November 21, 2012

Quantizer/Analog Shift Register take II

In 1972 as a resident at CalArts, Fukushi Kawakami made four modules as additions to the school's rather extensive Buchla 200 system. The modules are a Control Voltage Switching Matrix, two Control Voltage Integrators and what I believe is the world's first Analog Shift Register.

Since then, the world has fallen into disarray, computers have taken over, analog modulars have gone in and out of favor several times and those four Fortune Modules have ended up in Grant Richter's hands. Somewhere in the middle of all that, Serge made an analog shift register and wrote about it (under the nom de plume Arpad Benares) in Synapse. Even before the Fortune Modules, Buchla had made a rather amazing Control Voltage Integrator called the 155, but that's another post.


Analog Shift Registers are a bank of Sample and Holds. In fact, using only the first output, it is a Sample and Hold. When a pulse is applied, the CV on the input is stored on output one. Whatever was on output one is moved to output two, and so on. 

In 1997 I made 2 copies of a module that was a dual four stage analog shift register as well as an 8 channel voltage quantizer. One of these is still in daily use over at OSI music and the other is rotting on a shelf in my shop. When this photo went around the forums and blogs some people suggested that rearranging the panel to allow the analog shift register outputs to be quantized via shorting bars would be a good idea. Point taken.


I had a couple of ideas of my own that could make it a better module. Sadly, it got back-burnered and never saw the light of day until now. The new version has rotary switches to select the scale to quantize to. The ASR outputs can be plugged into the quantizer with  shorting bar. There is no longer a "slave" switch to chain the two ASR's together, but a cable and shorting bar can now do that too. Some new ideas have come up as well, like using the quantizer to look-up the voltages from the "random" voltage sequences from the 266. It's obviously not as glamorous as an oscillator or filter, but it will come in useful to some people.

Spring Chickens

In the golden age of analog synthesizers, everybody had a spring reverb in their system. In fact, sounding like the Radiophonic workshop is often as easy as adding spring reverb to simple sine and square wave sounds. Buchla was no exception including the 190, 275 and part of the 212, 208 and 227. The 275 in particular is a rare and interesting beast, offering voltage control of the wet/dry mix as well as simple EQ on the dry signal. 

The module itself sends out and receives a line level signal to an external reverb unit. Most likely, the signal is sent to a 17" x 7" x 3" box that has the reverb drivers and recovery amplifiers built inside, along with 2 15" dual spring tanks.  Any line level in/out unit could be used with the module though. The brochure reads, "The reverberation electronics and delay elements are remotely mounted. Interconnections are at line level, permitting the use of the control module with reverb units of alternate origin (E.M.T.'s, for example)." In the case of the module in my hand above, the "remotely mounted" electronics were not included. Rather than connect an "E.M.T.", the owner and I decided to whip up some electronics to use a couple of reverb tanks pulled from a 190 unit.

The electronics inside the remote unit for a 275 have a discrete amplifier, envelope follower and a variable gain recovery amplifier. In other words,  a compander is built in.  The slowed down attack makes a bit of a pre-delay on the reverb and the gain reducing as it decays takes out some of the noise from the spring reverb. Sadly, the amplifier uses +/-24 volts, which I wanted to stay away from. I created a similar circuit, using the same amplifier IC that is used in the 208 and 227, LM380, with a 275-like compander. I attached it to the back of the module and ran pigtails to the tanks, mounted on the back of the cabinet. The trims control how much action the compander has. I just trim them to have unity gain out when the signal is full strength. I wish I had made a sound clip once it was installed.

This unit could be built inside a box with the springs like the original one was. Since it has line level in and line level, 100% wet output, it could be put on a send of an analog mixer or on an insert of a DAW, like the vintage Roland and RCL units I have right now on my ProTools rig. I use them in my sessions and since they don't have any controls on the front to change, everything comes up the way I left it. Kinda cool I think.