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Thread: Need advice/help for my school project!

  1. #1
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    Need advice/help for my school project!

    Hello, everyone! I am a high school student working on a school requirement, which is writing a physics lab on a topic of my interest. Since I enjoy playing the guitar, I decided to look more into guitar pickups. The research question that I decided on is "How does the frequency response of a guitar pickup change with the number of turns of wire?"
    To carry out this experiment, I used a circuit model from the blog, Electric Guitar Pickup Measurements | kenwillmott.com

    This is the circuit diagram that I tried to imitate:
    basic_measurement.png

    As a student who is broke and has only few equipment, I used alligator clips, 3.5mm jacks, USB sound card with input and output, 100ohm, 47k ohm, and 470k ohm resistors, a small magnet that I borrowed from school, enameled copper wire, and cardboard.

    1) Pickup: I watched this video (DIY Guitar Pickup | HACK A WEEK) to make this simple pickup. I basically attached the magnet shown to two pieces of cardboard, and then wrapped it with copper wire about 200 turns.
    img_2807.jpg

    2) This is a picture of the test coil. I attached a plastic tube to two pieces of cardboard. Then I wrapped it with the same copper wire about 100 turns.

    img_2808.jpg

    3) The whole setup of my experiment is unlike the circuit diagram that I posted. I didn't connect any resistors because the software, Right Mark Audio Analyzer, had errors
    img_2803.jpg

    Using this setup, I ran the software RMAA and got some results:
    test-1-setting.png
    test-1-graph.png

    My question is whether this result is valid. I still have a very basic understanding of the electronic concepts that are happening. From my understanding, the peak is the resonant frequency. What I don't understand is why the graph has many bumps (spikes?) unlike the results shown in the blog.

    In addition, I ran another software, audiotester v3.0, with the same setup but got a significantly different graph.
    audiotester-result.png
    Why are the two graphs significantly different? I have a feeling that the results are very inaccurate, but I just can't verify it myself.

    To sum up, I have very basic understanding of this topic. I am clueless to what I should do next and whether I'm on the right track. Please, if there is anyone who can guide me through this project, I would really appreciate the generosity.

    Thank you, everyone, in advance...

  2. #2
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    Maybe I was asking too much... Can anyone give me some advice on measuring the resonant frequency without using an oscilloscope and function generator? Unfortuantely, my school can't afford these equipment, so I need a cheap way to measure frequency response (resonant frequency)...

  3. #3
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    this is complicated..

    you would need to be able to measure the L (inductance) C (Capacitance) R (Resistance) then can be found with math.

    lot of guys around here can explain this stuff,. I'm not one of them;

    best read this

    https://en.wikipedia.org/wiki/LC_circuit

    https://en.wikipedia.org/wiki/Electrical_impedance

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    Hi Dhk, kudos for tackling something like this as a high school project with minimal budget and gear! You didn't pick the easiest subject here, and I'd reckon your teachers will appreciate that no matter what results you get.

    Because of the design of your pickup, the output is going to be very low. From the pictures, I'd say the magnet wire you used is quite a bit thicker than what is usually used for guitar pickups. This should result in a lower resistance (with the same number of turns, the thinner the wire, the higher the resistance will be). Also participating in the very low output is how few turns of wire you have here. A typical Strat pickup is wound with about 7500 turns of AWG 42 wire. A pickup like that will have a DC resistance of about 6000 kOhms. If you have access to a digital ohm meter at school, I'd suggest you start by measuring the DC resistance of your pickup. The resistance of your coil is correlated to its output (even though it's not a perfect prediction, as it also depends on the type of magnet you used, the shape of the coil, etc.) - the lower the resistance, the lower the output. The problem a very low output poses when it comes to measuring frequency response is that the level of the signal you will get from your pickup (measured in dB) will be about as loud as the parasite noise any coil picks up. That includes humming from electrical devices around you, including your computer. This will make the results hard to analyse, as you might have a lot more going on around the same level as your signal.

    This said, on the first graph, you DO get a response at 1kH. The many smaller spikes after the first one look like octave harmonics of the initial tone. How "clean" is the signal you send through your test coil? Ie. is it pure 1kH or does it inclue upper harmonics?

    I am no expert in all of those fields, but I am sure there are a lot of guys here who can point you in the right direction. Don't give up, this is a cool project!

  5. #5
    "Thermionic Apocalypse" -JT nickb's Avatar
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    The signal level when using RMAA is too high. The red warning indicating clipping are showing. Also, you are exciting it with 1Khz and yet you get a series of odd harmonics. That is a sure sign of clipping.

    The resistors serve the purpose to isolate the test coil from the audio card. They, or some other means of isolation is required, for example a simple op amp based buffer will do it with no loss (attenuation) of the signal. The problem with the resistors is they attenuate the signal to 47/(470+47)= 0.09. This big loss might be why RMAA was not happy. You might try the microphone input in that case or adjust the audio settings to boost the signal again.

    The two things you need to accomplish is a clean drive to the exciter with no clipping and similarly a big enough signal into the audio card, but not so big that you overload it.

    Good luck
    Experience is something you get, just after you really needed it.

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    Thank you for your generous advice! One question is where or how can I get a simple op amp based buffer? Is it possible to make one with a low budget? Also, would the buffer be enough to get rid of clipping?

    Thank you~

  7. #7
    "Thermionic Apocalypse" -JT nickb's Avatar
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    You can build a buffer with some gain for less than $2. It will not help clipping. You may not need it anyway. Let's keep this simple, at least for now.

    First try connecting the test circuit output to the line input of the audio card. Using you audio card's management tools ot the windows one, make sure the line input is enable and the gain is all the way up. Next use RMAA and attempt do a test. You'll get the test screen up where it sends a test signal to your circuit and listens to the output. Look at the VU meters and adjust the audio card output level to try to get the VU meters in to the -1dB to -3dB range. If you can't get the VU meters high enough try connecting to the microphone input instead of the line input. Play with the input and output levels to get the VU meters in good range.
    Experience is something you get, just after you really needed it.

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    Quote Originally Posted by dhkstyle View Post
    One question is where or how can I get a simple op amp based buffer? Is it possible to make one with a low budget?
    Is your school providing you with any resources at all? Are there any school labs, that might have some equipment you could use?

    A buffer (in this context) is an amplifier circuit, which you need to place between your guitar pickup, and the input to your sound card. The reason is that the characteristics of the pickup coil might otherwise be affected by the low input impedance of your sound card, so you will end up measuring the wrong thing. As was previously mentioned here, using the 470k and 47k resistors provides the pickup coil with the high impedance it wants, but has the side-effect of also cutting down the (already weak) signal.

    So, my first question is, is there any sort of amplifier at your school - or home - that you might re-purpose for this experiment? You mentioned that you play guitar - what sort of equipment do you play it through? You might have a guitar effects pedal that might work as the buffer you need.

    Another thought: your school probably has some sound-reinforcement (loosely, P.A.) audio gear, used for assemblies, or theatre, or other school functions. Is it possible for you to get temporary access to any of that? If your school has an audio mixer with at lest one high impedance input, you may be in business; you could put your audio software on a laptop, and take that with you to where the mixer lives.

    It certainly is possible to make your own buffer, but that becomes another project of its own. It also requires some fairly expensive equipment - like a soldering iron, solder, wire cutters, et cetera - that you may not have. That's why I suggest looking for some existing equipment that can serve as your buffer.

    Quote Originally Posted by dhkstyle View Post
    Also, would the buffer be enough to get rid of clipping?
    You have to find out where the clipping is occurring now, before you can answer that question. My bet is that the clipping is at the other end of the chain - not at the pickup coil, but rather, at the part of the sound-card that's driving your test coil. Did you put that 100 ohm resistor (R1) in series with your exciter coil? That resistor needs to be there. Without it, your exciter coil will behave like a short-circuit, and cause overloading of the headphone output of your sound card / audio interface.

    One other question: do you, by any chance, have one guitar with single-coil pickups, and one guitar with humbuckers? Or a guitar with tapped humbuckers? If so, you already have two pickups with different numbers of turns in the coils, and you could conduct your experiment using your guitar, rather than your home-brewed pretend pickup coil.

    (The pickup coil you wound has far fewer turns of wire than the real thing, and that will cause the signal from your fake pickup to be very, very small.)

    -Gnobuddy

    Edit: Nick posted while I was typing up this post.

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