Design Changes For Guitar

Our group met again yesterday (as you can likely see from the video and images posted recently) and we finally were able to test our circuit using the guitar!

It should be noted that we were able to test our circuit design last week and verified that its actual behavior was very similar to the theoretical behavior we saw on multisim. A few pictures were posted earlier from this day, but to sum things up: We built our circuit, hooked it up to a function generator in the circuits lab room and hooked the output up to a speaker in order to see how we did.

One very important alteration to our circuit was the addition of another amplifier after the filter. Originally, we connected our speaker directly to the output of the filter, but this completely ruined the desired intent of the circuit. The output voltage essentially would never change in this configuration. We decided to try adding another amplifier (with a theoretical gain of 2) after the filter and connected the speaker to its output. This solved our issue successfully.

Our circuit actually seemed to work quite well in our testing. Using the oscilloscope, we determined our peak output occurred right around 110Hz as desired and our 3dB frequencies occurred around ~120Hz and ~105Hz. This was very similar to the theoretical bode plots we generated through multisim. Hooking it up to the speaker, we found that for a decent volume at 110Hz, the output became nearly inaudible at ~90Hz and ~130Hz. Since this two closest notes are at ~82.41Hz and 146.83Hz, we figured our circuit had accomplished the desired goal of isolating our single note. After testing with the guitar, however, we soon discovered this was not the case. Although this picture was included before, here is our setup from that day of testing:20170420_200056

Now, back to our meeting from yesterday. Yesterday we were able to test our circuit with an electric guitar to see if our filter worked as well for the frequency output of the string compared to a nice clean sine wave from the function generator. Our first problem was figuring out how to use the 4 pin jack we had purchased. The part came with no documentation whatsoever so it required a process of experimentation to determine which pin needed to be connected to ground and which pin was the output from the guitar. After trying all possible combinations, we were able to determine this. If anyone else is ever using a fender 4 pin guitar jack, the output is pin 2 and ground should be connected to pin 1.

After resolving this issue, we found that the output voltage from the guitar was way to small to be fed directly into our filter (we were getting no measurable output), we resolved this by adding an amplifier at the beginning of the circuit to amplify the signal enough to feed into the filter. After some experimentation, we settled on an amplifier with a theoretical gain of 3. This is accomplished by having an R2 that is ~2x that of R1 based on the figure below. We used R2 = 2.2k Ohms and R1 = 1k Ohms.

noninverting opamp

We then fed the output of this circuit into our filter and were able to get an audible output through the speaker! The only issue we had now was that essentially every string would play through the speaker. Confused, we tried using the analog discovery and found again that only right around 110Hz was audible as expected. Upon switching back to the guitar, again every string played through the speaker (although the higher strings were definitely quieter than the lower strings). We examined the signal being input from the guitar and found that it hardly resembled a sine wave at all. Due to this, our group concluded that our filter design although it works for clean sine waves, is quite ineffective for isolating the different strings of a guitar (due to the shape of the input waveform).

We may try implementing a 2nd order filter to see if it helps at our next meeting.

Lastly, here is what may be our final circuit design:

design-final

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