Jupiter One – Update

Panel Mockup

Overview Back in 2018 I bought an original Jupiter 4 Voice Card with the plan to turn it into a complete single voice synthesizer in a Moog 60 HP case. I added a replica Controller PCB and a new Pot PCB that holds the switches, LED’s and potentiometers. In 2020 I got as far as a prototype but with some problems:

  • Voice Card too close to Controller PCB, causing shorts
  • The 4053 in the LFO was wired to +/-15V
  • There were errors in my schematics
  • It sounded rather thin and weak, with only one VCO
  • BA6110 signals are too hot (+3dB over BA662)
  • The Controller board is too complex for a synth with no presets
  • I could not find suitable 4-way slide switches

Pro Mars Tune

Revised Plan I decided to simplify the design and strip back the Controller PCB, remove the Bend Modulation section, and align the design to be used with an Arturia Key Step which has a mod output (and arpeggiator). The 4-way slide switches were replaced with a rotary switch (LFO waveform), or potentiometers (Key Follow and PW).

To solve the thin sound I took a look at the Roland Pro Mars and realised it uses a second VCO card and not a full voice card. I could do the same, add a second VCO, especially as I had already proven my VCO deign in the AM8400. It requires a small modification to the Roland voice card to enable the second VCO to be patched in before the VCF. I have added the TUNE-A and TUNE-B  potentiometers for VCO2 and the slide switch.

I reworked both the Panel and Controller PCB’s in the Spring of 2021 and 2022, and added a VCO card. Here are the changes:

  • The Voice Card mounting has been reversed with component side facing out
  • Implemented the JP-8 LFO with CMOS switch waveform selection, and a S&H output
  • Implemented the JP-8 LFO Delay circuit (VCA based)
  • S&H modulates the VCF via a dedicated slider
  • PW is a slider rather than 4-way switch
  • LFO waveform selection is on a rotary switch
  • Key Follow is a slider rather than a switch
  • Key Follow circuit removed
  • VCF Envelope output patched to KF0 and then onto PWM
  • Removed the voltage control from the VCO, VCF and VCA modulation
  • Implemented SMD AS662 VCA’s to eliminate the +3dB boost of the BA6110
  • Moved to PTL30 slide potentiometers with LED’s
  • Added a Gate Boost circuit
  • Added VCO2 controls to the front panel
  • Added front panel audio output (as well as rear)

This involved quite a lot of work and the release of the AS662 chip inspired me to get the job done. The synth is really now a Pro-Mars without presets, or at least a Jupiter Two!

Outcomes I ordered the VCO2 PCB in March 2021 and the revised PANEL and CONTROLLER PCB’s in May 2022.

Behringer 1050 Mix Sequencer

B1050 module

Introduction During 2019 I worked with Behringer to develop their range of 2500 modules, and one of the most challenging was the 1050 Mix Sequencer. This post describes how it works and how I designed it, ably assisted by the Behringer Manchester team to achieve production status in late 2021.

The original ARP design dates back to 1969 and uses a large number of TTL chips. They draw a lot of current (500mA in total) and only the original 7474 chips from 1970 will work in the circuit (which are impossible to source). I built my own 1050 replica in 2017 and knew of the various issues in trying to recreate it. The CMOS version consumes only 130mA of power, the majority to run the CMOS logic and LED’s.

B1050 rear

The Design Rather than using TTL logic or a micro-processor I decided to use CMOS logic. I have written PIC assembler code to replicate the 1050 but Behringer use ARM chips, so it was quicker to go down a hardware route. The CMOS logic requires an inversion of the negative logic signals used in the ARP design to positive logic. It all requires a lot of IC’s! Over 30.

The original analog clock circuit has been retained, and miniaturized into SMD, with a BJT pair replacing the unijunction transistor. There is a small user accessible trimmer to adjust the base frequency of the clock, this is factory set and should not need adjustment. The clock runs from 0.5Hz to 150Hz and  generates a square wave, which is divided down by a 4024 binary counter and drives the heart of the circuit a 4555 Dual Binary to 1-of-4 Decoder/Demultiplexer. The limitation of not using a UJT is that the lowest clock frequency of the original (1 pulse every 30 seconds) cannot be achieved.

The analog circuit consists of eight inputs which are controlled individually by level pots and mixed into either a 4:2 or 8; 1 configuration with two outputs. 4580 dual op amps are used for the mixing of the analog signals and 4053 analog switches used to select whether the signal is ON or OFF. A toggle switch controls the 4:2 or 8:1 setting and sets the DG403 DPDT analog switch to the correct mixing positions. I originally specified high quality analog switches in both locations, but Behringer have gone with the 4053 for the input channel switching (lower cost).

B1050 Module

The digital circuit is a bit more complex. The clock divides down via a 4024 binary counter (only 3 stages are needed) to create the 8 channel positions as BCD. The 4555 chip is used to create a sequence of 4:2 or 8:1, with the individual channels switched ON/OFF and latched by the dual 4013 flip flops. These eight signals are than combined (OR gates) with the 4555 outputs and the external 12-pin IDC inputs from a B1027.

An eight position switch selects the number of positions before the sequence is repeated. The manual incorrectly states the rotary switch as 9-way. The LED’s in the white button switches are controlled by these 8 channels with transistor drivers, and the red buttons enable a position/channel to be activated, typically used for auditioning.

The EXTERNAL GATE toggle switch digitally enables the 10 gate positions fed in via the 12-pin IDC socket from a B1027 sequencer. A 12 pin cable is used to avoid confusion with the 10-way power cable. The clock has a toggle switch to select ON/OFF, and a single position advance. An external advance jack socket also enable step advance.

B1050 Switch

There is a 3P3T toggle switch (especially sourced by Behringer) for selecting 8:1 or 4:2 for the analog mixing and the digital sequence, so all 3 combinations are possible. The original used an even bigger 4P3T switch but the CMOS logic does some of the switching. The analog path in the B1050 is not AC coupled, so it can be used for mixing bi-polar CV signals up to 20v peak to peak.

There are three tests points on the rear of the PCB:

  • TP1
  • TP2
  • TP3

Conclusion The CMOS version of the ARP 1050 has turned out really well, with a reliable module at a low price with acceptable power consumption. A microprocessor version would be cheaper to manufacture, use less power but require software development and debugging.

Integrated Circuits Used There are over 30 IC’s in the B1050; ranging from Op Amps to analog switches and CMOS logic. The majority of the chips are on the rear of the PCB with only two on the front. Here is a list of the chips used and the functions:

  • U1 HCF4053 – analog switch
  • U2 HEF4013 – dual flip flop
  • U3 DG403 – analog switch
  • U4 HEF4072 – dual 4 input OR
  • U5 HEF4013 – dual flip flop
  • U6 HEF4013 – dual flip flop
  • U7 CD4071 – quad 2 input OR
  • U8 MC14081 – quad 2 input AND
  • U10 CD4071 – quad 2 input OR
  • U11 RC4580 – dual op amp
  • U12 HEF4013 – dual flip flop
  • U13 HC4024 – 7-stage binary counter
  • U14 HEF4069 – hex inverter
  • U15 HEF4069 – hex inverter
  • U16 HEF4011 – quad 2 input NAND
  • U17 HEF4013 – dual flip flop
  • U18 CD4555 – dual binary to 1-of-4 decoder/demultiplexer
  • U19 CD4071 – quad 2 input OR
  • U20 HEF4013 – dual flip flop
  • U21 HEF4011 – quad 2 input NAND
  • U22 RC4580 – dual op amp
  • U23 – not used
  • U24 – not used
  • U25 HEF4071 – quad 2 input OR
  • U26 HEF4071 – quad 2 input OR
  • U27 – Dual NPN transistor (LED driver)
  • U28- Dual NPN transistor (LED driver)
  • U29 – Dual NPN transistor (LED driver)
  • U30 – Dual NPN transistor (LED driver)
  • U31 HCF4053 – analog switch

 

All about the IR3109 chip

IR3109 Chips

Introduction One of the key components in vintage Roland synthesizers is the IR3109 four stage OTA chip. It enabled polyphonic analog synthesizers to be created due its smaller footprint and it helped create the sound of the 1980’s Roland analog synth, until digital filters took over in 1987. This post explores the technology inside the chip and where it was implemented from 1979 – 1986.

The Technology The IR3109 is intended as a synthesizer filter chip with an exponential CV converter, 4x OTAs, and 4x buffers (one after each OTA). It is very similar to the CEM3320 and SSM2040 chip in objectives, but with slightly different technical detail.  Roland provided a minimal datasheet in the Jupiter-4 service manual: The IR3109 contains four variable transconductance amplifiers designed for VCF applications in musical instruments. The device is equipped with four high input impedance buffers , and anti-log circuitry (VIN to VOUT) which controls the conductance of for amps.

  • wide transconductance variable range (1uυ -10mυ)
  • low offset voltage (less than +/-3mV)
  • high impedance MOS P channel buffer

It is an open design which means the stages can be used independently, so the chip can be configured as LP, HP, All Pass, as a SVF or used as four VCA’s but always under a common cutoff frequency CV which uses the exponential converter. At Roland the chip was used as a 2 and 4 pole low pass filter, 2 pole SVF with HP and LP outputs, and as a dual and quad VCA. At Boss the chip was used in phase shift mode in the PH-2 pedal and the RPH-10, and the IR3109 and D80017A appeared in the the GR-300 and GR-700 guitar synthesizers.

The technical design is assumed to be based on OTA’s that are BA662 cores, and the exponential converter is based around a NPN/PNP pair. The P-MOS buffer transistors are likely to be DMOS devices, which is unusual and may explain the difference in sound to a CEM3320. A Modwiggler thread about the IR3109 engineering is here.  The chip is frequently used with external temperature compensation.

Which synthesizers used it?  Roland started using the IR3109 in the Jupiter 4 from March 1979 to replace the hand matched BA662’s. The implementation was a standard OTA based 24dB filter with no Q compensation, heard in many Roland mono and modular synthesizers during the late 1970’s. The Jupiter 4 filter may sound a bit noisy because the signal levels into the IR3109 were rather lower than the headroom available, this “mistake” was not repeated in later designs.

The development of the Jupiter 8 in 1980 took full advantage of the new IR3109, with both 2 and 4 pole low pass modes, and Q compensation, where the input signal is boosted back into the chip when high resonance levels are dialed in. This compensates for a signal level reduction as resonance is increased.

This was unique at the time (January 1981), and not seen in the contemporary Prophet 5 and OB-X, which used the CEM3320 as per the datasheet. The Jupiter 8 was also different in that the filters would not self oscillate, although this can be dialed in using PCB trimmers. Later on Curtis made filter chips with inbuilt Q compensation; the CEM3372 in 1982 (Prophet 600) and the CEM3328 in 1984.

The IR3109 was then used in the Juno 6 and 60 (1982), the Jupiter 6 (1983) and the early MKS-80 models (1984). The Jupiter 8 configuration was simplified for the Juno’s, with only one mode (24dB low pass) and a reduction in components. However it added the ability to self-oscillate, which was common across all 3 Juno’s, and it retained Q compensation. Whilst the single DCO needed a chorus to fatten the sound, the 24dB filter delivered a fantastic sound.

IR3109 in SH-101

1982 also saw the IR3109 used in a mono-synth, the massive selling SH-101 (over 10,000 sold), for the first and only time. This is a similar configuration to the Juno with a 24dB low pass filter with self oscillation but no Q compensation. Once again it defined the Roland sound of the early 1980’s with some special magic that is hard to replicate without the IR3109.

In 1983 the IR3109 implementation in the Jupiter 6 broke new ground for a Roland filter, and hailed a different approach that would last for another 3 years. It was configured as two 12dB SVF’s with HP and LP modes, lots of good sounding resonance but no self oscillation. A BP mode could be selected by pressing both front panel buttons together. The Jupiter 6 implementation sounds significantly different to the Jupiter 8, especially at high resonance levels, and has both found lovers and haters of the SVF sound.

The resonance of the filter can be adjusted by a PCB trimmer but this is aimed at getting consistent sounds across the 6 voices and its not a way to dial in lots of self oscillation in. Q compensation is included on the first SVF, with the input signal feed into the resonance control OTA, this actually increases the signal level at low resonance settings – the reverse of the technique with a standard low pass filter. There are documented hacks to increase the signal level at higher resonance levels (by feeding the resonance CV into the VCA) and for removing the low resonance Q compensation.

IR3109 inside JX-3P

Roland also released the low cost JX-3P in 1983 at £1000 with a simplified filter implementation based on the IR3109 in 24dB low pass mode. There is no Q compensation or self oscillation, and the resonance is controlled by two transistors rather than a VCA. Full resonance is set quite low in volume, which is noticeable when compared with the Juno range. It can be sorted by adjusting the resonance PCB trimmers to provide a higher signal level from the filter (800mV+ rather than 600mV). The JX-3P has become regarded as having a thin tone due to the mild maximum resonance.

The MKS-80 of 1984 was launched with the same Jupiter 6 circuit but with only one mode (24dB low pass), which was rather expensive and component intensive for just the one mode! So Roland switched over to the IR3R05 chip half way through the production run (REV 5 version), to reduce costs and save PCB space. However the MKS-80 did introduce Roland customers to the sound of a 24dB low pass filter created from SVF’s, which became the basis of their polysynths for the next 3 years. It may also explain why the MKS-80 never sounds quite like a Jupiter 8.

Roland released the 16-voice MKS-10 analog piano module in 1984, using 16x IR3109 in 24dB low pass filter mode with variable cutoff and a fixed low resonance. This 2U rack unit has inevitably become a source of IR3109 chips to rescue vintage Roland synths, like the Jupiter 8. The MKS-10 was quickly replaced by the far better sounding digital MKS-20.

Summary The sound of the early 1980’s Roland synthesizer was dominated by the IR3109 chip, implemented as a 24dB low pass filter, with Q compensation in the Jupiter 8 and Juno polysynths. Each synthesizer has a different implementation and sound, although there are strong similarities between the Jupiter 8 and Juno 6/60.

The final days of the IR3109 in analog synthesizers were in the last SH-101’s manufactured in mid 1987. However the chip lived on inside the Boss PH-2 phaser pedal, which was manufactured until 2000 possibly longer. This was a fortunate for people needing IR3109’s for repairs, as the PH-2 could be bought at low prices in 2010 and the IR3109’s salvaged. The prototype AM8060 filter was built from a PH-2.

With over 50,000 chips manufactured in THD and SMD (possibly more), this was a Roland success story both in terms of sound but also in keeping costs down. The need for further cost reduction was to influence the next stage of Roland filter chip design.

IR3109 SMD chip

The IR3109 in SMD The D80017A chip appeared in the Juno 106, MKS-30 (1984) and the MKS-7 (1985). It was a further miniaturization of the VCF and VCA sections of a synthesizer with a SMD IR3109 chip and two SMD BA662F chips, soldered onto a ceramic carrier dipped in black resin. Whilst the SMD chips have proved long lived, the resin and printed circuit traces were a fatal mix and caused many failures over the years.

Replicas of the D80017A soon appeared from 2000 onwards to keep Juno-106’s alive, typically using LM13700 OTA’s and TL072 JFET buffer op amps. The schematic of the D80017A is the same as in the Juno 6 and 60 (24dB low pass filter), with Q compensation and the ability to self oscillate.

IR3R05 Chip

The IR3R05 In 1984 the IR3109 chip gave way to a re-modelled version of the chip as the IR3R05. This contains two state variable low pass filters, which are configured in series, with onboard resonance control and a VCA. This filter type was first seen in the Jupiter 6, where the IR3109 was used to create two SVF’s, with a second IR3109 performing resonance control.

The IR3R05 packed even more analog circuitry into a 14-pin DIL chip, with the resonance CV control and a VCA with both linear and exponential control. This chip enabled the next range of Roland analog polysynths (JX-8P, JX-10, MKS-70, Alpha Junos, MKS-50) to be manufactured at lower price points and created the Roland analog sound of the mid 1980’s.

However the downside to this implementation was the lack of self oscillation, which reduced the range of sounds that could be created from the filter. This was done because it is hard to tame the SVF’s when they go into self oscillation, with distortion and clipping a real issue. There is no resonance PCB trimmer, which makes it easier to build the synth, as the chips were consistent in quality across voices, but you can not dial it back in without making resistor changes. The IR3R05 can be driven into self oscillation with a larger resonance CV signal, which is what we have done with the AM8105 Super-JX module.

AM8109 JP-8 Filter

AMSynths Modules Back in 2010 we launched two module that replicated the filters in the Jupiter 8 and Jupiter 6, as the AM8109 and AM8060 respectively, using original IR3109 chips. We also located a stock of NOS IR3R05 chips and launched the AM8105, which like the AM8109 had the ability to go into self-oscillation at high resonance settings. The eventual shortage of these chips meant the modules went out of manufacturing, and new additions like the AM8106 did not get released (Juno 106 filter).

In 2022 the availability of a low cost and great sounding IR3109 clone from Alfa Rpar (AS3109) meant we could restart production of the AM8109 and AM8060 modules, as well as new modules such as the AM8101 (SH-101 filter), AM8160 (Juno filters) and AM8180 (MKS-80 REV4 filters). The upcoming AS662 will also enable more Roland VCF’s and VCA’s to be added to our range of modules.

Filter Capacitors The sound of a filter is partially reliant on the type of capacitor used in each filter stage, and how closely matched they are. Expensive polystyrene capacitors sound the best when closely matched at 1%, which means the filter points are the same in each stage, which results in nicer sounding resonance and self oscillation onset. Roland only used these type of capacitors in the flagship SH-7 monosynth and went with cheaper ceramic disc capacitors in nearly every IR3109 implementation.

The Juno-106 uses SMD ceramic capacitors, which may explain a difference in sound to the Juno-60 which uses THD versions. In 2022 high quality PPS SMD capacitors are available for synth designers or THD MLCC ceramic and polypropylene. The AMSynths modules use 2.5% polypropylene capacitors in the AM8109 and AM8060, and 10% MLCC ceramic capacitors in the AM8101 and AM8160 and AM8180.

What do they sound like? There are plenty of demos of Roland analog synthesizer on You Tube, so you can listen to the differences there. Whilst I have owned the Roland Jupiter 8, Jupiter 6, MKS-70, MKS-80, Juno-60 and JX-8P, I never recorded any filter comparison demos! I will upload demos of the AMSynths filter modules onto the You Tube channel.

 

Jupiter 6 Waveshaper

Original Roland Chip

Introduction The Roland Jupiter 6 (and MKS-80) use a custom hybrid chip (EHM-S226W83S ) for transforming the CEM3340 oscillator waveforms to a consistent level of 5v centered around 0V (-2,5V to +2.5V). The CEM3340 creates waveforms that are at different levels (Pulse 0 to +12V, Triangle 0 to +5V, Sawtooth 0 to +10V) which need level shifting for mixing into the VCF. This is implemented by a mixture of SMD resistors and transistors.

The chip also contains a transistor based sync circuit, as Roland chose not to use the inbuilt CEM3340 sync (weak and strong), possibly because they wanted to control the sync on/off with a +5V programmable signal from the micro-controller and didn’t want use an analog switch.

Roland took this approach to reduce the footprint of the waveform translation to a minimum using a 12-pin SIP package. This enabled the Jupiter 6 PCB’s to be more compact than the Jupiter 8. An alternative approach is to use resistors and Op Amps but this is 5x the PCB size in the days before SMD components could be used.

Roland EHM-S226W83S

Pin Out The EHM-S226W83S pin out can be easily determined from the schematic in the Jupiter 6 Service Manual:

  1.  +15V
  2. -15V
  3. Pulse Output
  4. Pulse Input
  5. Sync Input from second VCO sawtooth
  6. Sync ON/OFF from microcontroller
  7. Sync Output to CEM3340
  8. Triangle Input
  9. Sawtooth Input
  10. Triangle Output
  11. Sawtooth Output
  12. GND

Tightly Packed!

How it works There are four separate circuits; one for each waveform and one for the sync. The triangle wave generated by the CEM3340 is the 0V to +5V and is level shifted to -2.5V and +2.5V by a PNP transistor (Q1) and a resistor network (R2, R5, R6).

The 0 to +10V sawtooth wave is reduced and shifted to -2.5V to +2.5V using three resistors (R1, R3, R4).

The 0 to +12V pulse wave is reduced and shifted to -2.5V to +2.5V using a more complex circuit of six resistors, an external capacitor and a NPN transistor (R13 – R17 and Q4.). The SH-101 service manual uses a similar circuit without the final level shifting. So it is easy to determine what most of the values are without measurement.

The sync circuit is also more complex. The sawtooth wave from the source VCO is switched off by grounding the input to a NPN transistors (Q2), or going high to enable the Sync. The sawtooth is then pulse shaped by a NPN transistor, 2 diodes, a capacitor and 3 resistors, The narrow pulse resets the VCO integrator via its capacitor (Q2, Q3, C1, R7-R12), creating hard sync.

Tear Down For my Jupiter 6 VCO clone I wanted to replicate the EHM-S226W83S circuit, along with the RA3A resistor array connected to the CEM3340. The array resistor values are easy to determine from the schematics and values in the MKS-80 service manual. The Jupiter 6 makes use of all 6 resistors in the array, whilst the MKS-80 only uses five of them.

The EHM-S226W83S was harder to replicate, as although the schematics are in the manual, the values are not printed and are a mystery. I bought one in April 2018 and removed the plastic encapsulation with acetone (took 2 days of soaking), so that I could then measure the various printed on resistors using a multi meter, some components had to be desoldered to be measured.

Prototyping Once the values had been obtained the full VCO schematics were laid out using Eagle CAD. Roland has implemented the VCO with some changes to the original CEM3340 datasheet  which have also been captured.

A prototype PCB with THD components was ordered in April 2022. The AMSynths Jupiter 6 VCO will operate with 12V power rails, so the resistor values will need changing anyway, as the original uses 15V power rails. I plan to keep with THD components rather than building SMD replicas.

 

Synthanorma SQ312

Prototype SQ312

Overview The first Synthanorma analog sequencer was famously used by Klaus Schulze in the Spring of 1975 in Berlin, during the recording of his Timewind album, which was released in the August. The track  Bayreuth Return uses the sequencer in a live recording to 2-track tape. Klaus first saw the SQ312 at the Frankfurt Music fair on February 1975, and he purchased the prototype.

The Synthanorma SQ312 was the first product made by the Synthesizerstudio Bonn, founded by Matten & Wiechers in the early 1970’s. They went on to build a 16 step sequencer for Tangerine Dream – the 316.

In early 1975 there was a limited choice of sequencers for musicians to buy in Europe; only the expensive Moog 960, or the basic sequencer in the EMS AKS. During the next 18 months the options improved with the ARP 1601 (at DM2,800) the PPG 213 (clone of the 960), PPG 314 and the Roland 104 and 717. But at the Frankfurt Fair in 1975 the Synthanorma was a new innovation.

Research Approach The Synthanorma SQ312 has been tricky to research and document, as there is not much information about this very rare innovation from 1975. None of the 20 or so built seemed to have survived into modern times, and pictures of them are rare. The front panels have no or minimal lettering, so I have used the later 316 model as a way to guessing the functionality. An article published in the German Keyboards magazine in October 2004 provides a rare overview of the Synthanorma SQ312 and the image is their copyright.

Production SQ312

Sequencer Features The SQ312 has the 12x step controls on the left, each in a column, and a control section on the right.  There are three rows of potentiometers which set the control voltages for the three channels, below this a multi-way rotary switch, a push button and a toggle switch.  Looking at the later Synthanorma 316 I am guessing that:

  • Rotary switch – 3 way (END, SHIFT, SKIP)
  • Momentary push button – SET, activates the step
  • Toggle switch – ON = STOP the sequence

There is a red LED at the top of each step column to indicate when the step is active.

The control section is harder to work out and changed over the production run. The prototype that Klaus used has less controls than the ones used by Edgar Froese. The first part of the control section is the CV row controls and indicators, a sequential switch. For each row there is a red indicator LED, a rotary switch and a momentary push button. I am guessing that:

  • LED- indicates when the row is active
  • Rotary switch – 3 way (SKIP, SHIFT, STOP) controls the row
  • Push button – SET, activates the selected row when used with the step SET

To the right of the sequential switch is the voltage controlled clock. In the prototype there are potentiometers from clock rate and pulse width. Below this is a group of three push buttons and a LED. I am guessing that they are STOP, START and STEP. Below the clock are four toggle switches and one push button. I don’t know what these are for.

In the prototype there is a set of 12 jack sockets below these toggle switches and a push button. At least three of these jack sockets are for the row CV outputs (upper left and maybe duplicated in lower left) and one for the clock output. On production models the number of jack sockets was reduced to ten, and clock section expanded with more potentiometers.

The SQ312 could be synchronized to tape, so some of the jack sockets must be for this feature.  And finally at the top of the control section are two toggle switches mounted sideways. I am guessing they are for transposing the sequence.

If you have any more detailed knowledge of the controls or technical design please use the comment section below. Much appreciated!

Versions The SQ312 sequencer sold in small numbers in Germany (less than 20?) and the design evolved from the prototype to the production model. Edgar Froese had a model which looks like it has two VU meters at the top right. Edgar bought his immediately he saw the prototype at Klaus’s house (which was next door to his in Berlin). I am assuming the sequencer was based on low power CMOS logic, which was becoming available in the mid 1970’s due to its use in high volume digital calculators.

Klaus used his Synthanorma to sequence his ARP 2600 from 1975 to 1977, and used the sync feature when recording to his early 8-track tape recorder. The 2600 and Synthanorma were replaced by a set of PPG modules and dual sequencers.

 

Oberheim Xpander – 2

Pot and Display PCBs

Overview In 2021 I acquired a set of original Oberheim Xpander PCB’s – the Voice and Processor boards, so that I could build my own Xpander at around £2000. This is an expensive and crazy project that requires lots of difficult to source parts.

In 2021 I bought the CEM3372 and CEM3374 chips, and waited until the “Elephant in the Room” had been addressed. I also bought wooden ends so I could get some of the panel dimensions, and panel switches and buttons. I have not located all the correct caps.

Membrane Display

Membrane Panel This is the “Elephant in the Room”, as I cant make a membrane and have to wait until one is remanufactured. In late 2021 Sweet Discrete announced they would make the panel if they got 200 orders (see mock up picture).

I jumped in at order 196 in February 2022 and there are only 4 more orders need to get the membrane into production. I am confident this will happen, so I can start planning the rest of the build. Sweet Discrete make a wide range of replacement membrane panels and are highly recommended.

Pot PCB A smaller “elephant in the room” is that I don’t have a Pot PCB, and I will have to make one, using the membrane panel as a template. This will take a few weeks to lay out and check before ordering one. A one off PCB this size is around £300, so its a major expense and something I have to get right. Whilst a NOS Display PCB is available, the new VFD displays mean I don’t need to use one.

Metal Chassis I need to design a metal top panel, which I will do in CAD software. This will be expensive to have made as a “one off”. The bottom panel may be based on a Roland D-50 base plate. I doubt anyone has an old Xpander chassis but I will keep searching!

Xpander Encoders

Next Big Purchases The next step, in parallel to the Pot PCB design, is to gradually buy the major components needed for an initial build and test;

  • New VFD displays £400
  • Switched power supply £400
  • Processor logic chips and OS chips £200
  • New set of encoders £100

The Xpander Mockup Panel was created by Sweet Discrete and the image is their copyright – More in Episode 3!

Yamaha SPX900 Refurb

Yamaha SPX900

Introduction I bought a used Yamaha SPX900 multi-effects unit in December 2021 to complete the FX rack for my main analog console, the Soundcraft 22MTK which has five AUX sends. The plan is to refurbish the SPX, and use it for delays with pitch shifting and the Symphonic preset. Launched in 1988 (at over£700) the SPX900 was Yamahas first full bandwidth effects processor with 16-bits at 44.1Khz.

With only a mono analog input and multiplexed stereo output it was eclipsed by the more powerful SPX1000 in 1989, which has 4x the sound memory and digital I/O but at twice the second hand price. The SPX990 is a 20-bit version of the SPX900, with the same DSP chip but more memory and effect types.

The SPX900 arrived without much packaging and the rack ears were bent back. It is always important to protect rack ears when packing a unit to avoid this.  I managed to straighten them out but they have cracks in the metal now.

Technical Design The SPX900 uses an 8-bit microprocessor the HD6306, which is a derivative of the Motorola 6800 running at 4Mhz and with a 32kbyte ROM  chip holding the main OS. There is a large main custom DSP chip, as well as custom chips for EQ and modulation, this is when Yamaha were designed their own chips. The main sound memory is 7x 256k bit DRAM chips, with equates to 1.3 seconds of stereo audio signal at 16-bits and 44.1KHz. Delays times of up to 1.48s are possible, with Stereo Delay providing 740ms. In Freeze mode up to 1.35 seconds of mono audio can be sampled and replayed.

The ADC chip is the PCM78 and the DAC the often used PCM56, which deliver the 16-bit performance. The Op Amps are medium quality bi-polar NJM4556/4558’s, and the ones in the audio path could be upgraded with care to faster and lower noise versions.

Effect Types The SPX900 has 13 different effect types, with the spatial effect generated by the DSP chip, whilst others involve the EQ and Modulation chips.

  • Basic Reverbs, Detailed Reverbs, Early Reflection Reverbs
  • Delay, Stereo Echo, Modulation
  • Noise Gate, Pitch Change, Freeze, Pan
  • Compressor, Distortion, Aural Exciter

The majority of the 50 factory presets are SINGLE effects (all with digital EQ), but there are of course some MULTI effects where the different types are combined in series, and DUAL effects where two are combined in parallel (limited to reverbs and delays). There are 49 user memory slots which I will soon fill up!

Custom Early Reflections Preset 17 (Programmable ER) enables access to four sets of user modified early reflection parameters, so you can design your own reverb space. They are edited using the USER ED EDIT function. The parameters are; room size, liveness, diffusion, initial delay, high and low pass filters. This feature is often overlooked as it buried in menus but it is covered in the user manual and is a great way to create new spaces.

New LED Display

New Display The original LCD backlight had faded, and I was keen to replace or even upgrade to an OLED. However the LCD is a narrow version, which reduces the potential replacements, and rules out an OLED. I have used a LED display with white text on blue (the Midas MC21605J6W-BNMLW-V2), just £11 from Farnell.  The height and width of the new display are the same as the original, and the new display fits snuggly against the front panel bezel.

The LCD mounting is a simple snap fit into the false panel, with Yamaha deciding not to use any machine screws. The LCD is connected to the SPX900 main PCB with a 14-way ribbon cable with a captive IDC header on the LCD and a plug/socket on the main PCB.

The new display uses the same cable and pin out but with 2 extra pins (15 and 16) for the power to the LED backlight. I desoldered the header from the old LCD and soldered it into the new display leaving Pin 15 and 16 unconnected. I then wired a 220R resistor from Pin 2 to Pin 15 (+5V) and connected Pin 1 (GND) to Pin 16.

Contrast Trimmer

The contrast circuit at R226/R227 needs replacing as it does not provide the right voltage to VO at Pin 3, the LCD shows no characters. I desoldered the resistors and replaced them with a small 10K trimmer that adjusts the contrast for the display.

The contrast can now be set to have a clear blue background with no ghosting of characters. A perfect blue display with light white lettering. Much better! The contrast adjustment means you can set it up for different positions in your rack, an external trimmer pot fitted at the rear would be a great upgrade.

Recapped PSU

Power Supply Recap The SPX900 has a standard linear power supply rather than a switched variety (as used in the SPX90). I replaced the main reservoir capacitors at R503/R504 (2,200uF 35V) and R505/R511 (2200uf 16v), along with the three 7XXX voltage regulators and all the 100nF bypass capacitors which did look worn.

  • Nichicon 2,200uf 35V
  • Nichicon 2,200uF 16V

I also replaced all the local 10uF/35V capacitors near the Op Amps, 1/2.2/3.3uF near the ADC/DAC chips and 10/33uF BP capacitors in the audio signal path. 100nF ceramic capacitors were also replaced through out the main and power supply boards.

Yamaha SPX990 I bought a cheap SPX990 soon after the SPX900 purchase, which needed no refurbishment as it was in better condition and had less “miles”. The SPX990 lives in the left of my studio in a 12U 19″ rack where it is the fifth FX unit connected to the Soundcraft 22MTK, this enables the 18-bit quality and wide range of effects to be used by my collection of poly synths.

The SPX900 is mounted in a 19″ rack to the right of my studio, where it is used on the FX sends of my Behringer RX1602 mixer. This provides a synth sub mix into the Soundcraft for my Guin Guin MME, Model D, Pro-One and Korg Odyssey (Mk3).

 

Moog Modular Routing

Inside a Moog CP4

Introduction The Moog Modular system has internal signal busses, which from 1968 onwards were controlled by utility modules, both full height like the 992 and 993, and half height like the CP3. These modules are very useful as they replaced external patch cables, provide switches for routing and potentiometers for mixing. The original control panel range of modules was reworked in the System 15, 35 and 55 as CPxA modules.

The Behringer System 55 includes some of these utility modules (992, CP3A-M and CPA-O) but there is no internal bus structure, and some Moog modules have not been replicated. The objective with  my “Big Moog” is to get as close to the Moog Modular workflow as possible, so I have designed a set of four AMSynths Eurorack modules, and an optional Bus Board, to fill these gaps and provide the much needed internal bus structure.

Moog Modular Wiring

CV & Gate Control The original Moog modular has a set of internal busses and external “trunks” that are pre-wired between the back of all the modules using edge connectors, to reduce the amount of patch cables needed. In a System 55 there are three sets of CV signal (CV1, CV2, CV3) and three sets of S-Triggers (ST1, ST2, ST3), They are typically connected to external keyboards, ribbon controller or a 960 sequencer.

I have replicated the Moog bus structure with a 6-pin cable that runs behind the modules, with IDC sockets on the new AMSynths utility modules. There are three sets of CV signal (CV1, CV2, CV3) and three sets of V-Triggers (VT1, VT2, VT3) which can externally patched into from Eurorack keyboards and sequencers (including the 960). The conversion of V to S-trigger for the Behringer 911 modules is done within the AMSynths AM-CP1 or AM-CP4 modules which are used to drive the bus from external signal sources. The receiving modules are the AMSynths AM-CP3 and AM992, which are used to connect VCO’s and VCF’s to the CV signals (CV1, CV2,CV3).

Trunking Explained

Trunk Lines A Moog Modular system has up to eight “trunk lines” which are simply wires from a front panel module (e.g. CP2, CP3, CP3A, CP35) to rear mounted 6.35mm mono jack sockets. They are not universally popular with owners but they enable connections to and from out board equipment (FX, Tape Recorders). They are usually marked as TRUNK LINES on the panels and are sometimes connected to multiples.

This feature was obviously left out of the Behringer CP3A-M (which is actually a replica of the earlier CP3 with transistors), as Eurorack has no rear jack sockets and customers are used to patching everything into the front of modular synths. The AMSynths AM-CP3 module does have “trunk line” capability, with two pairs of front panel jack sockets available via internal connectors to be wired to rear mounted 6.35mm jack sockets.

AM-990 This 4HP module is based on the Moog CP1 console panel, which provides front panel jack sockets connected into the Moog bus for three CV signals and three S-Triggers. The AM990 does the same job, with six jack sockets with LED indicators for V-trigger On. A 6-pin IDC socket on the back of the PCB enables connection with AMSynths utility modules, using a 6 way ribbon cable or via the AM900 BUS BOARD PCB.

Only one AM990 is needed in a Moog Modular Eurorack system, to bring the signals onto the 6-way IDC bus. If interconnection is needed with additional cabinets I recommend using a rear mounted 6-pin DIN socket and cable set, wired via a ribbon cable to the IDC bus on the AM900 BUS BOARD. The AMSynths utility modules take the three sets of CV and V-Trigger signals and apply them to VCO banks (AM996), VCF banks (AM992, AM-CP4) and VCA/Envelope Generator banks (AM993, AM-CP4).

AM-CP3 This 20HP module is a recreation of the mixer section of the Moog CP3 console panel, which is intended as a four channel mixer for VCO outputs. The oscillator CV input routing is provided by the AM992 module. The original CP3 was used in the Moog Modular I,II,III and then upgraded to the op amp based CP3A in the 15, 35 and 55 models.

The left hand side of the panel provides accurate CV selection and mixing for a bank of AM901A/B’s or Behringer 921A/B’s. The first three jack socket inputs are normalised to the internal IDC 6-pin bus whilst the fourth CV input goes via an attenuator potentiometer and is mixed into the other threes signals using a precision Op Amp. All four inputs have slide switches for selecting on/off with LED indication. The output of the mix is available as an internal connection for AM901A VCO Controllers and as a jack socket for Behringer 921A VCO Controllers.

The AM-CP3 has a faithful reproduction of the Moog transistor based four channel mixer, with input jack sockets at the top of the panel. The mixer has a master gain potentiometer and dual normal and inverted outputs. The click filter from the original has been retained, with an on/off slide switch and amber LED indication.

The front panel also has provision for Moog style “trunk lines” with two pairs of jack sockets, which can be connected from the back of the module to two rear facing 6.35mm jack sockets in the cabinet which the AM-CP3 is mounted in. The levels of the outgoing and incoming signals can be adjusted with potentiometers, which use the CP3A Op Amp circuit. There are also four jack sockets arranged as Multiples.

AM-CP4 This  16HP module is recreation of the Moog CP4 console panel with filter CV routing, and trigger & envelope control for two sets of adapted Behringer 911 and 902 modules. The CP4 was used in Moog 1C and IIP, and the nearly identical CP4A was used in the Moog 35. This handy module reduces external patching by providing CV and trigger routing via switches. with LED indicators. The AMSynths version contains dual V to S-Trigger converters, which are Eurorack compatible.

The left hand side of the panel provides filter CV routing with four independent inputs that can be switched on or off (red LED indication). The fourth input has an attenuator potentiometer and a green LED indicator. All inputs are mixed together at unity gain using a precision Op Amp and can be used to drive VCO’s as well as filters. The output CV can be manually patched into the CV input of a 904A or 904B filter module, to control frequency cutoff. The output is also available internally for patching to an AMSynths 904 or 904B filter module, which have internal inputs.

The right hand side of the panel provides trigger selection from two V-trigger inputs, which are converted to S-Triggers and internally patched to two 911 modules. At the top of the panel there are two toggle switches (red LED indication), to select the trigger signal inputs. The LED’s indicates the presence of a gate signal when in the switch is in the on position.

The left hand column of switches (orange LED indication) route the selected S-trigger signals to two modified Behringer 911 modules (LEFT and RIGHT). The right hand switches (with green LED’s) connect the DC control voltages from the 911’s to their respective 902 Voltage Controlled Amplifiers (LEFT and RIGHT).

AM992 This 8HP module replicates the Moog original with four front panel CV inputs, selected by slide switches with LED indicators. The first three inputs are connected to an internal IDC 6-pin bus and normalised to the front panel jack sockets. The fourth CV input goes via an attenuator potentiometer and is mixed into the other three signals using a precision Op Amp. The original Moog used a passive mix approach. The AM992 has replaced the Behringer 992 modules in my “Big Moog”.

AM993 Module

AM993 This 8HP module replicates the Moog original with eight slide switches and indicator LED’s. It is designed to work with a set of modified Behringer 911, 911A and 902 modules to provide signal selection and routing. An AMSynths AM997 provides the V-trigger to S-trigger conversion and is easily internally patched into the AM993. Daughter boards are provided to enable the supplied internal cable harness to be connected into the Behringer 911, 911A and 902 modules.

The Moog 993 was used in both the IIIP and System 55 to provide routing for a set of 3 VCA’s and Envelope Generators. In smaller system the Moog CP4 provides routing for two set of VCA’s and Envelope Generators and is replicated by the AM-CP4.

AM996 This 8HP module replicates the CV mixing section of the Moog CP3/CP3A with four front panel CV inputs, selected by slide switches with LED indicators. The first three inputs connected to an internal IDC 6-pin bus and normalised to the front panel jack sockets. The fourth CV input goes via an attenuator potentiometer and is mixed into the other threes signals using a precision Op Amp. The original Moog used a passive mix approach.

The AM996 has replaced the Behringer CP3A-O modules in my “Big Moog” and complements the Behringer CP3A-M modules.

Typical System Setups A smaller replica Moog modular will be based around one or two oscillator banks, a single filter and a pair of 902/911’s. The AM-CP4 is an ideal companion for this setup along with a AM-CP3. A larger system with 3 or more oscillator banks, more filters and three 902/911’s deserves multiple AM-CP3’s with both AM992 and AM993 modules. The AM-CP1 enables the bus to be expanded across multiple cabinets.

Availability This range of four new AMSynths utility modules (and internal AM900 BUS BOARD PCB, are available from our webstore, and the AMSynths 904A, 904B and 904C will be launched in 2022.

Guin Guin MME

Guin Guin MME

Introduction Part of my “Big Moog” setup is a Behringer Model D, which occupies the space where the third bank of 921A/B VCO’s are in the replica Moog IIIP. This added the Minimoog sound which Klaus has used for many years and it avoids the setup issues in the Behringer VCO’s. In 2022 the Model D is being moved and replaced with a bank of AM901 VCO’s, with the MME acting as main lead Moog, whilst the Model D sits on top of the “Big Moog”.

The plan is to add a SCI 700 Programmer (3U) replica above the “Big Moog” to enable preset patches with the Model D. and traditional ADSR envelopes, to broaden the sound. However this approach needs separate VCO CV inputs, which the Model D does not have. I did consider adding a patch bay to the right of the Model D but then I came across the 60 HP Guin Guin MME.

The MME replicates the Minimoog circuit with THD components, except the power regulator chips and 3046 which are SMD. It has CV inputs for the VCO’s, VCF and VCA and is a perfect match for the 700 Programmer. It also enables me to make component changes. The Model D sounds very good and uses SMD PPS capacitors in the filter, so it will be interesting to compare.

I ordered a panel and PCB set in October 2021, and watched them travel across to the UK from France. Very excited to get this project started!

The Build Buying most of the components looked simple initially, especially as Tayda had nearly everything in stock. However the SMD LDO’s were not in stock anywhere, and with 1 year delivery timeframes. However I did find them at UTSource (which I have success with in the past), so I ordered them. The required 38x 2N3392 transistors were also hard to locate, I found 15x at UTSource and the rest from eBay. I  later found that Futerlec have loads at a low price!

The Micro Match connectors, which connect the Control Board to the Jack Board proved difficult to source, with very long lead times for some parts. In the end I used a pre-built cable from Farnell to finish the build. The connectors are small and the plugs are difficult to close over the cable, I used a vise.

Original Moog VCF

Component Choices The Minimoog went through a number of revisions during its manufacturing life. The MME PCB’s are designed with the 3046 based VCO’s which makes it an “old” version produced from 1972 to 1978 and I am making my replica as a 1972 version. I have used 5% carbon resistors in the build, except for where 1% metal resistors are needed for precision. Some resistors would have been carbon composite in 1972, but there is no space on the board for them.

I have matched all the transistor pairs in the ladder filter and VCA, which was typical of an original 1972 Minimoog. Later versions reduced the hand matching in the filter to save production costs. The capacitors in the VCF and VCA audio path were polyester in the Minimoog; either Tropical Fish in the early 1970’s or polyester box in the late 1970’s.

  • C62, C65, C66, C67 = 68nF – VCF ladder
  • C63, C64 = 220nf – VCF buffer
  • C51 = 330nF – VCA input

I fitted 68nF Tropical Fish capacitors in the VCF ladder and upgraded some of the electrolytic capacitors. The Minimoog and MME make extensive use of the 741 Op Amp in the VCO’s to gain stability over the previous all transistor design. Today we have options about whether to upgrade some of these to further improve frequency stability, as the wave shaping remains all discrete. The VCO Buffer Op Amps at IC4, IC8, IC15 are obvious candidates.

Powering On By the end of November I had received all the components and populated the boards, it was about 5 days work to solder all the components in. The Micro Molex connectors and control knobs proved hard to find, but success in the end. I powered on the MME with all boards connected and set the operating voltages. All the SMD regulators worked perfectly.

Initial power on faults, with resolutions:

  • VCO3 dead – Dual JFET pins swapped
  • Broken VCO2 frequency pot – bought new one
  • VCO2 frequency too high due to above
  • Noise level low – trimmed louder
  • VCO1 PW goes to 0% and 100% – incorrect pot fitted
  • No sound through VCF and VCA – reflow of solder joint
  • No glide – incorrect pot fitted
  • VCO3 scaling fails – cleaned under Dual JFET

The schematics do not come with the kit, but can be requested. I used the original Moog schematics to help fault find. The components are very tightly packed on the Analog Board and its very easy to make an unwanted solder bridge. I checked the top of the VCF ladder for -10V and with some re-soldering got the VCF and VCA working. I broke the VCO2 pot when fitting the panel, so I ordered a replacement. VCO3 is not oscillating, so I checked all the transistors and came across the problem – the dual FET had its legs crossed!

The noise level was be adjusted with the trimmer, if this isn’t sufficient I will replace the modern 2N3904 with an old 2SC828R which provides a loud and wide noise spectrum (it was used in the TR808). Sorting out the PW is more complicated, once I had all 3 VCO’s working I compared the wave shapes and checked the resistor values – all ok. Turns out I had put 5K pot in Glide and 1MA in VCO1 PW! Easily corrected! The glide pot should be really 5MA rather than 1MA, so I may switch this over to give longer glide times.

Calibration This is carefully explained in the build notes and the VCO’s scaled up accurately with only a few cents error over 5 octaves. I set my analog synths at C1 =32Hz = 0V. There are additional trimmers for the VCA which need setting. The MME uses the “old” Moog oscillator design with 3046 transistor arrays but I noticed no temperature or scaling drift once warmed up.

Outcome A very nice Minimoog clone with added features and a fantastic filter sound thanks to the Tropical Fish capacitors. The use of precision resistors has helped create an accurate set of VCO’s and I don’t think I will upgrade the buffer Op Amps. The build documentation is very good but some explanation of the new features would be useful (see below). It is a very large and challenging build, which requires lots of care in soldering, but delivers a very accurate Minimoog in 60HP!

MME Extra Features The MME has a very useful set of additional features over the Minimoog (and Model D) plus some omissions;

  • VCO2 and VCO3 Sync to VCO1
  • PWM on all VCO’s with external PWM inputs
  • Modulation has a level pot
  • VCO Mod and Wheel CV inputs
  • Individual CV inputs for the VCO’s
  • VCF audio input with level pot
  • VCF and VCF CV inputs
  • Envelope CV outputs
  • No VCF input switches (use the level pots)
  • No power on LED indication
  • No headphone output or 440Hz reference tone

 

Moog 901A/B VCO – Part I

Moog 901A/B

Overview In December 2021 one of my customers asked me if I could build the 901A/B VCO’s as an AMSynths product. In the past I had rejected the idea as they are relatively unstable and I cant get anywhere close to a Behringer 921A/B price point. However I have a small stock of CA3019 diode array chips (and they are still available new), and there is space in my “Big Moog” to fit a set of 901A/B/B/B, so let’s have a go! I will also add the rare 901C module which creates a LFO from a 901B.

The early 901’s used diodes whilst the later versions used enclosed diode array chips. The stability issue is caused by the uncompensated exponential converter, which was never corrected (MOS-LAB have  produced 901 VCO’s with a modern expo). The limited pitch range is caused by using a unijunction transistor as the sawtooth core of the module, 3 octaves of reasonable scaling appears the limit. I will be measuring this once my design is completed.

Research and Design The 901A/B went through many schematic changes in its life to make it more reliable/stable, so I will base my replica on the later 1970 version, to hopefully capture the best ideas from Bob Moog. This later version is the one in Klaus Schulze’s 1969 IIIP “Big Moog”. The modules have been successfully cloned by various 5U/MU manufacturers, and of course Moog have re-issued them as exact replicas. I haven’t seen one squeezed into EuroRack but the clones do seem relatively easy to get working.

My design criteria is to follow the original Moog schematics and use the same components, except where I cant find them or I can chose a better part. I will improve the power supply design as that is clearly what Bob was trying to achieve but without access to modern components in 1965! I have squeezed the designs into 8HP EuroRack modules with 3 PCB layers. No SMD components will be used (except possibly in the power supply), all THD, so I can solder them up myself.

Moog 901A – 1969

Stable Power The power supplies of the original are -6V, -10V, +12V and +11.5V, with the 901A powering the tracking circuit of the 901B from its +12V supply. Precision power rails are very important to the front end of a VCO and I have used modern LDO regulators across the modules.

The -6V is generated by LM2991 LDO regulators from the EuroRack -12V supply and a LM2941 LDO is used to generate the +11.5V in the 901B. The +12V power supply is interconnected across the modules using the BUS connector – see discussion below. The -10V is delivered from a LM337 adjustable linear regulator.

Voltage Divider The 901A uses a resistor ladder to generate the initial frequency control voltages, with a set of 12 voltages from -5V to +6V. I have used a precision voltage source (REF02)  to provide +5.00V, which is then buffered and inverted by a LT1013 Op Amp. VCO transposition is therefore limited to 5 rather than 6 octaves, but I doubt I would ever transpose up 6 octaves!

Moog 901B – 1969

Semiconductors The transistors used in the later versions are 2N3392 and 2N4058, which I will use as they are easy to obtain. Switching to modern transistors with higher HFE needs changes in resistor values which I wish to avoid. The transistors need to be hand matched in some of the circuits, for example the exponential generator.

There are two selected transistors; the 2N2646 UJT at Q10 and the 2N4058 at Q7. These will be hand selected using the Moog calibration process of the correct voltage at Test Point A and the sine waveform symmetry. The 2N2646 is surprisingly easy to obtain new, 50 years after it was used in the Moog 901B’s!

The later 901A modules use two CA3019 diode array chips as temperature compensation (heated like the CA3046 in the Prodigy) and are an improvement over the early versions which had no temperature compensation at all. I have located a source of NOS CA3019 chips and ordered a batch. There are five 1N34A germanium diodes, which are fortunately still available. The unobtainable CL-1 regulator diode is replaced with a contemporary 1N458A, which is what Moog used in the Minimoog re-issue.

Resistors Whilst the original made use of carbon composite and carbon resistors, with precision resistors in the voltage ladder. I have used 1% metal resistors throughout, with 0.1% in the voltage ladder of the 901A and the tracking circuit in the 901B’s (to match the tracking across the three 901B’s).

Trimmers All trimmers have been upgraded from single turn in the Moog to multiturn Bourns. The later versions of the 901A have Scale, High and Low compensation trimmers, with the Mid compensation trimmer removed (as Moog did). I have added a Pulse Width trimmer to the 901A to get accurate pulse widths. The 901B has a Tracking trimmer, as well as Sawtooth offset, Triangle symmetry and Sine offset and symmetry.

Capacitors The timing capacitor values are critical to the octave matching of the 901B and have the following values;

  • 10uF – low mode, not critical
  • 200nF – use x2 100nF in parallel
  • 100nF – use 100nF
  • 50nF – use 2x 100nF in series
  • 25nF – use 22nF and 3n3F in parallel
  • 12nF – use 10nF and 2n2F in parallel

The first three octaves will be accurate and use hand matched Wima MKP2 polypropylene capacitors, although the upper two octaves will be out a bit, This can be corrected manually by the frequency pot and its unlikely I will be switching octaves live. These capacitors have 5mm lead spacing to get them to fit on my 901A panel PCB.

There is a second set of capacitors on the  2P6W rotary switch that go into the Triangle output (to help symmetry?). These were independent of the timing capacitors in early 901A’s but later versions only added a capacitor at low and a high range points, and reused the timing capacitors for the first 4 octaves.

The rest of the capacitors are either ceramic (pF) or polyester (nF).

Mechanical Design The 901A and 901B are each built from three PCB’s, with the circuit spread across all available board areas.

  • POT PCB holds the Alps rotary switch
  • JACKS PCB holds the jacks sockets and pots.
  • MAIN PCB holds the power supplies and 10-pin connector.

Traditional 1P12W and 2P6W rotary switches are used for the frequency ranges, they require more space between the PCB and panel than my usual designs. This created the 3 layer PCB approach which also provides more room for the circuits. The 901A and three 901B’s are inter-connected using a 6-pin IDC cable, this replicates the Moog design but with a more direct dedicated wiring system.

901C Output Stage This is a rare Moog module with only 15 made on perf board in 1967. It provides complementary outputs (one normal, one inverted), for each of the four waveforms and further 2 inversion modes for the pulse waveform. The circuit is transistor based and provides waveform amplification as well, with a dual gang gain potentiometer.

The circuit requires a 3P6W rotary switch which is expensive and simply too big to fit in an 8HP module. I have used CMOS 4051 switches to replicate the functionality, with a modern Alps rotary switch controlling the six signal paths on three layers (input, output+, output-). 2N3392 and 2N4058 transistors are used, along with metal film resistors. There is one ceramic capacitor and single turn trimmer to adjust the initial gain.

 

 

 

 

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