INTRODUCING THE MOOG ONE-J! (Adding Some Personal Expression to my favorite instrument...)

I love the Moog ONE. 

Having worked with it for a couple of years now, I've found a lot to like in the concept and realization of this amazing instrument. It has an incredible feature set, with a deep and complex group of modules and routing capabilities, and one of the most elegant user interfaces I've ever had the pleasure of using. 

Its sophisticated UI is the perfect way to harness the ONE's capabilities without needing a huge learning curve to master it. There's tons of knobs, and a minimum of menu diving to easily and quickly accomplish just about anything you can dream up. 

Now, that being said... 

One thing that I didn't bond with were the pitch and mod wheels. 

For my taste, they just seemed out of place with the rest of the instrument. They're made of metal, and are all smooth surfaces. The mass of the metal made them feel sluggish, and the smooth surfaces made them slippery and difficult for me to control accurately. I tried applying some self-stick rubber dots to the wheel divots, and that did help improve the grip, but the weight still made them difficult to control, and they also tended to wobble on axis. I used some thin kapton tape to better shim the fit of the wheels to the potentiometers, but even after all this work I was already starting to think that there must be a better way for me to use the left hand controller. 

Years ago, I created a custom joystick controller for my brand-new Prophet 5. It was a fairly ugly looking stand-alone box that plugged into a connector I added to the back of the P5, and was mounted on the keyboard stand beside the wheels on the unit. 

 It also housed some custom-designed electronics, including a dedicated LFO for vibrato, which freed up the P5 LFO for PWM and other modulation duties, and a sample and hold that could be assigned through the P5 poly mod and used either a noise source or the LFO waveshapes as the sample input. 

I found a nice 4-axis joystick that was built for an R/C airplane transmitter by Kraft, and with a bit of work, design and planning, and some questionable manufacturing skills, my project came to life. 
After a few upgrades and revisions to the original circuitry, I had an advanced (for the time) accessory that really worked well for me and meshed very well with my vision for using the synth, greatly expanding its capabilities for personal expression. 
Now, many years later, it's no longer just an accessory, but a crucial and organic part of the instrument. 


As I continued to program and become more familiar with the capabilities of the ONE, it became apparent to me that I still wasn't being as expressive as I wanted with the left hand controller wheels. After several months of using the instrument this way, the concept of creating an updated joystick controller to replace them began to look more and more attractive. 

I thought quite a bit about what I would want in an updated controller like this for a long time, and realized that given the much deeper Moog ONE interface capabilities, and unknown nature of the hardware involved (Moog won't release schematics, unfortunately) that this had the potential to become a huge and complicated project. Finally, I decided to take the leap, and spent the next several months examining and measuring the various wheel and CV interfaces, reverse engineered the circuitry that was involved, and made a few different proto boards to try out various hardware interface designs. When I was happy with my results, I took a deep breath, dug in and started work in earnest to create what I now refer to as the Moog ONE-J (patent pretending, all rights reserved, all rites preserved, all right!!). 

The ONE has a wealth of CV input and output jacks on the back. There are inputs for two CV expression pedals, two more dedicated CV inputs, and four CV outputs. These CV inputs are all assignable through the extensive mod matrix and are programmable per preset. The CV outputs are globally assigned, and these assignments can be saved per user space. 
This made the idea of a multiaxis controller even more attractive, as it could be be completely and easily remodeled/reimagined with each preset change (or new user space), if so desired. 

As it turns out, each of the jacks on the back had an unused normalling connection, so the joystick axes could easily interconnect through them, gaining access for all the functions through the mod matrix. If any jack actually had a CV cable plugged in, that would automatically overide that particular joystick function, truly giving me the best of both worlds. 

The ONE mod matrix allows up to 20 programmable connections per each of the 3 synth layers. Each modulation source can connect to multiple destinations with adjustable independent scaling for each one, and each destination can accommodate multiple sources, so this would allow for each programmable axis of the joystick to be incredibly powerful and expressive. 

 I found a nice 5-axis joystick suitable for my purposes, and began to design the circuitry to allow it to interface with the ONE. A multi-talented friend designed and 3D printed a cool bezel for it, and a few coats of black crinkle paint gave it a sleek retro appearance and texture, which looked at home with the rest of the left hand controller hardware. 

I used a Dremel cutting wheel to carefully cut out the wheels from the LHC box, leaving the perfect space to mount the joystick and bezel. 

A few miniature switches, an additional potentiometer, and an indicator LED were added to expand the controller assignment capabilities on the fly. More on this in a bit. 

The electronics interface for the controller was designed to allow for complete and accurate adjustment of each axis so it could seamlessly interface to the ONE, and have the same scaling attributes as the the other controllers such as the keyboard, LFOs, envelope generators, etc. (the modulation matrix for the Moog is designed and scaled so that a modulation amount of 10% produces a change of one octave). 

I felt that this was especially important, as I wanted it to organically integrate with the instrument during programming and performance. 

I also designed the interface module to connect to the instrument through a series of multi-pin connectors, so it would be easy to remove for development and modifications as the project advanced. This required a bit more work and planning, but more than paid for itself over the course of development and fine-tuning. 
I used some groovy 70's rainbow colored ribbon cable to make the interconnect cabling, because that's how electronic music looks, right?
I also used my labelmaker to print out tiny white-on-clear labels for the trimmers to make tuning and calibration easier. 

The module was built on a piece of perfboard (with a handy ground plane on one side, perfect for these types of projects) using a combination of through-hole and surface mount parts. All trimmers were precision 25-turn types which I had recently stocked up on cheaply through eBay. I used thin wire wrap type wire to make all the interconnects between components. 
This type of construction makes it easy to make changes and modifications as needed. 

One particular advantage of the original wheels was that the mod wheel typically isn't spring loaded and could be set to a particular position, like a standard potentiometer. This would allow you to set a continuous amount for vibrato, or create a set amount of PWM or other modulation that would add animation to sounds which could be very useful. The joystick axis for the mod wheel input (twist left) was spring loaded, so I decided to add a small potentiometer to the bottom of the LHC to allow me to duplicate this function. I used one with a pull switch so that it could be activated when pulled up, and the function would revert to the joystick when it was pushed down. This also allowed me to preset a mod amount that could be activated on demand if wanted. 

Groovy. 

The system design was built around a 5 axis joystick. I found this one on eBay for a very reasonable price. 

I liked it so much that I ordered a few more for other projects, and as spares if needed. I have seen them available with 5K ohm and 10K ohm potentiometers. I got a few of each, and built this project using a 10K ohm example. All of the axes are spring loaded, return to center types, and it looks like the forward/back and side/side axes can be (somewhat easily) modified to remove the spring loading so the stick will stay where you set it. This would make a very nice Pink Floyd-style Azimuth Coordinator (seen below) for panning sounds in a quadraphonic setup.

So, how do I use it (one might ask)?

The forward and back motion is assigned to pitch bend. The incredibly deep implementation of the ONE allows you to program different bend ranges for bend up and bend down, allows reversing their function, applying a different bend range for each oscillator, different ranges for each of the 3 synth layers, and also makes the bend CV available as a source in the mod matrix, so it can be used for many other purposes as well. This should give you just a taste of the depth and complexity available to an imaginative programmer. There's more; lots more. 

Side to side motion of the joystick in my system is normalled to the expression pedal 2 CV input (the expression pedal 1 input in my system is always occupied by an actual expression pedal, usually a custom-modified Vintage Moog or Ernie Ball). It's typically assigned to control various filter cutoff functionalities (and there are many to choose from) but is also sometimes used to bend one or more oscillators in the sync mode to create those familiar timbre sweeps. Of course, it can also be assigned in many other ways, but for my uses it's typically used to shape and modulate the overall timbre of the sound I'm controlling. 

Twisting the joystick to the left creates a CV that is assigned to replace the mod wheel, and it's also used in conjunction with the push/pull switch knob mentioned earlier. Like a typical mod wheel, it's often assigned to create vibrato or tremolo. The mod wheel is also available as a mod matrix source and freely reassignable if desired. Twisting the joystick to the right creates a CV that can be assigned through the CV1 input. I often use this as an alternate vibrato, like using a unipolar square wave for a trill effect, or even sample and hold for a completely different type of modulation. Another great use is to access an audio rate LFO to add "growl" to a sound. 

The 5th axis (5th dimension!) is a momentary push button, located on top of the controller. 
I created circuitry with a small toggle switch mounted beside the joystick to allow it to be a momentary control, or latching (push on/push off). An added red LED on the LHC panel lights when the control is active. Another small toggle switch allows the CV created by the push button control to be routed to the CV2 input or alternately to the sustain pedal jack to allow for chord hold effects. Both of these switches are the short bat versions which are big enough to switch easily, but small enough to resist getting accidentally smacked into and broken. I used a nice dress nut for both switches, which I feel gives them a more pro appearance. 

I once again used the labelmaker to create a few small white-on-clear labels to identify these added controls. As I've mentioned in previous blogs, trimming the labels as closely as possible (along with proper font and size selection) makes the overall effect look much less like a label, and more like an organic part of the unit. 

I call this the "anything can happen" control. Its superpower that makes it so valuable is that it can create subtle or dramatic changes that can happen immediately, and without any undue noise or glitches (as can sometimes happen when changing presets). It can be programmed to completely retune the oscillators on command during a solo, bring in other layered sounds, supercharge certain settings for a pinpoint emphasis, make everything momentarily pan around furiously, and probably a million more things you and I have not dreamed of. Yet. 

The LHC panel was designed so that it could be opened to allow easy access to my module for tuning and calibration once everything was installed in the system, and the unit was turned on and thoroughly warmed up. 

As you might imagine, having all of this control immediately available under your left hand as you play can add a tremendous amount of expression and nuance to your playing, especially while soloing. You also have quick easy access to the X/Y touchpad, and just in front of the joystick are the octave transpose buttons, which can be easily accessed using the index and middle fingers while still maintaining control of the joystick. 

The ONE has the capability to independently save your mod matrix settings as its own preset, and I've found this to be invaluable to me. I can name my work and save it so that I can add a particular setup to any new preset I'm working on in an instant. There's even a section for adding detailed notes so I don't forget any important/arcane details about adjusting things. This is especially important because this feature saves the modulation routings, but not any settings of the modules that are used, so it helps to make notes of anything that's important to recreate the desired effect. Once a modulation preset is saved, it can be easily added into a new preset you're working on, much like adding a favorite spice to a stew. 

The path to making something like this truly expressive involves envisioning and designing a layout on the various axes that complements your particular workflow and thought processes as you play, and then practicing with it while fine-tuning the ranges and sensitivities to fit your style. This can be an ongoing process; it certainly has been for me (pretty much every time I sit down to carve a sound).

However, the added capabilities and depth of expression has made the amount of work involved more than worth it to me. I've gained a deeper understanding and appreciation of the incredible work that the team at Moog put into creating this vastly programmable instrument platform, and I've had the satisfaction of adding my own creative touches to help mold it to my particular style of expression. 

I'm more than happy with the results. 

For those readers who might be interested, I'm providing a link for a short time here to a small collection of presets from my personal library, as a gift to my fellow Moog ONE enthusiasts.

https://www.dropbox.com/s/e25yrfvznhc8xs4/SYNTHGUY%20GIFT%20LIBRARY.zip?dl=0 

Be sure that your unit has the latest firmware, which is 1.4 as of this writing. Some of the presets will require this to function properly. 

Enjoy!

Moog's Incredible Flagship... The Moog ONE!

For the past few years, I've had the incredible pleasure of working with Moog Music to develop sounds for their incredible new flagship, the ONE.

It's truly an analog dream machine, with an extensive set of features and abilities, and it's massive flexibility allows you to approach it as a polyphonic synth, a mono synth, and even a highly capable modular system.

Here it is in my home studio during a break from a marathon programming session...

More recently, I've been taking a little time to actually create some music with it. Although there's a number of things in progress, here's the first one to make it across the finish line...

This was an improv that revolved around using the ONE as a modular system. Everything was recorded directly to my iPad, my preferred way of working these days. I used the oscillators waveshaping and voltage controlled mixing capabilities to create timbral changes as the sequence progressed, and used the built-in effects to create ambiance (no external effects were used). I also used a pair of expression pedals to control things like panning and filtering during the recording process.

There's more to come...

How To Own A Real Moog Modular System For Less Than $30!!

I've been an iPad enthusiast for several years now, to the point of using it pretty much exclusively in my home studio as a recorder, and for many other tasks, including electronic instruments and effects. The quality it brings to these tasks is astounding, and it just keeps getting better as time goes by. If you haven't gotten in the game with this yet, you should seriously consider it.

Moog got into the iPad app world early on, and their latest offering is an incredible accomplishment.

The Moog M15 precisely duplicates the form and function of their Model 15 modular system, a $10,000 synthesizer, and can be bought for the iPad for $29.95. All of the original modules are there, and they even threw in a few new ones, as well as a stereo digital delay. You can create and save your own patches (an incredible luxury) and every aspect of it performs just as you would expect the actual hardware to, including very realistic audio rate FM modulation (no mean feat!).

You never run out of patch cords (think about that for a moment...)

 

AND, this beast can play your patches back in 4 VOICE POLYPHONY!!!!

Let me tell you, the experience of a polyphonic Moog Modular is something you Don't Want To Miss!!!

 

I eagerly created a large personal library drawing on my years of work with my own synthesizers, and now Moog has made a choice set of patches available to others in a collection I call Mercurial Strategies (yes, a nod to Eno's Oblique Strategies card sets for creative unblocking decisions).

Here's the info on this latest update to a truly incredible app...

 

 

Moog has updated their already excellent Model 15 app, and along with substantial updates to the Audiobus capabilities, Midi and the built-in recorder they have included three new collections of presets available as IAPs, including a collection I designed specifically for this app called Mercurial Strategies.

I've created a demo for some of these sounds that can be auditioned here:

https://m.youtube.com/playlist?list=PLvwSQaX_3FcmF7u1kf_tE-i0YEDkDkaTU

These recordings were made by running the M15 played live via an Arturia Launchpad 25 keyboard through Audiobus and directly into Auria on an IPad pro.

No additional EQ, effects or apps were employed during the recording process.

This collection contains 75 presets from my personal library that I've created for this app, which are each carefully crafted to display a number of powerful capabilities and programming strategies that might normally be overlooked by other programmers. This is not a collection with multiple variations of the same basic patch routing (or "strategy"). Most of these presets utilize a unique realization of module arrangement to produce their sound and each is capable of providing many more useful variations of the example presented when a user explores the options represented in that particular "strategy".

These cover a wide range of offerings, from lush pads to cutting leads to surreal soundscapes and effects, and each has documentation embedded in the preset description discussing such things as unusual programming techniques involved, sound concept genesis, advantageous adjustments, and useful playing suggestions to get the most out of each patch.

In them, I explore interesting and unique ways to create velocity sensitivity, stereo manipulation, unusual and dynamic filtering, creative techniques for the built-in digital delay, controllable feedback networks, differential and phase modulation, alternative uses for the Mod Wheel, and many others. A number of these patches specifically utilize the incredible polyphonic capabilities of the app. This makes exploring each preset aurally interesting and gratifying, as well as serving as a handy tutorial for users to incorporate these useful and powerful concepts into their own work.

I'm a 30+ year owner of a Moog System 55 (as well as many other synthesizers by Moog, Sequential, Oberheim and others) and have developed a number of interesting patches for it both on my own, and through interactions over the years with the many fascinating and talented individuals I've had the good fortune of meeting and collaborating with. As you might expect, I've been able to scale and transfer some of these to the Model 15 platform, with what I think are very good results.

I hope that these patches will inspire you to create your own great electronic music!

 

 

Posted with Blogsy

Sequential Circuit's Forgotten Past...The Model 700 Programmer

Before they became world famous for creating the Prophet 5 synthesizer, Sequential Circuits built accessories for popular synths. An interesting and under-appreciated one is the Model 700 programmer. Designed to be hooked up to a Minimoog ( or other CV controlled synth) it's easy to see how this unit was the seed of the concept that eventually led to the creation of the P5.
Print ads from back in the 70's show the unit along with Sequential's other product at the time, the model 800 Digital Sequencer. Here's an early ad featuring no less than Undisputed Minimoog King Rick Wakeman, not too shabby of an endorsement for a fledgeling company that at this point appears to be based at yet another interim address in Silicon Valley, possibly one of Dave Smith's apartments.

Companies did a fair amount of wanderin' back in those days, it would seem.


These devices both offered an interesting alternative to what Oberheim was offering at the time. Oberheim had been building a digital/analog hybrid sequencer since 1974, and offered a rudimentary programmer as an addition to their four and eight voice polyphonic systems, but not as a stand alone device to control other synths.



The address for this ad shows yet another location for the early operations of the company before they settled into their main headquarters on North First Street in San Jose...


...After which, one assumes that they had a nice party to celebrate. The Digital Sequencer was apparently well-shielded against such real world hazards as cake frosting and delicious candies, while the Programmer appears somewhat less ready to deal with such tasty dangers.

The unit has 64 user definable presets to control various key aspects of the host synthesizer.
There are three quantized CVs that can control oscillator pitch, and two programmable envelope generators (including delay) that can control the filter and final VCA. Switches for each oscillator allowed the inclusion of an external CV source like a sequencer (perhaps the Model 800, eh?) and the filter envelope section included an offset control for initial filter tuning. Although there's no way to set various wave forms or mix or mute individual oscillators, this was still a big step forward for those Brave Souls grappling to successfully control a Minimoog or the like in the heat of a live performance.

Imagine the pressure...

"Tune Those Oscillators!"

"Trim That Filter!"

"Adjust Those Attack and Decay Times!"

"Flail Randomly at that Keyboard!"

"Dance, Monkey Boy, Dance!!!"



If you hold your head at the proper angle and squint your eyes at it just so you can sort of see some the control panel for the P5 lurking there, just under the surface.



I picked up my first Model 700 in Sacramento in the early 80's, where I found it sitting dusty and unloved in the corner of a store, for $75. I used it to give the Moog 55 some quick programmability, and got a fair amount of fun out of it over the years. It was easy to connect into the system in various ways since it was equipped with 1/4" jacks, and the programmable DADSR envelope generators were a nice addition, although the programmable steps (64 for each segment) were a little coarse. I couldn't resist customizing the preset selector switches with color coded ones, to match what I had done with my Prophet 5 (more on that in a future blog).




To get the maximum amount of control with a Minimoog required some modification of the instrument, and typically a multi pin connector was added to match the one provided on the back of the Model 700. This made connecting things quick and easy, but the hassle and expense to get this done probably discouraged some potential customers at the time (not to mention the units $1200 price tag).

Back then, for a few hundred dollars more, you could actually buy a Minimoog.


I remember reading that Howard Leese used one with Heart, and I can see him wailing on a Minimoog during "Magic Man" in my mind's eye as I look at this unit.



An interesting performance feature is a footswitch jack that lets you step through the presets, a great idea. There's a front panel knob that sets the number of steps (1-8) the unit will cycle through in a bank, so you could preprogram a manageable sized preset cluster to jump around through during a song if desired, without having to take your hands off the keys. It's Magic, Man!

Now, if you look at this feature set closely, it begins to look more and more familiar.

Hmmm....programmable voltages, switch steppable, selectable cycle length...

Hey, It's an (up to) 8 step sequencer!

Each step has 3 quantized pitch CVs, and two programmable envelope generators, as well as filter and VCA settings, making this this a pretty powerful sequencing tool. The footswitch input is even switch selectable to accept V or S triggers. What more could a young synthesist need?
Let the Sequencing begin! Yee-Haaa!




Well, in reality, the stock unit took a finite amount of time to update all the parameters when switched, which made running faster sequences tricky, as you could often hear the pitches slewing to reach the notes in time.

I got a schematic for the unit (mucho thanks again, Riley!) and noticed that the design was pretty straightforward...no microprocessors here! I found that by speeding up the system clock, I sped up the overall response time, which reduced the slewing effect at faster sequence speeds. I carefully adjusted the clock for the fastest performance possible without things going horribly awry during preset changes, and this boosted the usable sequencer speed considerably. Not as blazing fast as the Moog 960, but reasonably zippy nonetheless, and with the wealth of features available and some thought and planning, the sequences could be incredibly deep and detailed.


For those looking for even more control, a simple mod allows the internal Delay CVs to be routed to a set of jacks on the back, providing the user with two more programmable CVs per step. By using a normalled jack for this, the Delay function of the DADSR generators is available until you plug in a cable, which sets the delay to zero and makes the CV available for other nefarious purposes in your system. Push that pulse width? Bully a VCO? Totally programmable System Transposition per step?

Hey, WHY NOT?!!?

Besides, how often are ya gonna actually use that delay function, Chukko?


An interesting quirk of the unit is that the rotary selector switches have positions that are unused, and just labeled as OFF (maybe they got a good deal on the extra position switches). The step position selector was like this, so I decided to press these unused positions into service somehow.



After some tinkering around, I devised a circuit that could switch preset banks at the end of the 8 step sequence (much like using the sequential switch on the Moog 55) so now the unused rotary switch positions could be used to select a longer sequence by combining banks, making 16, 24, 32, up to full 64 step sequences possible.



Now this was starting to become a pretty formidable sequencing tool, as well as a super cool programmer.

Who knew?



There were two versions of this unit produced during its lifetime. My first unit was the later version, which had some improvements and used SSM voltage controlled envelope generator chips and VCAs to handle the programmable side of things.

Years later, I discovered that there was an earlier version, when I picked one up in a store in San Jose, also at a very reasonable price (thanks, Barney!). This one had engraved numbered preset selector buttons, and a discrete component envelope generator inside, probably predating the ready availability of the SSM chip.





As you can see, this is a pretty early example (SN15!!) almost certainly built in Dave Smith's apartment (you can almost see him at his kitchen table laboring deep into the night soldering the circuit boards by hand...) The board and component assembly looked much more, er, rustic with a more California handmade/organic quality to it.

The Waves sticker on the back is an interesting touch...I've never been able to find any info on them, but the thought of good Napa Valley wine and synthesizers is a very appealing combination. Any info out there, anyone?

This one I've left stock. Particularly interesting is the large number of CA3080 progammable OTA chips inside, being used as the many sample and hold cicuits and VCAs for the various CVs inside.

Dig all that OTA goodness, Bunkie!



Not to mention the gold-plated diapers, babies!

Woof!




"I've gotta fever, and the only prescription is MORE SCI 700!!"

The REAL Moog 905 Reverb Schematic Revealed!!!

The Moog 905 reverb module is one of those parts of the modular system that typically never got as much attention as some of its flashier counterparts like the filters or oscillators. For those in the know however, it went on to play an essential role in helping to create the patches that have become a part of the popular canon of sounds and effects that the instrument has become known for.

Back in the 60's, reverb springs were a pretty new and exciting way to spice up the sound of an electronic instrument. Guitar amplifiers, home organs and even PA systems utilized them to add spaciousness to an otherwise dry sort of electronic sound.

This was a relatively cheap and easily portable solution to mimic the sort of reverberation effects available in a professional recording studio, which usually used a large hard-surfaced reverberant room or specially treated acoustic chamber with a speaker and microphones inside to create their sound.

Reverb springs work by using one or more actual metal springs suspended between two electrical transducers. The input transducer is hooked to circuitry that actually shakes or twists the springs in response to an electrical signal...the bigger the signal, the greater the movement of the the spring(s). The signal travels down the springs, and is picked up by another transducer that converts this motion into a small proportional electrical signal that is then amplified and mixed with the original signal. The delay in this signal caused by the time it takes to travel down the springs, plus the fact that the signal "bounces" up and down the springs several times as it decays gives it its reverberant quality. Shake or smack the springs, and you get a great explosion of sound as they clang around!

 

 

Bob Moog realized that this technology would be valuable in creating space and realism (or even unrealism) for electronic music, and so the 905 Reverberation Unit was born.

 

 

Using it was simplicity itself...it had an input and output jack, and a single knob. When turned completely to the left, only the dry sound of the instrument was heard, along with some coloring from the electronics involved (more on this later). As the knob was rotated clockwise, the dry sound was slowly lowered, and the reverberation effect from the springs was slowly added to the signal. At the full clockwise rotation of the knob, only the output from the reverb springs was heard. This allowed for a wide range of effects that added the reverberated sounds to the original dry signal in various amounts.

 

There has always been some confusion about the actual circuitry involved in this module.

This early 905 schematic, dated from mid-1966, details most of the circuitry fairly well, but the value of the resistor connected to the collector of transistor Q3 is missing from the document. The mix knob is either drawn incorrectly, or missing the necessary info to show that the dual-ganged potentiometer elements must be connected to operate opposite from each other for proper blending of the two signals to occur.

 

This module's schematic was also included in the Norlin Modular Service Manual, but it was reproduced with a number of nasty mistakes (as were some other schematics In that collection as well) that have continued to circulate around the web. Some resistors were shown improperly connected, some resistor values were not representative of what was actually being used in later production models, and the dual element blend pot was again drawn incorrectly connected so that no blending of the two different sounds would occur. The resistor connected to Q3 is still shown here with no listed value. The input to Q2 is shorted!

So, not much help.

Trying to build a working unit from this schematic would be frustrating, to say the least...

 

Over time, the schematic has had some much-needed corrections made (shown here in red) and was reposted by various individuals, but there are still errors present in these modern day versions, including a resistor on these newer models that is not shown in any of these schematics.

 

After a lot of sleuthing on the web, examining various circuit board photos of these modules, and more lately seeing some excellent photos at the Moog Forum posted by an individual called Rezin, I've been able to piece together what I believe is the most accurate version of this schematic to date.

 

For those like myself who are planning to recreate one of these modules for their own system, this may be very helpful in capturing the magic that this module brings to the party!!!

 

 

After examining the various photos available, I created this stuffing guide using a picture of the copper side of the board to enable me to see just how the various components were really connected together "under the hood" in an actual production model unit.

At this point, I began to notice some differences.

In all of the schematics up to this point, the circuitry connected to the output of the springs was shown as using the +12 volt buss as the positive supply, and the spring chassis and the return of the associated components was connected to the -6V buss. All other electronics in the module used only the +12 volt buss and ground for their connections.

Looking at the 2.2k resistor and 75uf capacitor connected to the emitter of Q3, you can see that they are actually connected to the ground trace on the PC board, not to the -6 volt supply rail.

The mounting plate of the reverb assembly and the chassis of the springs are connected to the uppermost trace on the board... you can get a better look at how the plate is mounted in these pictures of the unit. Notice especially the soldering lug that connects one of the plate mounting screws to that uppermost trace...

 

So, at this point I was thinking, "Perhaps this trace is the -6 volt rail shown in the schematics?!??"

 

Looking at some better pictures of the various modules available, it looks like that trace is connected to ground as well, through the shield of the cables attached to the reverb springs.

 

OK, so just where is the -6 volt supply rail on this unit???

 

In fact, as you can see here, pin 3 of the power connector (-6 Volts, 3rd tab from the top) is not connected to the circuitry at all in these modern examples of the 905 module. You can however see a red wire connected to pin 1 (+12 Volts, top tab) and a green wire connected to pin 2 (ground, 2nd Tab from top) if you look closely.

 

 

This makes sense to me because if -6 volts was actually connected to the main chassis and springs, it would be fairly easy to short this supply to ground, as the springs tend to poke out quite a bit inside of the module.

 

Yikes!

 

This might be one of the reasons this was changed in ongoing versions of the module.

Having the extra headroom that connecting this circuitry to both supplies provides doesn't make a lot of sense here anyhow, as the signals that are generated by the spring transducers are fairly small, even if you shake the springs vigorously. It would probably make more sense to do this at the front of the circuit, as the single transistor is driving a fairly low impedance load and could use the extra "oomph" from a larger supply, and the input to many spring assemblies is often isolated from chassis ground and left floating for just such a purpose.

 

I've read accounts from various individuals that this module adds a subtle but pleasant amount of coloring to the oscillator signals run through it, even if no reverberation is applied, and it is often used in this way as a "thickener" to the overall sound. This is probably due to the nature of the electronics being powered from a single ended supply, and the characteristics and biasing of the transistors used in the input and output stages. This is, in some ways, much like the acclaimed discrete transistor mixers used in in the earlier modular systems, which added their own pleasing element of distortion to the sound, delighting modern listeners. (Cha cha cha)

The electronics in the 905 module also invert the phase of the dry signal passing through it, so if it is mixed with the original signal source this will cause various cancellations as well, depending on the settings.

 

So, after all this investigation, intimation, instigation and implication, this is the schematic that I believe now represents the connections and component values of most production models of the 905 reverb.

 

 

Using the pictures of various production models seen on the web as a guide, there have been some component value changes from that early 1966 schematic, and one addition.

The input resistor R1 was changed from 33k to 15k.

The resistor R8 was changed from 100k to 220k.

The resistor R9 was changed from 220k to 330k.

The resistor R15 was changed a number of times during production. On some units I've seen it was a 330k, or a 470k. I show it as a 150k in these schematics because that seemed to be the most common, and that's what was used in the unit photographed recently by Rezin. Any other changes might have been dictated by the transistors used in that particular example, which are usually not readable in most of the photos I've seen.

A 100k resistor R17 (not shown in any previous schematics) was added from the module output to ground.

In the unit photographed and examined by Rezin, all transistors are NPN types 2N3391A.

The reverb springs available back when this was designed typically had an 8 ohm input impedance, and a 2-3k ohm output impedance. There's lots more choices nowadays, including multi-spring units with varying decay times, so some experimentation with different units might help you create your own perfectly reverberated customized listening experience as you roam this nation's highways and byways....

 

 

 

So, for those intrepid individuals who would like to build their own unit (or repair a janky one) I hope that this will help you in your efforts. It helps lessen any hum pickup if you keep the reverb springs as far away from the power supply and transformers as possible, and use good quality shielded cables to make connections to and from the unit.

Many, many thanks to Rezin and Tom from the Moog forum for their most excellent photos and insights into this mysterious unit (as the dry ice fog slowly clears).

 

So, join us again next time as we go ...

 

FORWARD INTO THE PAST!!!!!!

 

 

 

 

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