So I was exploring the internets when I found this: http://www.bugplan.com/a15.htm , a very powerful amplifier, designed by an electronics engineer with experience designing circuits for law enforcement, I obviously couldn't resist building it and I have to say I'm very impressed with the performance of this circuit, it doesn't only allows you to hear even the faintest of sounds, like the swinging motion of an HDD arm inside your computer, when used with other sensors like a tape head it can be used to hear the different electromagnetic fields around, detect mains cables, power lines...

For me, this makes this device a must have, apart from the malicious intent to spy on your neighbor it can also be used as diagnostic and troubleshooting tool.

IMPORTANT:This kind of circuits or/and their use might be illegal in your area, so it might be wise to check your local regulations before using it, use it responsibly, invasion of privacy is illegal.

Step 1: The Circuit

This is a fairly complex circuit to implement on a perforated board without errors, so I thought it would be easier to make a PCB with Eagle and print the circuit using the toner transfer method. This also makes things easier for people who want to replicate the circuit.If you use the toner method make sure to check for continuity between separated traces or traces and ground, small defects can easily occur using this method.

Breadboarding is also an option, but the circuit performance might be affected due the capacitance and inductance of breadboards.

For more detailed info about the circuit I encourage you visit his website:


I've attached the schematic and board files so you can print the circuit or modify it if you wish. The circuit is tightly packed, and although this doesn't seems to have any effect on it's performance (I made sure to use thick traces with a big clearance) you might need to modify it if your components are not small enough.

Step 2: Making the Microphone

For the microphone you can use a piezo, but many other components will work, like a simple coil of wire, I had no luck with an electret microphone I had lying around.

I soldered a couple of wires to the piezo disk, the positive goes onto the gray area, while the negative get's soldered to the disk, I placed the disk over a wet napkin to avoid overheating it, then I glued a knob with epoxy. I used some wire with tubing I had laying around, you really want the wire to be in a tube of plastic or fabric, since the vibrations on the wire get transmitted to the piezo and affects the quality of the sound received.

I also experimented using a VHS tape head, works quite well at detecting magnetic fields that fall under the audible range.

It seems using coaxial wire or even twisting the two wires together help to reduce hum, this is because the wires act like a capacitor, filtering out some noise.

Step 3: Enclosure

Once the circuit is finished and it's working properly, it can be placed in an enclosure to make it more convenient to use. I used a 8.5cm long, 5.5cm wide and 4cm tall box, it's slightly longer than what I needed, but it will do.

I've chosen to use a pushbutton instead of a switch because the sound can get pretty loud if you bump or scratch the piezo when relocating it, pushing a button seems more convenient than toggling the switch each time.

The circuit is tightly packed into the box and the switches, potentiometer and jack get screwed to the sides of the board.

Step 4: Tests

This is what it sounds like after placing the piezo on my laptop, you can hear the HDD making it's usual noises. The audio quality is bad since I had to place my earphones next to the microphone.

Step 5: The End

I hope this instructable has been useful. Use this device responsibly.

Thanks for watching.

<p>Hi Victor,</p><p>Petty cool project! I was thinking about building an SMD version of this. Do you know if the transistors are replaceable by the BC817 or BC847, I have them on hand and would prefer them over ordering other parts.</p><p>Which capacitors are ceramic in the circuit? I heared they have a piezo electric effect and might influence audio amps if used without caution. </p><p>Finally what would I need to do to be able to run this at 6V (to CR2032 batteries)?</p><p>Please excuse me for that many questions but I'd like to get it at least to work somewhat on first try.</p><p>Thanks for this instructable!</p>
<blockquote><em>Do you know if the transistors are replaceable by the BC817 or BC847<br></em></blockquote><p>I just took a look at the datasheet and it seems those SMD transistors have a higher gain than the 3904, I would suggest to compare the gain at different currents and find a transistor which fits or adjust the biasing circuit and maybe some other elements, sorry but I can't help you there. </p><blockquote>Which capacitors are ceramic in the circuit? I heared they have a piezo <br>electric effect and might influence audio amps if used without caution. </blockquote><p>4.7uF and up are electrolytic (4.7uF,10uF,220uF,470uF) the rest are ceramic. I made two circuits, one with monolithic ceramic capacitors and another one with ceramic disk capacitors and some monolithic ones. The circuit made with monolithic capacitors seems prone to feedback or some kind of oscillation, causing a deafening screeching noise when used without the filter or when the piezo isn't in contact with anything. I would like to read some papers on that issue, it seems disk capacitors are better, but I don't know why. </p><blockquote>Finally what would I need to do to be able to run this at 6V (to CR2032 batteries)?</blockquote><p>That would require redesigning the circuit to make sure the transistors are properly biased amongst other things, this circuit is complex, at least for me, and requires a good amount of time to analyze, so I can't tell you... It probably will still work without modifications, but the amplification power would probably be lower. </p><p>I'm sorry I can't help you as much as I would've liked, but I'm still learning about amplifiers and filters and it will take some time for me to be able to design circuits like this by myself, this has been mostly a project to try and see if I could make a powerful amplifier from an schematic and solve the problems I might encounter. </p>
<p>Hi,</p><p>First off... You have made a nice and well written instructable, unfortunately, you found a rather poor circuit to run with, but that's not your fault! Blame it on the designer.</p><p>No change of other components are needed to use transistors with a moderately higher gain.</p><p>Using 6V will be OK, if it can be accepted that the Total Harmonic Distortion (THD) rise from 2.6% to 3.3% (far from Hi-Fi) and that the gain is reduced from 2400x to ~1300x (measured at the output of the pot set at 100%) - and while that may sound as a big difference, it's only 5.1dB down, so no big deal at all.</p><p>Much worse is it, that the &quot;designer&quot; seems to have no clue, gaining up, then reducing, then up then down etc. and the collector resistors and collector-base resistors should really follow a progression from high to low(er) values when going from left to right - All the reduction and the liberal addition of caps was probably made in an uninformed attempt to get rid of the stability problems in this circuits - without luck unfortunately.</p><p>The attached Bode plot shows a lot about this circuit: -3dB point at 734 Hz and 1MHz, &quot;Max&quot; at ~4.5kHz, if it wasn't for the 6dB higher peak at 190kHz and the phase inversion (Phase Margin=0) at around 117kHz doesn't help either - in other words, a very inferior amplifier that probably gives you &quot;Radio Moscow&quot; reception and oscillations.</p><p>A Bode plot shows an amplifiers gain relative to frequency and the ideal is a flat band from the lowest to the highest frequencies that you want and as little as possible outside this band. Some allows +/-3dB, a 6dB variation in the pass-band, some as little as +/-1dB - my personal pref, designing amps, is +0/-3dB and way under 0.1%THD.</p><p>Take a look at the pic (click it) and you'll quickly see what's wrong. Always question schematics without documentation whether found on the net or in books/magazines.</p><p>You should try running a Bode Plot on the amp at your uni - try shorting R3 for starters :)</p><p>Have a nice day :) </p>
<p>Thank you very much for these very detailed answers, Victor805 and Omnivent!</p><p>I have to admit I know barely anything about amplifiers and I've never worked with any audio application either. The phrase &quot;designed by an engneer with more than 50 years experience&quot; caught my interrest and so I thought I give it a shot. But if this is circuit is as poor as Omnivent says, I've no doubt that you are right, I think I'll pass this instructable and focus on my projects which tend to pile up anyway.</p><p>Would you be ok if I contact you, Omnivent, if I decide to do some sort of audio project someday? You clearly seem to be an expert on that topic. :)</p>
<p>Hi,</p><blockquote>The phrase &quot;designed by an engneer with more than 50 years experience&quot; caught my interrest</blockquote><p>I've read &quot;over 40 years&quot; somewhere and also that he started out in 1979 (with this very circuit, so how much experience had he got when he started?). Well, what do I know, 'cept what I can test of course :)</p><blockquote>Would you be ok if I contact you, Omnivent,</blockquote><p>I hope so ;P</p><p>Just PM me if you want :)</p><p>I've attached a plot of a design that I'm just awaiting some bits &amp; bobs for. It has a gain of 60dB (1000x) as is, runs from 6V and use easily available components (with lower gain or lower input, it could be run at 3V, but I have better designs for 3V amps, although they need better/newer op-amps).</p><p>I might make it an instructible when I get the missing parts, although I don't like to just pour on, when there's plenty posted already. </p>
<p>Wow, thanks for the detailed explanation! I also thought about the resistors, it didn't make much sense to me to see the biasing resistors being so low at the second transistor and then increasing at the third, as you said, they usually decrease as the current get's higher at each stage, but I didn't think the circuit was bogus since I saw it being posted in several places, including a youtube video about it. I saw all the other circuits and immediately assumed it was flawless, as you said, I should be more critic about the circuits I find on the internet. </p><p>It definitely works, but I guess it could be much better if it was made properly.</p><p><iframe allowfullscreen="" frameborder="0" height="281" src="//www.youtube.com/embed/YfYlYJYIzXQ" width="500"></iframe>I for one would like to see an instructable about it, thank you the time it took you to reply and for the bode plots. </p>
<p>Hi,</p><p>You're very welcome :) It puts out sound yes, but I just watched your video and the sound quality, when he turns it on, is distorted and he obviously felt a need for dampening the high frequency occasionally - just imagine the handling noise in &quot;everyday use&quot;.</p><p>The best way to start whatever project is to find and note down the needed specs, without being unrealistic. With a &quot;Spy amp&quot; that would be something like: What gain is needed, what bandwidth is needed, what levels of THD can be accepted and so on and then make a circuit that cover that.</p><p>A few other notes that may help: Mount the piezo (pronounced &quot;pitso&quot;) disc at it's nodal ring (the circle which doesn't move when it vibrates - can be found with ground pepper ;)) for max mechanical output and the widest frequency range (piezo discs are notoriously &quot;narrow banded&quot; with a resonant frequency that will be much more sensitive, usually within the range of 2..4kHz) .</p><p>Mount the entire &quot;microphone&quot; assembly in cushioning foam (less the listening side of course) and curl the wires going to it a couple of times to lower the mechanical aspect of the handling noise.</p><p>Even the project box may benefit from some mechanical dampening and a dab of RTV silicone to steady the caps in the signal chain might do its bit too.</p><p>Consider it as a whole system, where each part has to be at its best behavior :)</p>
<p>Your advice it's worth its weight in gold, many thanks again Omnivent, I'm starting an analog electronics course and we'll cover how to design amplifiers I'm depth, I will hopefully know how to build one myself (or at least try :/ ) from the very beginning by the end of this semester. </p>
<p>I totally agree with Victor805, you should make an instructable if you find the time to do so! I've done a quick research and <a href="https://www.google.de/search?q=site:instructables.com+discrete+audio+amp&biw=1858&bih=988&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiR55aQgtvKAhXHoA4KHWr0DggQ_AUIBygC#tbm=isch&q=site:instructables.com+audio+amp">couldn't find</a> any instructable on an audio amp from scrach. The instructables published usually contain only the circuit from the datasheet of the main chip with slight modifications. However I'm most interested in the process of designing the circuit rather than just building things up. If you could include that it'd be fantastic! Plus points for measurement data and diagrams ;)</p>
<p>Hi,</p><p>No need to involve Google:</p><p><a href="https://www.instructables.com/howto/preamplifier/" style="color: rgb(3,71,190);">https://www.instructables.com/howto/preamplifier/</a></p><p><a href="https://www.instructables.com/howto/microphone+amplifier/" style="color: rgb(3,71,190);">https://www.instructables.com/howto/microphone+ampl...</a></p><p>You may try other words of course (the problem of finding stuff is, that often what we seek has got many names).</p><p>I might shoot for a bottom up approach, but some stuff is more like a priori knowledge, which, by definition, is hard to explain/teach - it probably goes with having seen/learned a lot of building blocks and their behavior over the years - like for starters, how does an operational amplifier work and then, how can we make it behave to our liking etc.</p><p>But I know what you mean, and it saddens me too, that most of what is presented as designs these days are actually just gluing together a couple of ready made boards - if one just needs the end result, fine, but there's not much design in clicking together LEGO blocks or lifting a circuit from a datasheet (in some instances without even reading the datasheet and getting the circuit right).</p><p>Hope you don't mind if I accidentally post a couple of LED projects first - they seem to be all the rage (don't worry, won't be another lame ring of Neo-Pixels with next to no real usability, like everyone and his uncle seems to think the world needs more of ;))</p><p>Oh forgot to say Thank You for the follow :D</p><p>Have a nice day!</p>
<p>What kind of microphone is this?</p>
<p>It's a piezoelectric disk or &quot;piezo&quot; for short. It's basically a piezoelectric crystal sandwiched between two metal disks, when the crystal recieves vibrations it creates a voltage between the plates.</p>
what do the LSP connection points connect to?
<p>You have to open the image to see the annotations. </p><p>3 and 4: Piezo connected here, positive to 3. </p><p>2: 9V in</p><p>10 and 7: 10k potentiometer</p><p>8 and 5: Outputs</p><p>1, 6 and 9: Ground connections </p>

About This Instructable




Bio: I'm an electronic engineering student. I don't usually have much spare time but I like to work on random projects to keep myself ... More »
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