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.