Musical SSTC/DRSSTC interrupter

All credit for this article goes to Martin Ebbefeld from ctc-labs.de, after he closed down his website we made an agreement of me hosting some of his content. I translated his article as best as I could from German to English and added a few more details or information. The pictures are unfortunately in a low resolution, but it is for now all that is available.

A first few thoughts on how music can be played with a DRSSTC. The high current primary waveform that is used in a DRSSTC can not just be frequency modulated to amplify a analog signal as played back from a regular music source. The stress would be too great, the heat dissipation would destroy the IGBTs.

So what do we do?

A lot of clever and experimenting amateurs around the world have implemented different methods to be able to reproduce tones with a DRSSTC, tones that are equal to what a piezo speaker can produce, but almost all music is not that simple.

The high frequency resonant signal that is used to drive the bridge comes in small packages of 100 to 300 us length, this does not leave us much head room to modulate in. What we can do instead is to modulate the time that lies between the pulses, the so-called off time. Either by MIDI signals or from a simple analog to digital conversion signals as it is done in this article. (To be able to understand the following text and piece it all together, it could be necessary for the reader to study basic operation of a DRSSTC)

The simple circuit used here is a development of a circuit earlier made for other purposes, it could now be customized to work as music interrupter. We can not simply play back our favorite piece of music, we either need it in MIDI format or as a raw melody where we will have to edit it by hand.

Although there are already programs freely available for micron controllers to process MIDI signals in real time to a series of pulses to drive a DRSSTC over fiber optic cable, that method can be too complex for some experimenters. This article uses old and easy to understand analog components.

A normal DRSSTC will operate with a break rate of around 120 to 200 Hz (off time). This means that the time between pulses is 1/200 = 0.005 = 5 milliseconds. The characteristic buzzing sound is a product of the 5 millisecond off time and if we adjust the off time to be less, the distances between the pulses will be shorter and the sound will rise in pitch and appear as higher tones.

The idea is to match the input to a list of real tones, a awesome and brilliantly simple method. So how is this implemented in the modulator, the following examples and diagrams will explain this.

Tabel 1: Almost entire piano key list. Middle C (C4) and Tenor C (C5/treble C) with their respective frequency in Hz
Note C1 C2 C3 C4 C5
C 32.70 65.41 130.81  261.63 523.26
Cis 34.69 69.29 138.59 277.18 554.37
D 36.71 73.42 146.83 293.66 587.33
Dis 38.91 77.78 155.56 311.13 622.25
E 41.2 82.41 164.81 329.63 659.26
F 43.65 87.31 174.61 349.23 698.46
Fis 46.25 92.49 184.99 369.99 739.99
G 48.99 97.99 195.99 392 783.99
Gis 51.91 103.83 207.65 415.3 830.61
A 55 110 220 440 880
Ais 58.27 116.54 233.08 466.16 932.33
H 61.73 123.47 246.94 493.88 987.77

Middle C, called C4 is the octave called one-lined and Tenor C (treble C) is the octave called two-lined. The above table shows the frequencies in Hz for the different scales and are within those that a Tesla coil could play without going into extreme conditions. The inability to play higher tones lies with the ratio of on time and off time that becomes very unfavorable for the system.

A proportional limit between on- and off-time should be kept to maximum on-time to be 10%. This means that the highest playable note that a Tesla coil should play to avoid any damage would be maximum 10 times the maximum allowed on-time.

To understand this, lets look at the following illustration.

The simulation above shows a interrupter signal with 10% duty cycle which represents pulses that are 200 us long, which approximately corresponds to the pulse length in a normal DRSSTC, this is the period of time where the coil operates at its resonant frequency. Some coils operate with shorter pulses due to faster ring up time in low impedance primary circuits or some need longer pulses if they have a primary circuit with a higher impedance.

With a duty cycle of 10% and on-time lasting 200 microseconds, the length of the off-time ,until the next rising edge of the on-time, is 1.8 milliseconds.

\mbox{1.8 ms}=\dfrac{1000}{1.8}=555.55 Hz

555.55 Hz almost corresponds to the tone Cis in C5.

The reason to not play notes higher than 555 Hz has to be found in power dissipation limits, tolerable temperature rise and overall stress of the IGBTs, read my IGBT design guide for more details on this. The closer together the on-time pulses are, the more often the bridge is on feeding energy to the resonant circuit of the DRSSTC. More heat is dissipated, it is a larger load and power consumption rises dramatically. If the off-time is short enough, it may happen than that pulses start to overlap the ring down period of the damped oscillation and the current in the primary circuit no longer settles back to zero, but oscillates between the lower and upper limit which eventually will lead to failure in one way or another.

Modulation

We need to use a source that can feed our modulator with tones according to the sheet in table 1. This can be a synthesizer, whether it is software or hardware does not matter. We can however not use a normal music CD, it would only result in a chaotic mess of pulses that would not produce any good results on a DRSSTC. To get the best result we have to take out a part of the song, preferable the melody or lead guitar, something that is simple and recognizable for the song. With the method this modulator uses it is possible to convert the length and volume of each tone to a pulse length and exact pitch that the can used to drive a DRSSTC into playing audible tones.

Another example will follow.

In the above illustration the signal can be seen, from which the modulator has its pitch and tone length. Using the internally controllable on-time of the modulator it can vary the volume.  The highlighted area in the illustration is about 2.154 milliseconds long and that corresponds to:

\mbox{2.154 ms}=\dfrac{1000}{2.154}=464.25 Hz

464.25 Hz almost corresponds to the tone Ais in C4. With a deviation below 2 Hz.

These pulses was created in a music software where it is possible for a synthesizer to output square wave pulses. The tone lasts for 65.034 milliseconds and thus corresponds to a 1/16th note, since 1000 divided by 65.034 gives 15.376 and it roughly is 16 bangs per second, if you include the short breaks between each stop note.

In the above illustration we will take a look at a low tone for comparison.

The length of the note corresponds to the earlier illustration, it is also about 1/16th note, but with a much more coarse structure.

The length of one period of the pulse is here 8.66 milliseconds

\mbox{8.66 ms}=\dfrac{1000}{8.66}=115.47 Hz

115.47 Hz almost corresponds to the tone Ais in C2. With a deviation below 1 Hz.

This is a very low tone and DRSSTC does actually handle low tones quite well, also the so-called base line notes known from base guitars.

However these square wave pulses lasts for too long and the duty cycle is up around 50%, which is much higher than our initial design specification of a maximum 10%. So we need something in the modulator to make a more useful signal out of the very high duty cycle signals.

Circuit

Generating the tone sequences described above by the means of software is the most difficult part of this modulator. Without properly prepared signals from soft- or hardware there will be no singing Tesla coil from this modulator.

The simplicity of the circuit makes it easy to debug and possible also to expand with more features. It consists of a operational amplifier that works as a pre-amplifier so that it is possible to use low level audio signal from a CD, PC or keyboard.

The input audio signal is filtered and amplified by the LM741 operational amplifier. It is further amplified by the two BC547 transistors and converted to a square wave equal to the output of a Schmidt trigger. So it is possible to feed this modulator with sinusoidal signals and get a proper square signal to drive the DRSSTC with.

The most import part of the circuit is the 555 timer, the main task of the modulator is handled here, which is to ensure a correct length of the pulses as to not exceed the 10% duty cycle.

In the above illustration we have the input sound signal in green, the signal that is fed to the LM741 operational amplifier. This signal is conditioned, amplified, inverted and inverted again before it lands at the trigger input of the 555 timer.

If the trigger input of the 555 timer is held high, nothing will happen on the output and the oscillator circuit output made with the 555 timer is on standby. When a series of pulses are put into the modulator, the second BC547 transistor connects the 10 nF capacitor to ground and the trigger input of the 555 timer is momentarily pulled to ground. This causes the 555 timer to output a pulse of precise length determined by the potentiometer and the 22 nF capacitor. The signal shown in red is the output signal from the 555 timer.

The distance between the rising edge of the red pulses corresponds exactly to the distance between the rising edge of the green pulses, that was our input, so the important information as pitch is followed through. Everything that is done with this circuit is to cut off the unnecessary part of the square wave. The on-time, the width of the red output signal, can be regulated with the potentiometer to determine how much power the coil is allowed to use.

In the following illustration is can be seen that varying pitch and sequence of the input signal is used to generate a series of pulses that can be used to drive our DRSSTC with.

Chords

A last sensitive issue needs to be addressed. Very pleasing and cool sounds can be made by playing more notes simultaneously, also known as chords, but it does present some interesting issues with this simple modulator.

A square wave in itself is not able to express a chord as it can only represent one state. Different synthesizers overcomes this problem in different ways and some uses a intermediate frequency where it alternates between the two notes used in a chord so that for each X’th part of a chord it plays the two notes like this A, F, A, F, A, F, A etc. This method can also be simulated in some software synthesizers.

Chords does however also last longer and a short allowed on-time might give interesting results where the sound is not faithfully reproduced to the source. Chords are not recommend with this modulator, but experimentation could get interesting.

 

Demonstration

Music is played from the speaker output of a childrens toy keyboard connected to the input of the audio modulator. The Tesla coil demonstrated in these videos it is the Kaizer SSTC 1 in a state where it was being rebuild to the Kaizer SSTC 2 where the implementation of the modulator is shown in the schematics.

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