the Wavestation where an LPT42 may fit (again with a mounting PCB). I will also complete the design of a switched mode power supply and PCB for my Roland MKS-80.

 

Oberheim Xpander – Episode 1

Oberheim Xpander

Overview In 2020 I researched the original OB-Xpander analog synth and posted my story about the prototype here. I concluded that whilst it might be possible to recreate it from OB-8 PCB’s and modified OS software, the effort was not worthwhile for a 4-voice synth. However it might be worth recreating the production Oberheim Xpander with its 6-voices but the time needed to design the five PCB’s meant it was never going to be viable for me, although people have recreated the Jupiter 8 and Memory Moog from scratch.

Then it all changed! In Spring 2021 I noticed that Syntaur had NOS Oberheim Xpander PCB’s for very reasonable prices; both the Voice and Processor but no Pot PCB. They also had the Power and Display boards but these are not needed as I will use a modern PSU and new VFD replacement displays. Before buying the PCB’s I carefully researched the availability of components, especially rare DAC’s and old ROM, RAM, and concluded everything was easily available except the 14-bit DAC. Even the numbered buttons could be bought from Mouser (if 16mm square), and the CEM3374 and 3372 are available at just £20 each.

I priced up a full BOM, which came to a total of around £2000. This is an expensive project but cheaper than a second hand one at £4000, and I would get a brand new PSU and set of displays, so my new build would be future proofed. The downside is the resale value is probably low but then I don’t plan to sell it!

Xpander Panel Mockup

The Elephant This all sounds promising but the “elephant in the room” is the panel. Whilst I may be able to get a metal case manufactured, and I can buy new wood sides, I sure can’t make either a new Mylar film or a silk screened panel. The completion of the project is reliant on someone else doing this, as replacements for existing Xpander panels.

As at Spring 2021 there are two possible contender projects which seem active, although it could be a long wait for these projects to be in production! However it will probably take me 6 months to build the Processor and Voice boards, so I am hoping the timing works out! I will build a plywood base to mount the Voice and PSU and work out a temporary panel to hold the Processor and Pot Boards.

A smaller “elephant in the room” is that I don’t have a Pot PCB, and I will have to make one. This is tricky to get right in terms of dimensions before I have a proper and accurate front panel, so I will probably use a set of smaller prototype PCB to prove the new Xpander works and then get a full sized one made when I have a panel. A one off PCB this size is around £300, so its a major expense and something I have to get right.

Progress in April The Processor and Voice card PCB’s have been ordered from Syntaur and the CEM3372 and CEM3374’s acquired. I have started to identify the type of capacitors needed in the Voice board using detailed photos of the Xpander and Matrix 12 PCB’s.  During May I hope to acquire the new VFD displays and some of the hard to find components (DAC’s, RAM, ROM’s). In terms of critical path I can build the Processor Board first but really need a Pot board working to see if the CPU boots up and loads the software. I also need a power supply!

Xpander Mockup Panel was created by Sunshine Jones and its his copyright – More in Episode 2!

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!

 

Obie-Wan – Episode 1

Introduction The Oberheim OB-1 was the first analog mono synth with presets, closely followed by the PPG 1003 Sonic Carrier. Announced in January 1977 at Winter NAMM and released back in November 1977 the OB-1 is a rare synthesizer, even though hundreds were made, it is even more rare in the UK where Oberheim had no distributor and the UK price was over £1200.

The OB-1 was innovative in 1977 but its market started to be eroded in 1978 as analog poly synths with patch memory entered the market. Whilst the Prophet 5 and  OB-X were considerably more expensive, the Jupiter 4 (1978) and 2-voice Pro Mars were closer in price to the OB-1. With only eight presets the OB-1 looked dated by 1979 as did its pre-microprocessor design. Moog did attack the preset mono synth market in 1981 with the Source but ditched real time control.

The two oscillators have a SEM heritage, and the VCA uses a CA3080 OTA. but the legacy ends there. The filter is a world away from a SVF with a Roland-like 2 or 4 pole OTA filter. The envelope generators were initially OTA based but replaced with CEM3310’s in the Mark 2 OB-1.

The modulation options are fantastic, with noise as one of the sources. The OB-1 is a precursor to the OB-X and OB-8, which harness the full set of CEM chips to great effect.

Why an Obie Wan?  I had the idea of building a replica of the OB-1, initially as a wide 84HP euro rack module.  So why is the OB-1 an attractive synth to replicate:

  • Variable waveform VCO’s.
  • Sub octaves on both VCO’s.
  • OTA 2/4-pole filter (not a SEM SVF).
  • Fast and snappy envelopes (CEM3310).
  • Fantastic bass and lead sounds.
  • 8 user presets, upgradeable to 64!

How hard could this be?

Inside an OB-1

Analog Boards The first and easy task was to design a set of analog PCB’s, one for each major component (LFO, VCO, VCF, VCA+ENVS). The schematics are easily located and I have a high resolution copy of an original service manual. Each PCB is 70 x 105mm and this is familiar analog electronics which is easy to get right. A set of PCB’s were ordered in late February 2021. The LFO board will need to be revised to include the performance controls – see below.

Digital Boards The OB-1 uses a ton of CMOS chips to scan the switches and pots, store presets, strobe the memory and S&H’s, and light the switch LED’s – as well as controlling the analog synth. Although the Z80 was launched in July 1976, and I remember the impact very well as I was studying electronics in the UK – it took another year for the Z80 to be accessible to synthesizer manufacturers, as software development was needed – which was more expensive to set up than using CMOS digital logic. You needed a computer to create the software!

The Z80 powered the first set of poly synths in 1978, starting with the Prophet 5 and the Jupiter 8. Prior to this PPG, SCI, ARP and Oberheim were using CMOS logic in their synths. and making use of high cost RAM memory back in 1977, which seriously constrained the size of storage. The OB-1 is frugal on memory and uses just 1k bit of CMOS RAM. This means that serial processing and 6-bit values for potentiometers is required. The Oberheim OB-1 case is large and for a reason, there are a lot of circuits in there!

Game Change Replicating all the CMOS logic in the OB-1 processor board was daunting and I knew it would take up  too much PCB space to make the project viable. Thanks to Tauntek the CMOS processor has been replaced by a modern PIC18F2525 processor on a smaller PCB. Whilst this PCB is still too big for my project, it can be redesigned to fit. The new processor PCB effectively upgrades the OB-1 into the 21st century with 64 preset patches, no need for a battery, although we still have the 6-bit data values with 64 values and only half of the MIDI standard of 128.

Waldorf KB37

The KB37 Host The Waldorf KB37 is a fantastic piece of design and engineering, very well made and with space for 107HP of Euro Rack. I initially used it with my SH05 modules, but it seemed wasted on a mono synth with no integration of velocity and aftertouch. Patch cables were needed everywhere and it was just a bit messy. I nearly sold it, until I released what it true role was – to host my OB-1!

Fitting the OB-1 into the KB37 was not as easy as I first thought. There is only 25mm of depth in the right hand side of the KB37 to clear the power supply. This restricts the PCB depth to use one layer, two with care. There is a Waldorf PCB at the bottom of the case facing the rear with the 5V and 12V regulators, Doepfer style power headers and the digital circuits for keyboard scanning and MIDI.

I decided  to tailor my design to the KB37 and it is not transferable to a rack mount case. I think the advantages of having the keyboard and performance controls outweighs the limitation of not being an 84HP super wide module. The KB37 brings MIDI in and out and instantly makes the OB-1 MIDI capable, it includes an on board arpeggiator with glide and the aftertouch and velocity from MIDI or the keyboard can control the OB-1 filter cutoff and the volume by using an updated performance control module.

More in Episode 2!

 

PPG 314 Sequencer

PPG 313 & 314

Introduction The PPG 314 analog sequencer dates back to 1976 and the beginnings of PPG when they were a modular synthesizer company. The first PPG sequencer in the 100 series was a straight copy of the Moog 960, but the 300 series sequencer was a more advanced design. It was still only 8 steps and 3 CV’s but it had dedicated controls for step timing and for moving around the sequence itself. Accurate switched step timing was also used in the Roland 717 and the Synthanorma around the same time.

In 1976 I was about to go to the University of Essex and study electronic engineering, whilst I had heard analog sequencers on records by Klaus Schulze and Tangerine Dream, I had never seen or owned one. I just had my trusty PE Minisonic synth! The PPG 314 which sold for £2,000 was way out of my reach, it was a years salary!

How Does it Work The arrival of CMOS logic chips in the 1970’s really opened up sequencer circuit design, thanks to its low power consumption compared with TTL (ARP 1050). The PPG 314 uses a decade counter at its core (4017) producing 8 steps, to drive 3 sets of CV’s that are summed using 741 Op Amps.  The value of each of the 3 CV’s is set by a potentiometer for each of the 8 steps.

The single sided Controller PCB is hard wired into the Main PCB and contains the 4017 counter chip and five 4011 NAND logic gate chips that buffer the 8 step positions into the CV’s and provide the logic control. There are also individual gate output for each step, which goes directly to the jack sockets.

The Main PCB is doubled sided (with no silk screen) and it holds all the potentiometers, LED’s and rotary switches, with the 6.35 mm jack sockets mounted directly to the panel.

Voltage Controlled Clock This circuit lives on the main PCB and is based on another 4011 configured as a VCO with variable pulse width. Whilst Emu Systems and Roland were using analog VCO’s in their sequencers, PPG opted for a simple CMOS version. There are Rate and Pulse Width (0 -100%) potentiometers and a LO/HI Range switch, as well a switch to turn the external control of sequencer rate OFF or ON. A green LED shows clock rate, and there are START and STOP push buttons which are mirrored as trigger input jack sockets. This enables remote control of the 314.

Step Timing The timing control for each step is preset via a 6-way rotary switch, with these timing divisions; 1, 1/2, 1/4, 1/8, 1/16 and 1/32. This is a great feature for setting up accurate rhythmical sequences, as the Moog 960 uses a potentiometer which makes timing accuracy difficult. With separate timing control the 314 retains three rows of CV control, without scarifying one row for timing. The timing is controlled via a precision voltage patched by the switches into the clock.

Programming The lowest row of 6-way switches are for programming how each step works, and provides another level of real time control. The six positions are;

  • LAST – previous step values are used for this step
  • GO NEXT – skips the step
  • GOTO 1 – moves to step 1, and starts again
  • STOP – stops at this step, and repeats
  • SEL TRIG – sends a trigger to the SEL TRIGGER output
  • NORMAL – sequences step by step

On the front panel the lettering is a bit cryptic with a solid circle as STOP and an arrow as SKIP. The GOTO 1 is not marked as available on Step 1 and 2 for obvious reasons, but its still wired up but it means you never move off Step 1.

AM314 Panel Mockup

AMSynths 314 As part of my “Big Moog” project I wanted to replicate the PPG modules that Klaus added around 1976/77 to his Moog IIIP. These consisted of a set of analog voice modules (VCO, VCF, VCA, etc.) and a pair of 314 and 313’s. I was lucky enough to have been given some photos of the inside of some of these modules including the 314, which meant I could reverse engineer most of the circuits.

The core of the sequencer is the standard 4017 decade counter but with the unique feature of being able to skip a step. Usually designers had to use a ladder of flip flop circuits to enable jumping seamlessly to the next step. Wolfgang Palm achieved this just using the 4017 which makes for a very efficient design. The 314 clock is based on a CMOS 4011 chip, and the timing switches use precision resistors and feed a voltage back into the Clock VCO. I have recreated all of the six function modes including LAST, SKIP and GOTO1.

The Euro Rack AM314 has condensed all the controls and circuitry onto 2 PCB’s, a Panel PCB and a Main PCB (Clock and Sequencer Controller). The prototype PCB’s were laid out in Spring 2021 and ordered in July. A 68HP wide panel was also mocked up to ensure all the switches and pots would fit and the lettering was in the right place. This leaves space for the 16HP 313 Seqeunetial Switch in an 84HP wide case,

 

 

Jupiter One Synthesizer

Overview The Jupiter One project is an original Jupiter 4 Voice Card turned into a complete single voice synthesizer, with the addition of a replica Controller PCB and the addition of a new Pot PCB that holds the switches, LED’s and potentiometers. The project started in 2018, but was paused for a year whilst I worked on Behringer projects, and restarted over the Xmas holidays of 2020. This post describes the work to get to a completed and full working synth.

I decided to replicate the Roland Jupiter 4 Controller functionality, which translates the 0 to +5V  potentiometer voltages into the CV’s required for the voice circuit even though I am not planning to add in the preset memory capability at this stage of the development. It also contains the LFO, Noise and half the ADSR’s, so I had to build at least 50% of it anyway! The Voice Card tends to use +15V on/off signals whilst the panel and DAC of the Roland design uses +5V. There are various level shifters on the main PCB which I have simply omitted and used +15V on the Panel PCB where needed.

A future project could be a replica Pro Mars but the Roland design is incredibly complex, very inefficient in terms of components and space and requires a lot of trimmers. The OS is locked up in a 8048 chip, so it not easy to modify the code to add more presets. A replacement microprocessor was available as the IO product but that appears to be history. I am replicating the Oberheim OB-1  preset monosynth instead, using a new microprocessor (already programmed) to reduce the complexity.

Controller PCB This new PCB holds the LFO and Noise circuits, plus the CV control stages for the VCO, HPF, VCF and VCA and six clock generators for the two ADSR’s. The LFO of the Jupiter 4 and Jupiter 8 are very similar and I have used the later design as it uses a CMOS analog switch to select the waveform. Initial tests proved the LFO worked well with a minor resistor value correction to get close to the original frequency range, but with a higher top frequency of 200Hz (not 70Hz) which I have retained.

The Noise circuit worked first time and uses the same 2SC828R transistor from the legendary TR808, which delivers a wide noise spectrum. In fact the noise level was too high and I had to trim in back to 2V p-t-p, as it would bleed through the JFET on/off switch at higher output levels.

Oscillation The VCO based on the ua726 chip worked first time, with the correct sawtooth and square waveforms, and frequency range selection, but the pulse width circuit needed some work as its uses a DP4T switch which I have not be able to source. The PW on/off switch was stuck on LFO modulation, and needed +15V as the reference voltage not +5V (level translation issue). Pulse width selection now works, with 50%, 40%, 20& and 10% widths, although the 10% is very narrow more like 5%!

The LFO bleeds slightly into the VCO frequency even with the modulation turned down, so this needs some attention.

Filters The HPF and LPF control voltages were initially too low and too high, although the filter did resonate correctly. The HPF control circuit works well in my 8104 filter module, so I correctly suspected a mistake by Roland at R111, which should be 47K not 100K. The LPF control voltage is being driven too high by the non existent Foot Pedal input which I have now removed. Both filters now work well and trimming is good.

The original filter S&H circuit uses the digital arpeggiator clock as a sample source, this is not available as it needs the 8048 microcontroller. I have used the LFO as the clock and S&H circuit from the Jupiter 8, with the front panel slider controlling the level of S&H into the VCF using a BA6110 VCA, so this could be preset in a future version with patch memory. My first attempt used an expo transistor pair which made the slider only responsive at the very top, I changed this to a single transistor linear response (as used in the JP-8).

Envelope Generators The original Roland design is unique, and a very complex way to get to a voltage controlled ADSR. It uses CMOS clocks and switches to get the voltage control and was replaced by a single IR3R01 chip in the Jupiter 8. The voice card contains half the CMOS circuit, so I have added the other half to my Controller PCB. I have proven the design by building the AM8007 Env Generator module, so I am confident I can get this working.

The filter ADSR worked immediately, whilst the amplifier ADSR was dead. I replaced the buffer Op Amp at IC21 which is a 4558 chip.

VCA and Output The voice card contains a BA662 VCA with a volume control on the front panel, and its own dedicated ADSR. This was easy to get working properly. This is also a final analog volume control before the output signal goes to the rear mounted Output jack socket.

LFO Modulation The original Jupiter 4 has a rather nice modulation panel to the left of the keyboard. I have replicated this using a thumb joystick that provides vertical pitch bend and horizontal LFO modulation depth to the VCO, VCF and VCA using the same three selection switches, and two rotary knobs.

Inputs and Outputs My Jupiter 01 has four 3.5mm jack socket inputs; traditional pitch and gate, and then X and Y inputs which over ride the Thumb Joystick control voltages and enable velocity or aftertouch to be patched into the synth. There is a single 6.35mm jack socket mounted on the rear PCB which also houses a Meanwell power supply.

Pot PCB My initial design used DP4T slide switches for the PWM, LFO waveform and Key Follow selection. But the switches could not be wired as dual four way, so I need to rethink.

Chorus Options The Jupiter 4 used a BBD chorus circuit to add some much needed depth to the single VCO sound, which I suspect I could do with as well! I could recreate the MN3004 fixed rate chorus as an internal PCB or go with a BBD-320.

Outcomes More work is needed on my PCB design. This set of tests proved many of the circuits but also showed up some issues – the 4053 LFO switcher needs +/-8V rails and the mounting of the Roland voice card needs to be extended as the gap between the PCB’s is too tight. So this project goes onto the back burner for a few months, before I get a final set of PCB’s designed.

Prologue Fast forward to Spring 2021 – and a redesign. The 4-way switches will be replaced with pots and the LFO waveform with a rotary switch. This means some mods on the Controller and Voice PCB, and some panel layout changes. The big change is to add a second VCO to give more depth to the sound, just like the Pro Mars. There is space next to the original voice card where I could squeeze the VCO in, and its all digitally controlled in terms of waveform and range. Just need to add a detune knob.

 

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