Overview of the low-noise microphone pre-amplifier circuit.
The Microphone Input is on the left-side, the optional Low-Z Output (headphone) is on the right-side, and the Line Level audio output is along the bottom.
The pre-amplifier is nothing really fancy, just an OP27 op-amp with an active split-rail bias for added stability. An active bias can both source and sink current, while a resistive divider can not. This also eliminates the need for a negative voltage supply.
The OP27's feedback network is configured for around 60 dB of gain in the "speech band." It rolls off anything above 7 kHz and below 100 Hz. The amplified microphone audio output from the OP27 is split into two legs.
One leg feeds a TL071 op-amp to act as a buffered Line Level output and the leg other feeds a JRC NJM2113 (or Motorola MC34119) low-noise audio power amplifier for driving standard low-impedance (8/16/32 ohm) headphones or a speaker.
The following pictures of the circuits may vary from the schematics due to tweaking, but the schematics are correct. TL071 op-amps were used to ease debugging, but they can be replaced with TL072 or TL074 op-amps to reduce the component count.
Mounting the low-noise microphone pre-amplifier circuit inside an old printer switch case.
Banana jacks on the left supply the circuit's regulated +12 VDC input. There isn't an onboard voltage regulator as the pre-amplifier will actually work over a large range of voltages. Just be sure the DC power is clean.
Above the banana jacks is a RCA jack for the Line Level output signal.
To the right of the banana jacks is a panel-mounted 1/8-inch stereo headphone jack. This jack's ground tab MUST be isolated from the metal case when using the NJM2113. There are special little plastic washers made for this purpose, but they may be hard to find. An audio isolation transformer will also work.
The panel-mounted 10 kohm potentiometer controls the headphone volume from the NJM2113 audio amplifier. It doesn't effect the level of the Line Level output signal. The potentiometer has a built-in power switch.
On the right is another 1/8-inch jack for the input from the Sony ECM-CZ10 directional microphone and a panel-mounted LED as a power indicator.
1% metal-film resistor should be used and the audio coupling capacitors should be non-polarized and polyfilm-based to avoid microphonics.
Alternate view.
Note the addition of an optional isolation transformer (1000 ohm to 8 ohm, center-tapped at 500 ohm) on the output from the NJM2113.
If you don't have the proper plastic isolation washer for the 1/8" stereo headphone jack and you are mounting the circuit in a metal case, you MUST use an isolation transformer.
The isolation transformer shown above is similar to Radio Shack 273-1380. It's probably not ideal, but it works and it also acts as an additional bandpass filter for the final audio output signal.
When using the Radio Shack audio isolation transformer, connect the black/green wires to the output of the NJM2113 and the red/white wires to the headphone jack.
All the wires connecting the audio jacks and volume potentiometer should be as short as possible and twisted together to prevent oscillation.
Optional ECM-CZ10 microphone mount.
It's just a small piece of aluminum plate with a hole drilled in it which was then fitted with a 1/2-inch rubber grommet.
Attach it to the rear of the microphone pre-amplifier's case.
Completed low-noise microphone pre-amplifier with an attached Sony ECM-CZ10 microphone.
Be sure the slots along the microphone's phase tube are not covered up.
Overview of the variable audio delay circuit.
The Audio Input is on the left-side and the Delayed Audio Output is on the right-side.
The MN3005 and MN3101 normally requires operation at -15 VDC, but we are going to cheat a bit here by reversing the Vdd and GND connections via isolation diodes. This will allow the chips to run at postive voltage.
Be sure to take this into account when reviewing the datasheets for the MN3005/MN3101 as the connections will look "backwards." The polarity of any polarized capacitors in the circuit should also be double-checked.
The delay time of the MN3000-series chips is set by an external MN3101 clock generator:
DT = 0.5 * N / CLK
DT = Delay time in seconds.
N = Number of on-chip delay stages. (MN3005 = 4096, MN3008 = 2048, MN3007 = 1024)
CLK = Clock pulse frequency in Hertz.
The MN3005 has 4096 stages and requires a (dual) clock pulse frequency between 10 - 100 kHz. This works out to a variable delay time between 20 - 200 milliseconds. A 10 kHz clock pulse on the MN3005 gives the maximum delay time of 200 milliseconds.
For speech jamming, longer delay times seem to work better. Randomizing the delay times works the best, but this adds complexity to the circuit.
Alternate view.
The MN3005 and MN3101 should be run at 15 volts, but they will work at 12 volts. This is regulated with a 78M12 voltage regulator on the main DC input. This helps to keep the MN3101 clock oscillator from drifting in frequency. The circuit should be fed from +15 VDC.
The 10 kohm bias adjust potentiometer should be adjusted for minimum distortion of the output delayed audio signal. Normally, this will be at or just slightly below 1/2 the Vdd voltage, which is 6 volts in this case.
The low-pass filter on the delayed audio output is used to eliminate any clock pulse feed-through.
Note that several software audio editing and recording packages have a feature to introduce a delay in real-time audio playback. This would eliminate the need for a hardware-based audio delay line, but where's the fun in that?
Mounting the audio delay line circuit inside an old printer switch case.
Banana jacks on the left supply the circuit's main +15 VDC input and a 78M12 voltage regulator will handle the rest.
RCA jacks are used for the audio input/output.
I added an optional DPDT relay to toggle the delay line circuit in-and-out of the audio path. When no power is applied to the circuit the audio passes through unaffected.
The panel-mounted 250 kohm potentiometer tunes the MN3101 clock generator from approximately 20 to 200 kHz. The MN3101 has an onboard "divide-by-2" stage which provides the dual 10 - 100 kHz clocks for the MN3005.
The audio delay line circuit is fed from the Line Level output on the microphone amplifier.
Overview of the experimental 40 kHz Pulse-Width Modulator (PWM) circuit.
The Audio Input is on the lower-left and the output 40 kHz PWM Signal is on the right-side.
This circuit takes an audio signal and pulse-width modulates it onto a 40 kHz carrier wave. This modulated carrier is then output via two MOSFETS in a half-bridge arrangement, similar to a class-D audio amplifier.
The modulator is based around a standard Texas Instruments TL494 PWM control circuit and an International Rectifier IR2111 half-bridge driver controllng two IRF630MFP N-channel MOSFETs.
Two multiturn 5 kohm potentiometers control the width and frequency of the TL494's pulse output. Since the transducers we'll be using for the speaker system are designed to work at 40 kHz, the TL494 will be tuned to oscillate at around 40 kHz.
A simple TL072 op-amp gain stage drives the TL494 from the remote audio source. A panel-mounted 100 kohm potentiometer controls the gain (1-10) of this op-amp.
The TL494 likes to see higher voltages (around 2 to 3 Vp-p) when "audio modulated" like this, so additional amplification may be required on the input signal.
Alternate view with the IRF630MFP MOSFETs installed.
The heatsink is probably optional, but makes a handy mounting point for the MOSFETs. The IRF630MFP MOSFETs have an isolated tab, so mounting them is quite easy and doesn't require special isolation hardware.
You can sometimes salvage IRF630MFP MOSFETs from old computer monitors.
This modulator design is based on the article "Ultrasonic Directive Speaker" by Kazunori Miura in Elektor, March 2011.
Mounting the 40 kHz PWM circuit inside an old printer switch case.
Banana jacks on the left supply the circuit's main +15 VDC input and a 78M12 voltage regulator will handle the rest.
The IRF630MFP MOSFETs should NOT be run through the voltage regulator as they have a high peak current draw, and this will also give you the option for running them at a higher voltage.
RCA jacks are used for the audio input and 40 kHz PWM output.
The panel-mounted 100 kohm potentiometer controls the gain of the TL072 op-amp input stage.
The pulse-width modulator circuit is fed from the Delayed Audio Output on the delay line circuit.
Setting the oscillation frequency of the TL494, 39.4 kHz in this case.
Measure the TL494's oscillator frequency at pin 9 to avoid loading the circuit down.
The transducers seem to work best when operated slightly above or below their resonant frequency of 40 kHz. This can be a problem when using lots of transducers in parallel as they all seem to resonant at a slightly different frequency, creating even more distortion.
You'll need to fine tune the TL494's pulse width and frequency potentiometers for minimal distortion on the final projected audio signal.
Overview of the unmodulated 40 kHz PWM output signal.
5 volts/division, 20 µS/division.
Overview of the Kobitone 400ST16-ROX 40 kHz ultrasonic transducers.
The plastic ring around one of the leads will be used for the "+".
The transducers are not technically polarized, but they'll all need to be in phase when fed in parallel.
Each transducer can handle around 60 volts peak-to-peak and has a capacitance of around 2400 pF.
Experimental speaker array.
The Kobitone transducers (Mouser 255-400ST16-ROX) are fairly expensive, so I only have eight of them for now. This severely limits the range (and effectiveness) of the parametric speaker, but makes for a good starting point.
You'll need a minimum of around 50 transducers for an effective parametric speaker array, and at least 100 of them if you want any sort of range outdoors. 2,000 of them will reach out to a kilometer...
The transformer (Radio Shack 273-1380) is for an experimental method to step-up the voltage to the transducers. It increased the 15 volt PWM signal to nearly 40 volts peak-to-peak in this application. The transformer did get fairly warm after awhile, so you may want to look for a step-up transformer with a heavier gauge wire on its windings.
Transducer wiring overview.
They are all wired in parallel.
The eight transducers looks like a 0.02 µF capacitor. It's possible to add a series 800 µH inductor inline with the 40 kHz PWM signal to make a resonant circuit. This is a simple way to step-up the voltage to the transducers without the need for a transformer.
Overview of the experimental parametric directional speaker array.
A directional speaker isn't really needed for speech jamming rebroadcast application.
A standard audio amplifier and speaker, like the Sony SS-TS502 on the left, will work. Cheap piezo horn tweeters (center) will also work and most can be run at 20+ kHz, eliminating the need for expensive ultrasonic transducers.
When using conventional speakers, you may loose the directional nature of the rebroadcast audio which is required for maximum effectiveness when speech jamming.
Completed GBPPR Speech Jammer.
Using modules for each of the main sections allows you to experiment with other configurations.
Some countermeasures to defeat this type of speech jamming include:
- Speak softly. If the microphone can't pick up your audio, it won't work.
- Concentrate on what you're saying. You can "talk through" the jammer if you know what you want to say ahead of time.
- Pad out your speech with a series of timed pauses or "Ahhs..." to allow you to regain your composure.
Combine the speech jammer with the "GBPPR MIL-SPEC Laser Dazzler" project in GBPPR 'Zine Issue #89 for increased annoyance.
