Analog Plasma Tweeter
Khalil Elbagarri
Ryan Hellar
Arash Kani
Department of Electrical Engineering
Massachusetts Institute of Technology
December 12, 2012
Contents
1 Abstract-AK
4
2 Overview-AK
2.1 Design-AK . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Building Process-AK . . . . . . . . . . . . . . . . . . . . . . .
4
7
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3 Circuit Description-AK
3.1 Block Diagram-AK . . . . . . . . . . .
3.2 Load Line Analysis-AK . . . . . . . . .
3.3 Gain and 3dB Points-AK . . . . . . . .
3.4 Input, Volume, and Tone-RH . . . . .
3.5 Preamplification Stage-RH . . . . . . .
3.6 Flame Control/Amplification Stage-RH
3.7 Output Stage-RH+KE . . . . . . . . .
3.7.1 Initial Design Choices . . . . .
3.7.2 Circuit Analysis . . . . . . . . .
3.8 Power Supply-RH+KE . . . . . . . . .
3.9 Corona-AK . . . . . . . . . . . . . . .
4 Mechanical Construction-RH
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1
5 Error Analysis-RH
31
6 Conclusion and Future Improvements-AK+KE
32
6.1 Conclusion-AK . . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.2 Future Improvements - Construction - KE . . . . . . . . . . . 33
6.3 Future Improvements - Design - KE . . . . . . . . . . . . . . . 34
7 Acknowledgments
35
A Full Circuit Diagram-AK
36
2
List of Figures
1
2
3
4
5
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10
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17
Axis response graph of a KEF AV3 speaker . . . . . . . . . .
Plasma Tweeter Block Diagram . . . . . . . . . . . . . . . .
Load line for 12AX7 triode of the first preamplification stage.
A black dot marks operating point. . . . . . . . . . . . . .
Load line for 12AX7 triode of the second preamplification
stage. A black dot marks operating point. . . . . . . . . . .
Input and Tone Control . . . . . . . . . . . . . . . . . . . .
Preamplification stage provided by two 12AX7 triodes . . . .
Bottom view of the case showing the preamplification stage in
practice. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Top view of the case showing the preamplification stage in
practice. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Stage . . . . . . . . . . . . . . . . . . . . . . . . . .
Linear Power Supplies . . . . . . . . . . . . . . . . . . . . .
T1 circuit in practice . . . . . . . . . . . . . . . . . . . . . .
T2 circuit in practice . . . . . . . . . . . . . . . . . . . . . .
High voltage power supply circuit in practice . . . . . . . . .
Self-resonant coil equivalent circuit . . . . . . . . . . . . . .
Two functioning coronas . . . . . . . . . . . . . . . . . . . .
Signal Path . . . . . . . . . . . . . . . . . . . . . . . . . . .
Linear Power Supplies . . . . . . . . . . . . . . . . . . . . .
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1
Abstract-AK
This paper outlines and describes the design and construction of a tubedriven plasma tweeter. Contained within, you will find the method undertaken to design and build the tweeter, in addition to a variety of add-ons and
improvements on a common design.
2
Overview-AK
Nearly all modern speakers are constructed by locating an electromagnet
attached to a membrane within the field of another magnet. The electromagnet uses current produced by an audio signal to create vibrations in the
membrane.These vibrations lead to pressure variations in the air, that people preceive as sound.This method is extremely suitable for low frequencies
because it is takes less much less work to move a significant mass at low
frequencies. At high frequencies, several issues can be observed with most
modern speakers. The inertia of the electromagnet and membrane is difficult
to overcome at high frequencies where these objects are moving with large
velocities. Additionally, standing waves can be observed on the membrane
which reduces the amplitude of the propagated soundwaves, reducing power
output. This effect is also more disruptive at high frequencies. The speaker
industry combats these undesirable effects by producing small, purpose-built
speakers called tweeters for high frequency audio.
Even so, speakers have other drawbacks to consider such as their direc4
Figure 1: Axis response graph of a KEF AV3 speaker
tionality. An individual standing directly behind a speaker can theoretically
hear no sound at all if the waves are not reflected back towards them. Figure 1 is a three-dimensional graph that shows the poor frequency response
in the off axis directions of a speaker.
Plasma speakers are an alternative to the convential speaker which can be
divided in a few different subclasses: the dc corona speaker, the rf corona
speaker, and the dc glow speaker.[2] These speakers were first discovered at
the end of the 19th century. William du Bois Dundell observed an irritating
noise produced by arc lamps at the time that was produced by the electric
arc the lamps created. Through experimentation Dundell found that he
could control the frequency of the sound emanating from the arc lamps by
changing the source voltage. Hermann Theodor Simon, a german scientist,
5
studied the same effect. Both scientists work resulted in the ’Singing Arc’
although Duddell invented his rendition earlier while Simon was the first to
introduce it.[3]
This paper is solely focused on the rf corona speaker which still has not
been the focus of much scientific work. S. Klein, did research in the use of a
speaker primarily consisting of a gas discharge from 1954 to several decades
later. His first design, too, was an rf corona speaker.[4] The rf corona speaker
functions by modulating an audio signal on top of a corona discharge produced by a high voltage self-resonant circuit. As the audio signal changes, so
does the corona. The resulting changes in size of the corona due to the audio
signal creates sound waves matched to the frequency of the audio input. The
plasma that is creating these sound waves is essentially massless. Therefore,
a plasma speaker has a greatly improved frequency response. Additionally,
the plasma does not change air pressure in a single direction, rather it does
so omnidirectionally. Therefore, there is no directionality. The frequency response of a plasma speaker is identical at any number of degrees away from
an arbitrary origin.
S. Klein’s work resulted in a few commercialized plasma speaker products.
The first of which was the ”Ionovac”. Other speakers by Klein received design
awards.[5] Nevertheless, plasma speakers are rare today which indicates that
they do possess numerous flaws rendering them unsuitable for commercial
use. Firstly, plasma speakers are complex systems that need routine main6
tenance. For instance, the electrode, where the corona is formed, needs to
be replaced. There are also health concerns as the plasma speaker produces
ozone which means they cannot be used in closed spaces. Also, the voltages
needed to produce the corona range from several hundred volts to kilovolts
which means that there is a non trivial risk of death when working on or
around a plasma speaker. Of less concern, plasma speakers consume a relatively large amount of power and create rf interference. Lastly, low frequency
sound, less than 2kHz, requires more air pressure to be created than a typical
plasma speaker’s corona can produce. This means that plasma speakers only
function well as tweeters in larger audio systems where conventional speakers
can be use for bass.
Nevertheless, the construction of a plasma tweeter is a worthwhile endeavor
with the proper knowledge and yields the best possible frequency response
for any speaker at high frequencies. This paper is made up of several sections detailing the following salient subjects explored during the process of
designing and building our plasma tweeter.
2.1
Design-AK
This paper demonstrates a thorough knowledge of the plasma tweeters mode
of operation and the processes by which we arrived at certain design choices.
It seeks to apply knowledge and make this particular circuit easily understandable by the student who may have only taken a rudimentary course in
7
analog design. The paper assumes a basic knowledge of tubes. The circuit
will be described in fundamental blocks to make it easier to understand.
2.2
Building Process-AK
The building process for this particular plasma tweeter is described in great
detail. It is of particular interest because of the high voltages involve in our
particular design and the extra precautions and attention to detail needed
when dealing with such high voltages.
3
Circuit Description-AK
To break this circuit down into more manageable parts, it is first represented
as an easy to understand block diagram shown below.
3.1
Block Diagram-AK
Figure 2 is a detailed block diagram of the plasma tweeter. It points out
all stages of the plasma tweeter and shows the propagation of audio signal
through those stages. These stages will be detailed in the following portions
of the paper.
8
Figure 2: Plasma Tweeter Block Diagram
Volume
Tone Control
Control
Tone Control
Line Power
300V AC-DC Conversion
1st Preamplification Stage
2nd Preamplification Stage
Amplification
Buffer
Line Power
4kV AC-DC Conversion
Output Stage
9
Flame Control
3.2
Load Line Analysis-AK
In our preamplification stage were three triodes connected in common cathode configurations. To determine the appropriate configuration, we used
load line analysis to assess the triodes operation. Our first pre-amplification
Figure 3: Load line for 12AX7 triode of the first preamplification stage. A
black dot marks operating point.
stages uses a 12AX7 triode and we can consider the load line drawn in figure 3. The load line drawn in figure 3 is the result of using a 350V supply
10
voltage as well as a 100kΩ load resistance. Inspecting the load line resulted
in the operating point listed:
Vgk = −1.2V
(1)
Vp(quiescent) = 190V
(2)
Ip(quiescent) = 1.2mA
(3)
Cathode biasing determines the operation point. The equation below was
used to determine the resistance that would be placed on the cathode.
RK = |
Vgk
Ip(quiescent)
|
(4)
Using the values in equations 1 and 3 we find that the cathode resistance
should be 1kΩ.
Similarly, our second pre-amplification stages uses a 12AX7 triode and we
can consider the load line drawn in figure 4. The load line drawn in figure 4
is the result of using a 350V supply voltage as well as a 50kΩ load resistance.
Inspecting the load resulted in the operating point listed:
Vgk = −1.7V
(5)
Vp(quiescent) = 250V
(6)
11
Figure 4: Load line for 12AX7 triode of the second preamplification stage.
A black dot marks operating point.
Ip(quiescent) = 1.7mA
(7)
Using the values in equations 5 and 7 we find that the cathode resistance
should be 1kΩ.
Our third pre-amplification stage uses a 12AY7 triode and because the
loading here was much more complicated than a simple load resistor, we
chose to use iteration to determine the correct functionality.
12
3.3
Gain and 3dB Points-AK
Preliminary gain values were also calculated for each stage. For an unloaded
12AX7, we can derive the following values from the datasheet:
µ = 100
(8)
gm = 1.6m0
(9)
rp = 62.5kΩ
(10)
The gain, µ, is an implicit value derived from gm and the resistance seen
at the plate, rp . The resistance seen at the plate, however, changes due to
the loading of the tube. The load resistance is seen in parallel with rp and
I chose to assume the tube feeds into a high enough impedance load that it
marginally affects the new rp value. For the first preamplification stage:
rp0 = rp ||RK = 62.5kΩ||100kΩ = 38.5kΩ
(11)
µ0 = gm ∗ rp0 = 1.6m0 ∗ 38.5kΩ = 61.5
(12)
For the second preamplification stage:
rp0 = rp ||RK = 62.5kΩ||50kΩ = 27.8kΩ
(13)
µ0 = gm ∗ rp0 = 1.6m0 ∗ 27.8kΩ = 44.4
(14)
13
Stage
1st Preamplification Stage
2nd Preamplification Stage
Amplification Stage
Input Voltage
50mV
50mV
50mV
Output Voltage Calculated Gain
2.33V
46.6
1.98V
39.6
1.32V
26.4
Again calculations for the amplification stage formed by the first 12AY7
were omitted due to difficulty in determining the precise plate resistance.
Given that the 12AY7 is a lower gain tube than the 12AX7, it was expected
that the gain be lower for the amplification stage than the previous two
preamplification stages.
The actual values were measured and the following table was formed:
The measured results follow the calculated values for gain. The first
preamplification stage deviates slightly more than the rest. This is most likely
due to the fact that the output impedance for this circuit was not a very high
impedance as assumed earlier. The second preamplification stage’s measured
and calculated gain values are essentially identical and the amplification stage
has a gain lower than both of the previous stages as expected.
Since the application for our plasma tweeter is ultimately good reproduction of audio, the various 3dB points are important to note in the circuit. Of
course, the capacitors in parallel with the cathode resistances create low 3dB
points. However, many of these 3dB values do not dominate the behavior of
the circuit. In later sections, these dominating 3dB points will be touched
upon.
14
3.4
Input, Volume, and Tone-RH
Figure 5: Input and Tone Control
Bass
Treble
Output
Tone Control
The first part of the audio stage accepts a line-level input from a headphone jack.The input audio is capacitively coupled to block any DC voltage.
There is a logarithmic potentiometer at the input in order to control the
volume, followed by a passive tone control. Since the gain of the preamplifier
stage is enormous, an active tone control wasn’t deemed necessary.
The circuit for the tone control can be seen in Figure below. The corner
frequencies were set at 480Hz and 15.9kHz. At low frequencies (bass), all
the capacitors in the circuit present a very large impedance, which means C19
and C20 essentially present open circuits, and using the log potentiometer
on the left, we can adjust the level of the signal at bass frequencies. At high
frequencies, the capacitors have very low impedance and essentially present
15
short circuits. Thus C18 and C22 both act as short circuits, and we can use
the log pot on the right to adjust the level of the signal at treble frequencies.
This is not an active tone control and so there will be some signal level loss,
but as previously mentioned, since the gain of the two preamp stages is quite
high, it shouldn’t be a problem.
3.5
Preamplification Stage-RH
Note: The plate voltages for both tubes in the preamp stage are provided
through the 100k and 50k resistors connecting to the 350V power supply
The tone control is followed by two gain stages in this section, both in
common cathode configuration. Each stage has a 1kohm cathode degeneration resister to set the grid’s self-bias point. There is also a 4.7nF bypass
capacitor across each degeneration resister to increase the gain of the stage.
Initially the first stage was capacitively coupled to the second gain stage to
block DC voltage. However, during testing of the circuit, there were significant oscillations caused by feedback from the second gain stage to the first
(through the capacitive coupling). Therefore, we designed and implemented
a high-pass (C4-R21), low-pass filter (R21-C17) in between the stages to
quash these oscillations. The high-pass filter’s -3dB point was set at 106Hz
in order to try and block 60Hz hum, and the low-pass filter’s -3dB point was
set at 22.2kHz, which is above the audible range, but certainly low enough
16
1.26V
212.5V
1.6V
257.5V
2.4V
174V
2.41V
177V
150-322V
155-322V
342.5V
Figure 6: Preamplification stage provided by two 12AX7 triodes
17
Figure 7: Bottom view of the case showing the preamplification stage in
practice.
to block RF frequencies. This did indeed quash the oscillations.
3.6
Flame Control/Amplification Stage-RH
The output of the preamp stage is capacitively coupled (via C7) into the
flame control/amplification stage. Here there is one more common cathode
stage of amplification, with the main difference being that the plate voltage
18
Figure 8: Top view of the case showing the preamplification stage in practice.
is provided by the current flow through the last triode (the follower) and back
through the 1k resistor (R9). The current flowing through the 1k resistor and
the subsequent voltage drop provides the plate voltage for this triode. The
final triode is a cathode follower, which passes the signal to the high-voltage
BJT follower through a DC blocking capacitor (C12).
The flame control of this circuit is set by the resistor and pot network
feeding into the base of the BJT. R15 and R18 set the starting bias point,
which is then adjusted by the pot. This network sets the DC voltage at
19
the base of the BJT follower. The goal was to get the DC voltage level as
high as possible in order to get a high level on the screen grid of the output
stage. The BJT follower buffers the output of the amplifier’s signal and sets
a buffered DC voltage for the screen grid of the output tube.
3.7
3.7.1
Output Stage-RH+KE
Initial Design Choices
(RH) As previously mentioned, one of the disadvantages of the plasma
tweeter is that it can’t operate well at low frequencies. However, to improve
upon the previous design we were building off of, we decided that to be
able to reproduce lower frequencies, we needed more plasma to create higher
pressures. To do this, we needed a higher power tube. We had a 4-400 tube
on hand so we implemented the output stage using that tube since it has a
higher power capability.
For the output coil, we used a ceramic former since it has low dissipation
at high frequencies and can also withstand high temperatures. It had 15
precut notches for the wire, and we used 14awg solid magnet wire to try and
keep skin effect losses at a minimum.
To prevent the high frequency oscillations from going back into the supply
and degrading the insulation, we have an RF choke to block the high frequency signal. (KE) This also makes the supply look like a high impedance
20
Figure 9: Output Stage
Feedback Loop
21
to the tube at high frequencies which increases the Q of the system. Without
it, oscillations would die down rather quickly because they would be shorted
to ground by the filter capacitors. (RH) We decided to wind our own inductor since there weren’t any available that fulfilled our specifications. Initially,
without thinking too much, we wound three layers of wire around a ferrite
core to create the RF choke. We kept the spacing between the wires to a
minimum to increase inductance and split it into two sections in order to
reduce the voltage stresses on the wire’s insulation. However, obviously, the
capacitance of this inductor is enormous since the wires are wound close together, and there are three layers of wire. At frequencies higher than 700kHz,
the inductor turned into a capacitor, which was obviously not ideal. After
realizing our mistake, we took a plastic tube, with air core, and wound a
single layer of wire down its length. We tested the response on the dynamic
system analyzer in lab and it behaved like an inductor to frequencies above
13MHz (the maximum frequency on the analyzer). Also, in addition to the
RF choke, for the signal thats not completely blocked, we have a capacitor
shunting it to ground.
For the breakout point, we needed it not to melt in the presence of high
temperature plasma, so we decided to use a relatively thick brass rod (since
Tungsten wasn’t available). Tungsten is preferable since it has a much higher
melting point. The addition of a large aluminum topload will help conduct
heat away from the breakout point.
22
3.7.2
Circuit Analysis
The BJT follower feeds the signal into the screen grid of the 4-400 tube, with
a certain DC voltage set by the flame control. The output circuit is selfresonant at 28MHz. The signal fed into the screen grid from the BJT follower
causes the gain of the 4-400 to fluctuate, which changes the amplitude of
the oscillations and hence modulates the flame, producing sound. Negative
feedback, to stabilize the flame at each new amplitude as the oscillations
change, is achieved by wrapping a freestanding wire near the end of the
output coil and feeding it into the control grid of the 4-400 tube. The feedback
can occur in this method because parasitic capacitance occurs between the
freestanding wire and the end of the output coil. The 10nF capacitor exists
to prevent DC voltages that might develop on the control grid.
As mentioned above, the output stage is self-oscillating. The question is
how exactly does the circuit oscillate? (KE) The circuit oscillates by having
the tube provide about 180 degrees of phase shift while, at resonance, the
resonator provides the other 180 degrees to get a sustained oscillation. If
the gain of the tube is high enough, the oscillation can get started with a
small transient. This could be either adding a small load to the output after
L2 or by slightly adjusting the voltages on the tube. The small load change
could be in the form of touching a small metallic piece to the output of
L2. We used an ungrounded screwdriver and for safety, it was taped to a
long insulating rod. It’s so easy to start because while not oscillating, it’s
23
in an unstable equilibrium meaning a small perturbation to the state could
start it oscillating. The output doesn’t run away with larger and larger
amplitudes as time goes on because as the amplitude of the oscillations get
larger, more voltage is developed at the end of L2 meaning the loading of the
resonator is increased. This makes the oscillations decay down to a stable
point. Another limiting factor of the amplitude is the inherent non-linearity
of the tube for large signals. The voltage on the anode can’t possibly swing
without bounds since it has a lower bound possibly around a couple hundred
volts when fully conducting. The feedback capacitor protects the tube so
that the output doesn’t arc to the feedback wire which would put the supply
voltage on the grid. This is quite bad for the tube so it should be prevented.
The value’s extremely large compared to the capacitance to the feedback
wire so it shouldn’t affect the oscillations noticeably. The 39k resistor holds
the control grid around 0V but should be self-biased negatively due to the
electrons from the filament striking the grid’s wire. It wouldn’t be positively
biased since control grids are placed relatively close to the filament to increase
the control of electron flow. This negative bias helps increase the efficiency
of operation but it could be improved more. This is touched on in the future
improvements section of the paper.
3.8
Power Supply-RH+KE
We needed four different voltage levels for this device. We needed 6.3V at
about an amp to provide the filament voltage for the preamp, amplification,
24
and follower stages (12AX7 tubes), 5V at 15A to provide the filament voltage
for the output stage (4-400 tube), 300V for the cathode for the preamp,
amplification, and follower stages, and a variable 0-4kV for the output stage.
The 4-400’s filament requires a fairly high current. Since there was no
transformer available, we removed the high voltage secondary from a MOT
(microwave oven transformer) and wound it with enough turns of thick wire
to get a slightly higher voltage than we needed. The current drawn by the
filament’s so high that wire voltage drops over the relatively short length in
the transformer and to the tube were not negligible. It’s very important to
keep high power tube’s filaments within 5Due to the lack of window space on
the MOT, the 6.3V winding was added to the 5V winding. This unfortunately
meant that the 6.3V winding couldn’t be used while the 4-400 wasn’t plugged
in since the voltage would be considerably higher. Such a voltage increase
would run the filaments too hot and would reduce their lifetime. For the plate
transformer, we used a 115V to 230V transformer since the peak voltage of
the output (230V*1.4 = 322V) is near the 300V we need. We used 1N4007s
in the bridge rectifier since they are cheap, available, and can handle the
voltage and current. To decrease the ripple of the 300V supply, we used
a RC pi network since the amplifier stages are sensitive to plate voltage
variations. The small fluctuations in an early stage of the amplifier would
later be amplifier and heard as hum.
25
Figure 10: Linear Power Supplies
26
Figure 11: T1 circuit in practice
Figure 12: T2 circuit in practice
For the 4kV supply, we used two MOTs in a non-conventional series configuration. Each MOT can supply 2kV at 500mA with a 120VAC input,
so they fit exactly what we needed. Since MOTs are designed to operate
very close to saturation, the two transformers had both their primaries and
secondaries connected in series. This reduced each primary’s voltage to 60V
pulling it far out of saturation reducing the unloaded current. Unfortunately,
this meant were back at 2kV so we added a voltage doubler to the output
27
Figure 13: High voltage power supply circuit in practice
to get 4kV.The addition of a variac to control the input voltage to the 4kV
supply allows us to control the output voltage (and thereby the maximum
power output of the tweeter). 4 1N4007s in series for each part of the bridge
rectifier was sufficient to satisfy the ratings. We used 14gauge wire on the
primaries of the MOTs, and 22 gauge, 50kV wire on the output of the 4kV
supply since the insulation on most wires isn’t enough to insulate against
4kV.
3.9
Corona-AK
The corona is arguably the most important part of a functioning plasma
tweeter as it is the entity which creates sound.The forms at the free end
of the self-resonant coil at the output stage. The self-resonant coil acts
as a very high Q filter which drives a very high potential at its particular
resonant frequency. The high potential breaks down the air which causes it
to form into ions that create a path to ground with reference to the tip of
28
the coil. An equivalent circuit for the self-resonant coil is shown below. The
Figure 14: Self-resonant coil equivalent circuit
inductance and resistance in this equivalent circuit are intrinsic to the coil.
The capacitance is the result of many different capacitances present in and
around the coil. Such capacitances are those between the loops of wire that
form the coil and the capacitance between the tip of the coil and free air.
The bode plot of this system shows a 180 degree shift in phase at the
resonant peak at a frequency that is determined by the capacitance of the
coil.
fresonant =
1
√
2π LC
(15)
Furthermore, capacitive feedback was utilized via the small loop of wire
around the tip of the coil where the corona is formed. In situations where
the coil could not sustain a corona, a screwdriver attached to a long pole was
used initially to start the flame as it provided a shorter path to ground. Once
the potential on the tip of the self-resonant coil is overcome, the potential
needed to sustain it is lower so the screwdriver could be retracted from the
system.
29
(a)
(b)
Figure 15: Two functioning coronas
4
Mechanical Construction-RH
We split up the power supply and the device circuitry to minimize magnetic
coupling from leaking flux to the tubes, and because of size and weight restrictions (The power supply is quite bulky and very heavy). The leaked flux
would cause the electron paths in the tube to differ from their near radial
path. Depending on the tube’s geometry, this may induce hum into the output. For the device circuitry, we used an aluminum case since it’s strong,
conductive, shiny, and lightweight. We used solder lugs to hold interconnections between components still so they won’t short to the case. All the
tubes and most of the output stage (RF choke, 15ohm 10W resister, output
coil) are located on top of the case so they can get adequate cooling and
30
due to size restrictions since the parts are either too large themselves or are
at a high voltage so proper clearances would be difficult. The case will be
grounded for safety reasons.
5
Error Analysis-RH
Even though our design has significantly improved the functionality of the
plasma speaker, there are several different directions further work can go in
the future so that the speaker has improved performance.
As mentioned previously, we had issues with clipping, especially on the
positive half cycle. This was almost entirely a result of not setting the bias
points correctly so that the output wasn’t centered, which is why there was
only clipping on the positive half cycle, and not the negative. If we had more
time, we would go through and adjust the biasing of each gain stage, and
especially the biasing of the flame control, in order to remove the clipping.
We were also not able to test our handmade inductor, L2, at frequencies
up to the level we were operating at. Our circuit was oscillating at 28MHz,
while we were only able to test our inductor at 13MHz. Since our oscillations
were so much higher than we were able to test, L2 might not behave very
much like an inductor at 28MHz. However, it is impossible to know this with
the equipment we had on hand, so this source of error is purely hypothetical.
The only way to test would be to wind another inductor with the turns spaced
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further apart to minimize capacitance, and see how that inductor affected
the circuit. We know that the choke was below its self-resonant frequency
since we wound another inductor with a self-resonant frequency that turned
out to be in the 100s of KHz and instead of the output portion oscillating,
it only drew a lot of current. This is because the inductor, at the frequency
the output would oscillate was a capacitor making it a low impedance path
to ground. This would quash any possible oscillations of the output so the
next thing it could do is draw a lot of current.
6
Conclusion and Future Improvements-AK+KE
6.1
Conclusion-AK
The goal for this project was to create a great-sounding and highly powered
plasma tweeter. Our circuits are truly analog being that they take advantage
of the qualities of vintage tubes and, in some case, were designed more so on
intuition rather than analysis.
The final design we achieved had a relatively large flame that, when stable, created beautiful sound. The frequencies it is capable of producing far
surpassed our hopes. Last, our tweeter’s volume was loud enough for typical
applications.
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6.2
Future Improvements - Construction - KE
There are many more additions that could be made to improve the performance and safety of the tweeter.
First of all, enclosing the high voltage supply would greatly improve the
safety and portability of this setup. It would prevent any wires from getting
caught on anything and allow for more high voltage stand offs to prevent
wires from moving about. Adding a faraday cage would also increase the
safety of the setup so that nothing could get near the plasma. Since the
output isn’t actually DC decoupled from the high voltage power supply, a
grounded object getting near the conductive plasma would allow the stored
energy of the caps to discharge through it. It would make quite a loud bang
and no one wants that from a tweeter when listening to music. The addition
of a faraday cage would also prevent coupling the RF carrier to any sensitive
equipment causing it to malfunction. As it stands we have a fan blowing air
longitudinally across the tube which, due to the placement of the output coil,
disturbs the flame. This is heard as a flickering sort of noise so proper cooling
of the tube would prevent this. The case would be kept at a positive pressure
and a chimney would direct the air flow from the base of the tube to the top.
This would also allow us to use a lower air flow rate which would reduce fan
noise while still ensuring that the temperatures don’t exceed the maximum
tube temperatures. Even though precautions were taken to reduce hum, it
was still audible on the output so increased shielding would help. It might
be necessary to separate the amplifier and the output stage to accomplish a
33
major reduction in hum since the wires carrying 15A for the 4-400’s filament,
though twisted, still probably leak some B field.
6.3
Future Improvements - Design - KE
With more measurements and design, it would be possible to add a filter
to the input to reduce group delay differences, which would be heard as
distortion, and also boost the bass frequencies. An active filter network might
be necessary instead of a passive one due to the need for extra gain. Fine
tuning of the tone control, increasing the power supply filtering, adjusting the
biasing of the amplifier stages, and increasing the emitter follower’s voltage
would allow us to create a cleaner, louder sound from the plasma. Biasing
the control grid of the 4-400 more negatively would allow for a greater anode
efficiency to a point since it would force the tube to stop conducting on
negative output swings. This would pull the tube away from operating near
class B and more towards class C. There is no necessity for the tube to be
able to operate in a wideband mode when its only supposed to operate at the
resonant frequency of the resonator. If the grid were biased too negatively
though, the feedback wouldn’t get the tube conducting for long enough to
sustain oscillations.
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7
Acknowledgments
We would like to thank Professor Byron Roscoe for the instruction throughout the semester, the assistance in ordering parts, and the advice in construction and debugging the circuitry. We would also like to thank Joe Sousa for
his help and advice.
References
[1] Rosco, Ron. Intro. to Analog Design Lectures and Labs. Fall 2012
[2] Ph. Bequien, K. Castor, Ph. Herzog, V. Montembault, Modeling plasma
loudspeakers, J. Acoust. Soc. Am. 121(4), 1960 - 1970 (2007)
[3] Ludwig Darmstaedter: Guide to the History of Science and Technology.
In chronological presentation. 2nd, umgearb. and amplified edition, with
the collaboration of R. du Bois-Reymond, edited by L. Darmstaedter,
Singer, Berlin 1908, p 971st
[4] S. Klein, Acustica, vol. 4, pp. 77-79, 1954.
[5] Ulrich
Haumann
Plasma
Speaker
Homepage”
http://www.plasmatweeter.de [Accessed 11 December 2012]
35
[Online],
A
Full Circuit Diagram-AK
Below are two figures that together comprise all the circuits that were used
in this project.
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1.26V
212.5V
1.6V
257.5V
2.4V
174V
2.41V
177V
150-322V
155-322V
342.5V
Feedback Loop
Figure 16: Signal Path
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Figure 17: Linear Power Supplies
38