Lock-in Amplifier and Scientific "Outstrument™" for Audio and Acoustics

Here is a low-cost lock-in amplifier suitable for teaching an introductory course on audio and acoustics.

We begin with a Teensy 4.0, high quality sound card, condenser microphone, amplifier, and speaker, and assemble into a device that can record, playback, and experiment with audio, as well as allow the student to see and understand and photograph sound waves, etc.

Starting with a speaker, some hookup wire, header pins (a strip of female and a strip of male 1-inch spaced square pins), Teensy sound card, Teensy, and audio amplifier...

You will need simple tools like soldering iron, wire cutters, etc..

First break off two pieces of male header pin strip the right length for the Teensy. Use the Teensy as a "ruler" by inserting as shown in the 2nd photo to see how much is needed. Male header pin strips are easy to break-to-length with bare hands.

Now you should have two strips the correct length. Insert them as shown in the 3rd photo.

Then solder as shown in the 4th photo.

Once the first strip is soldered, inspect it, as shown in the 5th photo.

Finally solder the other strip so they are both soldered on as shown in the 6th photo.

Make sure they are parallel and straight.

Now solder the female header pins to the sound card as follows:

Note that female pin strips are much harder to break-to-length by hand so it is best to "score" the strip with a sharp knife at the right point and then break it by hand.

The first photo in this Step 2 shows how to use the sound card as a "ruler" to measure the right amount of female strip to use, and then score it in the right place.

The second photo shows it broken to the right length (first piece).

Break off a second piece to the right length also, again using the sound card as a "ruler" as shown in the 3rd photo.

Now you can insert them both as shown in the 4th photo.

Solder them in place as shown in the 5th photo.

Inspect the final result as shown in the 6th photo.

Be careful about polarity, the two pins are different! One pin is ground and you can see it is connected to the outer housing as shown on the 1st photo of Step 3. Look carefully at the first photo and you can see the right pin is ground and the left pin is not ground.

Insert correctly as shown in the 2nd photo and then bend the microphone with pins parallel to the board as shown in the 3rd photo adding something underneath the board so it doesn't fall out. Here a piece of plywood is used.

Solder the 2 pins in place as shown in the 3rd photo, giving the result shown in the 4th photo. Make sure GND is connected to the ground side of the microphone and MIC is connected to the other side.

The amplifier is used to drive a loudspeaker.

The amplifier has a volume control that also contains the on/off switch when you turn to lowest volume it "clicks" off.

The bag with the amplifier in it comes with a nut for the volume control. The first photo of step 4 shows the nut on the workbench to the left of the volume control.

Be sure to stow (store) the nut on the shaft for later as shown in the second photo. You will need it later. Don't lose it. The best way to not lose it is to stow it on the shaft, tighten it, and then you can find it in the future (rather than putting it in a separate bag to get lost).

Use 3 wires, a red one, a black one, and a white one.

Connect the three wires to the amplifier as shown in the 3rd photo. Black is ground and red is +5v, side by side as shown in the photo. The white wire goes into one of the 3 holes to the left of the volume control in the 3rd photo. Specifically it goes into the hole closest to the red wire.

See the 4th photo.

You might find it easier to solder if you put the amplifier in a vice or holder.

The other end of the 3 wires goes to the sound card as shown in the 5th photo.

Notice the use of the holes for strain relief.

The white wire goes from LINE OUT of the sound card to the audio input of the amplifier.

Now cut off the connector (first photo of Step 5) from the loudspeaker (second photo).

Now add an extension wire to the speaker as shown in the 3rd and 4th photos. Some shrink tubing is helpful here.

Finally connect it to the amplifier speaker output as shown.

Test a short program that amplifies sound, records sound, and functions as a reverb unit.

Next, test a short program that functions as a lock-in amplifier.

The author wishes to acknowledge Kyle and Rocklen assisting with this project, and the students and staff of ECE446 and Myfab Makerspace.