Project Overview
Tape delays were one of the early sound effects added to guitars. They use cassette tape players to create the delay effect. In the most simple form, they take the output from the guitar and record it onto the cassette tape, then further along the tape the audio signal that was recorded onto the tape is read off and sent to a speaker. The physical distance that the tape has to move between where it is written and where it is read causes the delay effect. This project creates this effect using two different cassette decks. It also loops the signal creating an echo as well as the delay.
Step-by-Step Instructions
The first step is to get access the the "heads" that interact with the magnetic tape. To do this you must deconstruct the commercial cassette decks. I used two different brand cassette players. The one that I used to record was a Pioneer CT-W603RS, which is a large cassette deck designed to be part of a sounds system, and the other was Tomashi cassette recorder which was a small handheld recorder. Getting access to the Pioneer involved taking off the casing, and unscrewing the mounting plate that held the mechanism where the cassette tape is played from the rest of the casing. The mechanism to spin the cassette tape and move the head into position was fairly inaccessible and complicated, so I left the whole system wired together using the commercial controls on the cassette deck to control it. This was a little clunky, however when I tried to control them myself it required coordinating more parts than was practical, so this was the easier option. The Tomashi was much simpler, and involved only taking off the backing plate to get access to the circuit board.
In order to get the delay effect you need the two cassette players to play in series reading and recording on the same tape. To do this take two cassette tapes, remove the magnetic tape from them, you can discard one of the taped but keep the other (be careful to keep the spools of tape spooled and not let them get tangled) and use a hack saw to cut off lower right hand corner of one and the lower left hand corner of the other. Be sure to cut off only the bare minimum needed to thread the tape between them. This was little more than cutting off the screw hole in the corner. Then take the tape spool, put one side on the left most reel of the left cassette tape, and the other side on the right most reel of the right cassette tape, such that the tape unwinds from the left reel, passes over the space where the tape is read, then is routed to the other cassette tape, across the space for it to be read and is wound up on other cassette tape. Then build some way to hold the two tape players next to each other and have the tape run in a straight line. I found this easiest with some cardboard and hot glue. I cut a hole out of the middle of a piece of cardboard to hold each of the cassette players at the right hight so that the tape ran in a straight line. On the Tomashi tape deck, in order to allow the tape to run in from the side, I had to cut a small hole in the left side of the casing for the tape to come in through. One important note is that the some cassette tapes are write protected, which is a feature designed to prevent you from witing over what is on the tape. They have holes at the top on the top of the casing, that are there so the tape does not trigger a mechanical sensor in the cassette player that is necessary for the player to let you record onto the tape. The very simple, yet important solution is place a piece of masking tape over these holes.
The next step is to get access to the input and output signals from the two tape decks. The pioneer tape deck has RCA inputs for recording onto tapes, so I used a RCA to BNC adapter to send my input from a BNC cable to the RCA input. This was convenient as it was straightforward to get the microphone output to the BNC, and it let me test the system with my function generator to make sure that the tape deck was recording what I wanted to on the tape. The Tomashi was a little harder. It has a built in speaker, so It was straightforward to have it simply playback what it was reading from the tape, however in order to get the echo effect I need to send the output from the Tomashi back into to the input of the pioneer, so we need access to that signal. The circuit board in the player amplifies the signal before sending it to the speaker, and we need the unamplified signal, so instead of taking the input from right before the speaker I unsoldered the wires that lead from the playback head to the circuit board, and rerouted the signal to my prototyping board. Shown in the image, the two connection points circled are where the wires were attached, for reasons I will explain later I have other wires soldered in place there, but I drew an arrow to the wires that were originally attached there.
The next step was to wire up the electret microphone to record my input. I wired up the following circuit on my prototyping board. The value of the resistor does not matter much, but I chose a 10 kohm resistor. I found that a higher capacitance led to a clearer signal from the mic, so I used a 1 microfarad capacitor. I set the bias voltage to a 5V DC supply. The grounded output I left at ground, and the output from the capacitor is the one I used.
The signal that is sent to the Pioneer cassette deck to be recorded onto the tape is the sum of the microphone signal and the output signal, so I built a summing amplifier to add the two together. The resistance on each signal going into the amplifier will control its relative strength in the output, the lower the resistance, the stronger the signal. I wanted to be able to control the strength of the echo, so I used a 1kohm potentiometer to control the strength of the signal. I used a 200 ohm resistor for the microphone input and a 26kohm feedback resistor. These values were what I settled on after some experimenting to see what I liked best, and are up to personal preference.
Certain tape decks do better with higher or lower frequencies, I found the ones I worked with were significantly stronger in the lower frequencies, and thus the echo quickly lost the higher frequencies. Additionally, I had some lower frequency buzzing, that I wanted to get rid of, so I added a low pass filter after I summed the two signals, the values shown are again chosen experimentally to find a filter level that sounds good. Varying the resistor between the positive terminal of the op amp and ground will change the filter level.
The final step is to send the filtered and summed audio signal to the speakers. The signal needs to be amplified before it can be sent to speakers, however the Tomashi cassette player has an amplifying circuit built in, so to make things easier on myself, I soldered two wires onto the attachment points on the circuit board that I removed the wires from the playback head from earlier. Then I sent the output from my high pass filter to the positive terminal, and grounded the negative one, this amplifies the signal and sends it to the speakers built into the cassette player.
