Archive for the ‘Electroacoustics’ Category

96 Crickets Afoot

March 18, 2014

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I’m in the planning stages of a new electro-acoustic sculpture. I’ve made some prototypes of the basic circuit and drawn up some sketches of the final piece. The 96 speakers presented in a grid on the wall will click repeatedly at about once a second in interfering phasing patterns, much like car blinkers at a traffic light, or crickets. In this project, I’ve been interested in the use of identical ready made objects, and repeating structures – invoking. As always, I’ve involved waste in the narrative by employing off-castings of industrial manufacturing sourced from a surplus warehouse.

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The proposed sculpture comprises 96 oscillator units, oscillating independently. For the oscillator circuit I sought out the simplest possible design. The relaxation model fits the requirements. I discovered that a reverse bias transistor model could be used for a minimum of parts. The one transistor design cleverly makes use of a lesser known property of BJT transistors. Many common transistors exhibit a negative resistor effect when reverse biased.

Perpetual Ocean Machine

November 12, 2013

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The spring reverb had its day in many a recording studio as a cheap and convenient substitute for a real hall, equipped with loud speaker and microphones. I’ve been exploring the creative artistic possibilities of a giant spring reverb made out of a bed spring, and using it more like an instrument, rather than a mere effect. I intend to play it like a resonant cavity by putting it in an amplified feedback loop. It would then play in a harmonic series scale – like the shepherds horn, which by the way has a beautiful tradition of simple folk calls. At present the signal to noise ratio of my setup is too low to get a reliable oscillation, so for now I’ve been tickling it with my fingers instead of a transducer.

Take a listen to the rich layered undulations produced by the setup pictured above, consisting of the spring and a four channel looper built in PureData. This is an excerpt from a recent performance at Waugh Agency.

Pure Data Tutorial

October 3, 2013

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I’m making a (hopefully) helpful introduction to Pure Data programming for making your own electronic and experimental digital sounds.

Comments and requests welcome.

The Lumiphone

April 7, 2013

Basic Sawtooth Oscillator BreadboardedI’m at the refining stages of a design for a light sensitive classic analog synthesizer that can be played with hand shadows. It may also be the world’s most inexpensive and easy to build analog synthesizer, as it does not contain any specialized components or special calibration circuitry. The result is a fun to play musical instrument with lots of timbral possibilities – pretty much identical to a Moog style synthesizer. The synthesizer, which I will call here a “Lumiphone,” bears its parameters to the player for instant connection, unobstructed by buttons and wheels or programming. It’s all raw at your fingertips, like a violin. This enables the player to make more expressive music, compared to that produced by your average keyboard synthesizer. It’s also a lot more work to play. You can’t just push a few buttons and hit play to get an acid baseline like the famous Roland TB-303. For this reason, even though structurally they are equivalent synthesizer designs, the lumiphone sounds different from the TB-303 and the like. I think the lumiphone will be most adored for its vast timbral pallet – which are all accessible at the flick of a wrist. It will of course also be adored for its accessibility in price and ease of construction.

Forgo tonality, gain simplicity and affordability

basic sawtooth osc schematicI found that the greatest barrier to entry in making a home brew synthesizer was the expensive and rare parts. It turns out that those special parts aren’t essential to the actual sound producing circuitry. I had to do a lot of research to figure out how to redesign the circuits without temperature and linearity compensation. The key was eliminating the voltage control paradigm and replacing it with light as the common control signal. In turn, the circuits reduce to a simple set of designs that any beginner hobbyist could put together in an afternoon. The trade off is of course that the pitch of the synthesizer may vary slightly given changes in temperature. This is irrelevant if you play the circuit by ear, rather than with a keyboard with fixed notes. It is also irrelevant to noise artists and experimental musicians who actively step away from tonality.

Light control

Resonant FilterLight turns out to be a convenient and interesting control signal. You can play the lumiphone with hand shadows. You can also play it with a microcontroller like an Arduino by varying the brightness of an LED. You can even play the lumiphone with the brightness of a computer screen – which can be controlled by a custom program written, for example, in Pure Data or even Javascript. CDS cells take the light cast on them to produce a resistance. This variable resistance is used in the synthesizer to control different things, namely the Frequency of the Oscillator, and Cutoff of the Filter.

Onword!

Given the surprising versatility of this lumiphone, paired with it’s thrifty and accessible design I think this thing could be the beginning of something great. I hope it will be a useful design also for noise artists, circuit benders, and electronic musicians who will no doubt want to adapt it and borrow different elements to repurpose them for their own designs – like the Atari Punk Console. It also could be the beginning of a different kind of electronic music – a fresh start for the theremen. There’s still a lot of minor details to work out – like power supply design and deciding on the most accessible parts to call for – but soon I will be preparing some schematics and documentation for the project so that anyone with basic electronics skills can build one at home or in a classroom. In the mean time, if you click around on my blog enough you’ll be able to piece together most of what you need to build one.

555 LCO & JavaScript Get Freaky Now

December 13, 2011

Perhaps the cheapest and easiest baseline synthesizer ever – made from a light controlled oscillator driven by the brightness of a laptop screen. A sequencer written in JavaScript varies the brightness of the patch, controlling the pitch of the synthesizer. Though it’s not included here the light controlled resonant filter I built in previous post could be added to this circuit – for next time! At a decimal fraction of the cost of a x0xb0x or TB-303, this open source hack could put transistorized bassline goodness in the hands of anyone with a hand-full of Radioshack parts.

The script varies the brightness of a patch on the computer screen, stepping through the values of the sliders. The sequencer has eight steps, and a tempo control. The patch of the screen is pointing at the CDS cell (light controlled resistor) which is shrouded by a piece of black heat-shrink tubing. You can play with a copy of the sequencer code here http://tristandabbles.com/SliderTraverse.html. WARNING at present it only works on Chrome and other web-kit browsers. Full screen (F11) to see the light patches. It’s JavaScript so you can hack as you please. I’ve done a bit to spruce it up. I added a second light patch to control a filter. It pulses with a simple decaying envelope every beat, which gives it that classic TB-303 sound. There are other features one could add to the sequencer, like buttons for accent or tying notes, LFOs etc.

I built the saw-tooth oscillator out of a 555. It can be done, contrary to what I said in a previous post. The trick is to supply a regulated current to the timing capacitor, which creates a linear ramp. This is done simply with a single transistor configured as a current source. Unfortunately you also have to isolate the output with a high impedance buffer. For the experiment I used a little Radioshack amplified speaker, shown in the video, which seems to have a very high input impedance as it didn’t effect the frequency of the oscillator at all. For a stand alone design an op-amp buffer stage would be required. So it’s a toss up as to whether this design is any simpler than the one I posted earlier which uses two op amps and a JFET.

The Steiner Resonant Filter & A New Adaptation

June 15, 2011

I recently stumbled upon a document from the annals of 70s electronic music enthusiasm dedicated to the Steiner Filter. It’s a three mode (LP, HP, BP) resonant filter designed by the creators of the Synthacon, an obscure model of subtractive style synthesizer out of Utah USA that never went into large scale production. The synth itself never made a name for itself, and could have been lost to the tides of time and never to be spoken of again, if… it weren’t for its filter design, which due to its simplicity and unique sound has been floating around the DIY synth and guitar effects forums for quite some time. Here is the PDF N-Steiner VCF 1974.
The design is built around standard off the shelf parts – no special matched transistors are called for, or temperature compensated or precision resistors, or fancy transconductive op-amps from limited and deceased runs. And what’s more, the great simplicity and practical efficiency of the circuit in no way hinders it from producing some of the richest and most interesting tambers I’ve ever heard out of a VCF. This is I believe due in part to the influence on the Q or feedback of the circuit by the frequency control. That paired with a general unwieldy tendency to snap into oscillation makes it pretty grungy and slightly unpredictable – it’s almost instrument unto itself.
The Steiner filter is a perfect fit for my bigger over-arching project outlined here, which calls for simple designs using a minimum of components that are redly available off the shelf, and that highlight and carefully cultivate nonlinearities and manifold interactions, producing richer more interesting sounds, rather than compensating for and attenuating them into submission producing an auditory expression of perfected boring domestic sterility. But I digress…
I was especially interested in the concept circuit in the first part of the paper, because it is even simpler than the full fledged voltage controlled circuit. I fleshed it out in a circuit simulator to see how it worked, and it worked great. Here it is with a sawtooth signal going into the low-pass input. Note the ringing caused by a high Q.

And here it is with the sawtooth signal going into the high-pass input.

And here it is in oscillation with the Q set just at the point before it would start clipping as the signal maxed out at VCC.

I built the concept circuit presented in the first part of the paper, but modify it for light control so it could go with the light controlled saw tooth oscillator I still had kicking around from last time. Though the two variable resistors will change to vary the cutoff frequency of the filter they must be the same value. Whereas Moog or Steiner put a bunch of reversed biased transistors or diodes in place of the two resistors to make it voltage controlled, instead to make it light controlled, I replaced the resistors with a pair of CDS cells. The filter is then controlled with the shadow of the hand covering both CDS cells. If they are covered unequally interesting things happen. If R1 is covered mored than R2 the Q is increased. If R2 is covered more than R1 the Q is decreased. Here is the final schematic:

I added one more modification to the circuit consisting of two diodes set up in a clipping arrangement, which squelches the feedback when it reaches a certain magnitude (-+7V to be precise, luckily just a little within the bounds of the normal magnitude of the feedback signal) preventing the volume from increasing dramatically when the filter slips into oscillation. A side effect of this method of squelching is a rich harmonic distortion which sounds, well… awesome. The filter now also recovers from oscillation more quickly, making it more useful for playing “music.” Here it is shown in simulation:

The final result sounds pretty much like classic analogue synthesizer (for $10 in parts!). It sounds pretty great! Take a listen.

During performance the light controlled synthesizer exceeded all expectations. It turned out to be a powerful instrument suitable for beautiful music making. Please listen to this improvisation featuring the light controlled synthesizer accompanying an electric harmonium and electric guitar. The filter really starts coming in at around 2 minutes.

And if for some reason you can’t get enough, here’s another cut:

I think my next project will be to build an exponential converter which will make playing the light controlled synth a little easier.

A Case Against the 555 Timer

May 26, 2011

There are many 555 based noise making circuits out there in the DIY noise and circuit bending community. Its popularity is partly due to the fact that it is a simple chip to use; it provides all the facilities for a square wave oscillator. But it is also due to the fact that there are countless 555 schematics out there churned over and over from book to book and now blog to blog who’s first incarnation begin probably in the era of ham radio or on the graph paper pages of a Forrest Mims book. It makes one wonder if the 555 timer is so used not because it’s the right tool for the job but more because it’s what’s in the prevalent circuit designs, leading to a kind of stagnant force of habit in the community. My main complaint about these 555 circuits is that they can only generate square waves. Saw tooth waves sound much meatier especially when run through a low pass filter. They are the perfect wave form for subtractive synthesis. Further a sawtooth waveform can easily be converted to a square wave or a triangle or even a sine wave. This is not the case for a square wave. At the heart of your typical electronic synthesizer you won’t find a 555. You will find instead a voltage controlled sawtooth oscillator. But these synth schematics are sophisticated and inaccessible to someone just getting into electronics so they don’t lend themselves to the kind of viral proliferation of say the Atari Punk Console. I decided to see how hard it would be to design a simple sawtooth relaxation oscillator using a common TL082 dual op-amp chip, which like the 555 can be found at RadioShack. The only other active components are a diode and an MPF102 JFET transistor, also available at RadioShack.

I used whatever components I had lying around and scavenged others off boards from my junk pile. Here is the final schematic:

The theory of operation is as follows: The first op-amp (left) charges the capacitor in its feedback loop. The output of the op amp is positive going and increases from 0v to 15v linearly, at a rate dependent on the negative current through the input resistor. But before it gets to 15v the second op amp, acting as a comparator, discharges it. The moment the output of the first op amp exceeds 7.5v (half the supply voltage) the second op amp switches its output from -15v to 15v. This fires the JFET which shorts out the capacitor, reseting the circuit to its original state. The diode prevents the input of the JFET from going above 0v and the resistor to ground pulls it to ground when it’s not being pulled to -15. The capacitor in the second op amp keeps the comparator from changing state before the main capacitor is fully discharged.

Here’s a video of it in operation. The meter is connected to the main capacitor, showing it charging up and then quickly discharging when it reaches the 7.5v threshold. I have a potentiometer hooked up between -15v and ground with the wiper connected to the input resistor – a variable voltage divider basically. This allows me to input any voltage between 0 and -15v to the circuit, which gives a corresponding frequency tone.

Here is an example of the circuit put to practical use as a light sensitive theremin. I replaced the 10k input resistor with a CDS cell (light sensitive resistor) allowing me to vary the frequency with the shadow of my hand

And another one of the light theremin at a lower frequency.

So this circuit was pretty easy to build. It is a little more complicated than a 555 oscillator, but it’s also little more interesting. A 555 is actually a relatively complex device but for the beginner it is usually treated like a black box. This circuit however has all the mechanisms exposed, making it perhaps a better education tool than the 555. The dual supply might be a little off-putting, requiring either two wall warts or two 9v batteries. Could this be modified to work with a single supply? One thing is for sure though, it sounds good, I dare say better than a 555 circuit. One more important point, with this circuit frequency modulation is possible. We could build a few of these circuits and be able to generate very rich FM sounds. We could also build a slew limiter for the input, or a random signal generator. The possibilities are virtually boundless with voltage control. To fully realize these options we would want to invert the input, and perhaps while we are at it scale the input exponentially. This unfortunately calls for another op amp.

Exponential Converters

May 19, 2011

All classic analogue synthesizers have exponential converters built into their oscillators to convert the linear voltage scale we use to control them into the exponential scale we hear. The 1 volt per octave standard has this transformation built into it. This is a critical part of the synthesizer system because, without it, pitches would sound flat for a given voltage interval or fall entirely outside the control voltage range. This has to do with the fact that we hear pitches in an exponential scale. This can be heard using a slew limiter switched to linear on a classic synth. Exponential control of filter frequencies is just as important for the same reasons. So controlling the pitch in a synthesizer with an exponential function is important. Now, to date, the common way of achieving such a conversion uses some of the most uncommon parts and very sophisticated circuit designs that utilize matched components and little heater ovens and very fine adjustments with precision manufactured resistors and precision voltage and current references. This is all for the sake of accuracy, and elimination of drift caused by changes in temperature. We’re going to blow that all away and start with the simplest most accessible approach using the central mechanism at the heart of these systems.

At the heart of an exponential converter is a simple lowly diode. That’s it. A lesser known fact about diodes is that they have a nonlinear current to voltage relationship that pretty much matches an exponential function. See the exponential curve in the top right quadrant of the graph when V approaches Vd.

Diode-IV-Curve

So we have at our disposal a linear voltage to exponential current converter. This turns out to be an advantage because the typical sawtooth oscillator is actually current controlled rather than voltage controlled, because the frequency depends on how fast the timing capacitor charges, and that rate of charge is dependent on the rate of electron accumulation at the plate, i.e. current. More theory on relaxation oscillators can be found on my blog post.

The problem with the diode approach for classic synthesizer designs is that the V to I function is heavily temperature dependent. According to Hal Chamberlin (my elder in these matters) for a typical VCO design a 1 degree Celsius change in temperature would result in nearly a half step change in pitch. Pretty significant if you consider that the ambient temperature in your average terrestrial environment can vary between 4 and 40 degrees Celcius, which would result in a possible pitch drift of roughly 1 1/2 octaves. Is this too much drift for a chaotic noise box, or is it the range of instability that we find actually advantageous? If the pitches drifted into an inaudible range it would render the device useless at certain temperatures.  If you wanted the device to function in outer space it could be a problem, but it looks like here on earth it would produce meaningful values in any condition short of a sub zero meat locker. Good to know.

If it turns out that the drift is too much there is a pretty simple approach to eliminating the bulk of the drift using a second diode which cancels out the changes. There will still be nonlinearities and since we won’t bother to source matched components it won’t be perfect, but the bulk of the drift will have been eliminated.

If you’re interested in this topic I recommend checking out Hal Chamberlin’s book “Musical Applications of Microprocessors.” It’s out of print so you’ll have to search the library system, but it will be well worth the effort as it has an extensive section on classic analogue synthesizer circuit design. If you are thinking about building a synthesizer I also recommend checking out the ASM-1 site as they have a whole modular synthesizer design that is open source, developed and well tested.

A new project

February 4, 2011

I think it’s time to rethink the temperature compensation measures found in most modular analogue synthesizer systems. Though the work done to stabilize the frequency of oscillators and filters, and the amplitude of envelopes etc. warrants much respect and gratitude to our early modular analogue synthesizer gurus in the Moog era, I feel that we need to question the whole design requirement. We live in an era now as artists where chaotic forces are not evaded but rather relished and encouraged for their richness and depth and mystery. “Extraneous” variables are no longer extraneous but rather the very subject of our interest and inquiry. If we hope to understand the world as a system rather than a set of disconnected linear processes then there are no extraneous variables. They are not irrelevant or unrelated to the subject being dealt with but rather a part of the subject themselves. Thus where not too long ago the electroacoustic artist’s studio featured a linear path between input variables, on one side, sound synthesis modules in the middle, and an output on the other side now we are seeing more and more chaotic systems that feature feedback and interrelations that don’t have distinct inputs and outputs but rather taps, portals into the universes contained within that to this day undo the prowess of Newtonian physics.

For these reasons I’d like to propose a new synthesizer design where rather than eliminating temperature as a variable we may even go so far as to highlight it in our design by thermally connecting the temperature sensitive parts of the circuit to the housing, making the ambient temperature, and the heat generated by the circuitry, and even the placement and temperature of the user’s hands on the housing, key variables in the electroacoustic system. Also rather than a linear in-out flow the design would feature a system of interrelation and feedback through a modulation network rather than matrix, and taps for input and output.

I am happy to report also that such a device would be easier to build and hence proliferate in a growing community of “deep listeners” and Makers. Eliminating temperature compensation greatly simplifies the design process, and eliminates the need for specialized components such as matched transistors. It is conceivable that a circuit could be built strictly from some of the most common parts, available even at RadioShack.

 

Free Tube Amp

June 29, 2010

My friends Mikey and Wendy gave me this thing they found at a yard sale. They were hoping to sell it for something on ebay, but it turned out to not have any input jacks or knobs. A radio? but no tuning nob or switches. It was full of tubes and had speakers. Clearly it could be used as an amplifier somehow!

I traced out the tone and volume circuitry, and found it in the middle of a chain of four tubes. I disconnected the wire going from the output of the volume potentiometer (wiper contact) to the input (grid) of the next tube. I left the current limiting resistor. Then I just installed a 1/4 inch jack on the back, connecting the sleeve to the ground on the circuit board and the tip to the current limiting resistor going to the grid of the first of two amplifier tubes. And, there it was, a PA amplifier. It worked great with line level signals from my ipod etc, and I got a small amount of sound from a guitar, but it wasn’t loud enough to use as a guitar amp.

To boost the guitar signals I built a preamplifier circuit based on the 12AX7 tube. I found a useful schematic on the “cooperative tube guitar amp project” online at ax84.com. I just copied the preamp section of the basic amplifier design. I drew from the +300 volt supply already on the circuit board which I found by looking up the plate pins of the other tubes and metering it out.

With this addition the amp not only gets nice and loud but distorts when it’s turned up all the way. It sounds great too! Listen to my friend Andy shredding in this video:

The guitar (if I may add) was found in the dumpster behind a local thrift store – making this ear smashing hair thrashing metal machining rig virtually free.

If you enjoyed this article please check out my other Electroacoustics posts on tristandabbles.wordpress.com.


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