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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