AC Bias Tape Recorder

INTRODUCTION:

In this Instructables I am going to make an AC Bias Tape Recorder. You can use this device or project to record your voice or favourite on a Compact cassette. You can also use this device to erase a recorded compact cassettes. Sound quality of my AC Bias Tape Recorder is truly very good. So let's get started

BIAS:

Tape bias is the term for two techniques, AC bias and DC bias, that improve the fidelity of analogue tape recorders. DC bias is the addition of direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal (generally from 40 to 150 kHz) to the audio signal. Most contemporary tape recorders use AC bias.
When recording, magnetic tape has a nonlinear response as determined by its coercivity. Without bias, this response results in poor performance, especially at low signal levels. A recording signal that generates a magnetic field strength less than the tape's coercivity cannot magnetize the tape and produces little playback signal. Bias increases the signal quality of most audio recordings significantly by pushing the signal into more linear zones of the tape's magnetic transfer function.

AC BIAS:

Although the improvements with DC bias were significant, an even better recording is possible if an AC (alternating current) bias is used instead. While several people around the world rediscovered AC bias, it was the German developments that were widely used in practice and served as the model for future work.
The original patent for AC bias was filed by Wendell L. Carlson and Glenn L. Carpenter in 1921, eventually resulting in a patent in 1927.[11] The value of AC bias was somewhat masked by the primitive state of other aspects of magnetic recording, however, and Carlson and Carpenter's achievement was largely ignored. The first rediscovery seems to have been by Dean Wooldrige at Bell Telephone Laboratories, around 1937,[12] but the BTL lawyers found the original patent, and simply kept silent about their rediscovery of AC bias. Teiji Igarashi, Makoto Ishikawa, and Kenzo Nagai of Japan published a paper on AC biasing in 1938 and received a Japanese patent in 1940.[13] Marvin Camras (USA) also rediscovered high-frequency (AC) bias independently in 1941 and received a patent in 1944.[14] The reduction in distortion and noise provided by AC bias was rediscovered in 1940 by Walter Weber while working at the Reichs-Rundfunk-Gesellschaft (RRG).[15] The German pair received several related patents, including DE 743411 for "high-frequency treatment of the sound carrier". Possibly independently of Weber and Braunmühl, the UK company Boosey & Hawkes produced a steel-wire recorder under government contract during the Second World War that was equipped with AC bias. Examples still surface from time to time, many having been disposed of as government surplus stock. After the war, Boosey and Hawkes also produced a "Reporter" tape recorder in the early 1950s using magnetic tape, rather than wire, which was based on German wartime technology.

DC BIAS:

The earliest magnetic recording systems simply applied the unadulterated (baseband) input signal to a recording head, resulting in recordings with poor low-frequency response and high distortion. Within short order, the addition of a suitable direct current to the signal, a DC bias, was found to reduce distortion by operating the tape substantially within its linear-response region. The principal disadvantage of DC bias was that it left the tape with a net magnetization, which generated significant noise on replay because of the grain of the tape particles. Some early DC-bias systems used a permanent magnet that was placed near the record head. It had to be swung out of the way for replay. DC bias was replaced by AC bias but was later re-adopted by some very low-cost cassette recorders

HARDWARE LIST TO MAKE THIS PROJECT

• Cassette Mechanism (with capstan motor, pinch roller and capstan wheel)
• C-60 Compact Cassette (either blank or recorded)
• A Comact cassette player (here I am using LA4160 IC cassette player circuit)
• Bias Oscillator Circuit
• Record/Play head
• Erase Head
• 12V Lab bench power supply
• A microphone pre amplifier circuit with microphone( Electret condenser type)
• Li-Ion Battery
• 3.5mm Audio cable
• Speaker (4ohm/5W)
• Bias Trap coil
• Capacitors

1000PF - 3

• Resistors

4.7K or 4K7 - 1

27K - 1

12K

10 ohm

• Two pin screw Terminal

My cassette mechanism has 6 Buttons on the Top. The function and name of each button has been given below

1. Play/Pause
2. Stop
3. Fast Forward or FFW
4. Rewind or RW
5. Play
6. Record

Voltage for capstan motor is ranges from 6V to 12V

Bias Oscillator

An oscillator used in a magnetic recorder to generate the alternating-current signal that is added to the audio current being recorded on magnetic tape to reduce distortion.

I have already published an Instructables on LA4160 IC tape recorder and player. So if you want to know more about the LA4160 playback circuit then please visit my instructables with name LA4160 Tape Recorder. Here I am using this circuit to reproduce or playback the recorded sound. For playback of the audio the only thing that you need to do is that connect the playback head or record head to the input of LA4160 Playback Circuit.

During the Recording of audio signal I disconnect its supply wire (only positive wire). and for playback I connect its supply wire(only positive wire) to the positive terminal of my power supply.

A

Microphone Pre Amplifier:

microphone preamplifier circuit is consists of NE5532 low noise operational amplifier IC. NE5532 IC has two pre op amps on a single package. Here I am using only one of the two. I have used the NE5532 low noise op amp as an inverting amplifier configuration. In this configuration, the phase difference between the output and input signal is 180 degrees.

Parameters of Microphone Pre Amp

Gain - The gain of the microphone Amplifier circuit depends on the value of Feedback resistor and input resistor In my case I am using feedback resistor of 1M(megaohm) and input resistor of 47K(kiloohm) so gain of my amplifier is 21.27. you can calculate the gain of the gain of the amplifier using the given relation-

Av = - R2 / R1

here,

Av = closed loop gain in inverting amplifier configuration. R2 = feedback resistor and R1 = Input resistor

High pass filter

Resistor R2,capacitor C2 and C3 make a high pass filter and cut off frequency of the high pass filter is 3Hz, here cut off frequency means that this high pass filter blocks all the audio signal of the microphone which has frequency less than(<) 3Hz.

Low pass filter

Resistor R2 and capacitor C1 together form a low pass filter. Cut off frequency of low pass filter is 15KHz.

Making the Mic pre amplifier Circuit

Before soldering all the electronic components on a PCB, please test the circuit on a bread first and then you must solder all the components on the PCB. You can also take the help of pictures of my my PCB .