Sunday, November 1, 2015

Review: SSL 1250 Quad LFO

The Synthetic Sound Labs Model 1250 Quad Low Frequency Oscillator is what it says it is: four LFOs in one panel, formatted in the MU (Dotcom) modular synth format.  It's a pretty simple module.  The panel is divided into five sections: four sections are each for one LFO, and a bottom section contains the output jacks.  Each LFO has three controls: a rate knob, a waveform select switch (sine and square waves are available), and a peak indicator lamp which is also a pushbutton.  Pressing it switches the LFO between high range and low range.  The lamps are red LEDs and actually look much redder than in the picture to the right; I think the infrared filter on my camera prevented the deep red from registering.

This is an LFO meant to drive slow, evolving patches.  On the high range, with the knob full clockwise, the period is about 22 milliseconds, which works out to 45 Hz.  With the knob at 5 (straight up), the period is 50 ms, or 20 Hz.  As you turn the knob further left, the period increases linearly, which per the law of reciprocals means the frequency decreases exponentially.  With the knob at 2 (the 9 o'clock position), the period is 150 ms, a frequency of 6.6 Hz.  At the low end of the knob's travel, between 0 and 1, the change is much more than linear -- with the knob full CCW, I measured a period of 80 seconds.

If you want really slooooooooow, switch to low range.  With the knob full clockwise, the period is about 1200 ms, or around 0.8 Hz.  At the 5 setting, it's 3 seconds.  At the 2 setting, it's 8.5 seconds.  At the 1 setting, it's 36 seconds.  With the knob full counterclockwise… I was not patient enough.  After three minutes, it had climbed from zero volts to +0.45V.  If I've done my math right, that's a cycle time of about 45 minutes!  The cycle indicator light starts to light up when the sine wave rises +1.5V, and reaches full brightness by +3.5V; it goes out when the sine wave drops below 1.5V.  (This is true whether the sine or square wave is selected.)  At moderately slow rates, it's rather hypnotizing to watch.  I did a quick check of all four oscillators to make sure they were all calibrated the same, and didn't see any noticeable differences.

Looking at the waveforms on the scope: The square wave looks good.  The sine wave is a bit distorted; it looks a bit triangle-ish.  There's a distinct corner at the turn point, and the rise and fall portions look a bit straight-lined on either side of the horizontal axis.  (A perfect sine wave is straight only right on the axis; it has at least a little bit of curvature everywhere else.)  It's not as bad as that makes  it sound; most of the waveform looks like a good sine wave, and using it to modulate a VCO, I didn't hear any abrupt reversals in pitch rise and fall, as one would if the VCO were modulated with a triangle wave.  Also, the sine wave doesn't quite make it to the 5V rails; it turns at about +/- 4.5V.  The square wave looks good.  There are no visible changes or variations in the waveform with frequency.

The build quality looks good, up to SSL's usual high standards.  There is one main board and a smaller jack board, as you can see in the photo to the right.  (That blurry white cable with the colored wires coming out is my tacky homemade power cable.)  Most of the components are surface mount.  The main board is flush to the back of the panel, and the jack board only stands off about one inch (2.5 cm), so there should be no problem installing the 1250 in the most shallow cabinet or skiff imaginable.  The panel is standard MU construction and all of the dimensions are correct.

The SSL 1250 serves a basic but essential function in a modular synth: to avoid highly repetitive modulations that can become fatiguing to listen to, you need to be able to mix several LFOs to create modulation shapes that are more complex but not totally chaotic.  The 1250 does this job admirably.  And the blinkylights factor is high too.  The one improvement I might suggest is some onboard way to output a combined waveform without having to use a separate mixer.  If the output jacks were chained -- that is, the output of a given jack combines with the next higher numbered jack when no cord is plugged in -- that would be useful.

SSL is at  They sell both direct and through dealers. 

Thursday, August 27, 2015

Analog computers and synths

A few weeks ago, a poster at VSE asked a good question: To what extent, if any, did the design and use of analog computers in the mid-20th century influence the development of music synthesizers?  My first thought was, "probably not much".  Then I did some research...

First, let's go over what an analog computer is.  An analog computer, put simply, is a device that accepts input parameters which are represented by something inside the computer.  It performs computing functions through mechanisms and/or electronic circuits, and the outputs are expressed by quantities of something the mechanism can produce.

A Philbrick K-3 analog computer, circa 1950.  From the Philbrick Archives.

Analog computers preceded the development of electricity.  The first, simple analog computing devices go back to the Middle Ages, but significant ones started appearing during the pre-industrial scientific discovery period from 1600 to 1800.  Generally they relied on sliding or rotating parts to represent measurements which were input or output.  A simple but important example is the slide rule, invented in the 17th century.  A basic slide rule multiplies two numbers by positioning one operand on a sliding scale against a fixed scale; the amount by which the sliding scale is moved represents (by reading off of a scale) the product.

In the early 20th century, a number of powered analog computers were invented to do specific calculations.  An early driver behind the development of this technology was the need for a device called a "gun director".  This was a computer that computed the elevation and azimuth angles at which an artillery piece needed to be pointed in order to hit a target, given the range to the target, the wind, the weight of the shell being fired, and possibly other factors.  The Norden bombsight was a famous electro-mechanical analog computer deployed by the Allies during World War II.  To use it, a bombardier looked through a sight glass to find the target to be bombed.  From the pointing angles of the sight, and the rate at which the bombardier had to move the sight in order to keep it on target, the bombsight computed the heading that the aircraft needed to fly, and the time at which the bombs should be dropped.  In this computer, the quantities being computed were represented by the movements of levers or gears.  (The bombsight was usually coupled to the airplane's autopilot so it could actually fly the aircraft during the bombing run, and to the bomb racks so it could release the bombs at the right time automatically.)

Norden bombsight (top left) and servos controlled by the bombsight.

Electronic analog computers started to appear around 1930. As was generally the case of the mechanical analog computers, most of the early electronic devices were hard-wired to perform a specific computation; because of this, early uses were limited to problems that were both important and difficult, enough that it was worth the cost to build a computer.  An early example was a device known as the "AC network analyzer", which was built to solve problems that electrical power utilities were encountering as individual power stations were being combined into large grids.

In 1938, electrical engineer George A. Philbrick, then employed by the Foxboro Company of Massachusetts, wrote a proposal for an electronic analog computer that would model various types of closed-loop manufacturing processes.  One of the problems that Philbrick had to solve was how to design circuits that would perform the needed math operations in a general sense, that is, not specific to a particular problem.  In 1943, Philbrick was working on a contract with the U.S. Army to devise improvements to the M9 gun director, which had been built by Bell Labs.  It worked, but it was too slow to compute in real time.  Philbrick came in contact with Loeb Julie of Columbia University, who had devised the first experimental operational amplifiers.  (Yes, there were op amps decades before the first integrated circuits.)  Philbrick realized that Julie's op amps could be used to perform a variety of analog computing math functions, and he began working on his own improvements.

Philbrick K2-P op amp

After WWII, Philbrick started his own company, George A. Philbrick Researches.  The company was heavily involved in both analog computing and commercial op amp design and manufacturing.  The company published a widely regarded collection of papers and notes concerning analog computing -- system design, circuit design, programming, and operations.  Analog computers were becoming more compact, and general-purpose units were appearing that offered a number of function modules which could be interconnected by the user in any desired configuration using patch cords.  In fact, Philbrick's company developed the idea of a "modular computer", in which individual function blocks could be purchased and combined as needed to apply to a problem -- a concept very similar to the modular computers that would come later.  At some point Philbrick hired a certain young electrical engineer, one Alan R. Pearlman, who took an interest in the op amp business.  So much so that, in the early 1960s, Pearlman and another Philbrick employee broke away and established their own company, Nexus Research Labs, which continued their work in the op-amp and analog computing business.

Philbrick K3 analog computer modules.  From the Philbrick Archive.

If Pearlman's name doesn't sound familar, look at his initials -- A.R.P.  In 1966, Pearlman's group sold Nexus Research Labs to Teledyne.  The sale made Pearlman wealthy, and he used some of that wealth to found ARP Instruments.  Look at the photo above.  Looks vaguely familiar?  The Philbrick analog computer systems were modular.  There were about 10 function modular that the user could purchase and configure in a case as needed.  Compare to this:

ARP 2500 model 1947 voltage controlled filter

However... The first of what we consider synthesizers today didn't come from Pearlman.  The two men who are generally credited with developing the basic building blocks of the analog synthesizer -- the voltage controlled oscillator, filter, and amplifier -- are Robert Moog and Don Buchla.  Moog has an obvious, if indirect, connect to Philbrick via Columbia University, where Philbrick and Loeb Julie worked on the first op-amp designs in the 1940s, and where John Ragazzini and Rudolf Kalman had continued to work on analog computing concepts through the 1950s.  The Columbia-Princeton Electronic Music Center opened at Columbia in the mid-1950s, but it is not clear how much cross-fertilization there was between it and the analog computing labs.  Moog just missed experiencing the RCA Synthesizer, which was installed at the center in 1958; he had graduated in '57.

Little is written about what Moog actually studied or did at Columbia (far more is written about his theremin side business by which he paid his way through school), so further investigation is difficult.  He did get his degree there in electrical engineering, and in a mid-1950s electrical engineering curriculum, he most certainly would have had instruction on computer circuits, both analog and digital.  There were probably analog computers to use, and possibly they were Philbrick units like the one pictured above, thanks to the connection to the university via Loeb Julie.  Where did Moog come up with the idea to make his first synths modular?  Did he spend some time with a Philbrick analog computer at Columbia?  Did he, perhaps, try to coax sound synthesis out of it? 

Buchla is even more of a puzzle.  There is almost no information available on the Internet about what he did prior to founding Buchla and Associates in 1962.  It is known that he was involved with Morton Subnotick and the San Francisco Tape Music Center, which was a tape studio and had little if anything to do with analog computers.  He was involved in some way with the University of California, Berkeley (it's not clear if he was actually a student or faculty there or not), which at the time was the world's foremost center of nuclear physics research, a field in which a considerable number of analog computers were used to model nuclear reactions.  Buchla studied physics (along with several other fields) and probably would have come into contact with the nuclear physics program's analog computers.  To what extent this influenced his later thinking about synthesizers is difficult to say.

So to answer our question: did analog computers influence the development of analog synths?  The answer, at this point, is "maybe".  We know that Pearlman was heavily involved in analog computers, but he came in a little after Moog and Buchla.  We know that Moog was at Columbia at a time when the school was involved in both analog computing and electronic music, and we can see similarities between his modular synth designs and some of the modular computer designs that he might have worked with.  Buchla is less certain, but he probably would have at least seen analog computers at Berkeley. 

For more information about George Philbrick and his pioneering company (it's a worthwhile read for anyone interested in electrical engineering history), see the Philbrick Archive at