Any audio electronics hobbyist will be interested in a quality cassette copier to enrich his music collection. Here is a quality recording circuitry which has been tested and used for over six months and found working efficiently.
The whole circuit is in fact an integration of some well-known circuits. A cassette copier has four main sections. First, a head preamplifier to pick-up the signals from the playback head and, second, a high frequency booster to boost the amplitude of high frequency signals during recording. The high frequency response is deliberately boosted in order to give a high signal-to-noise ratio on playback and to increase the clarity of the recorded signal. In this circuit this requirement can be met by keeping bass flat (no bass boosting; potentiometer at mid position ‘0’) and increasing the high frequency by keeping the pot above ‘O’, the mid position.
Listening to the depth of the tone of the playback signal and one’s preference, the overall tone of the recording signal can also be controlled using active tone control circuit besides high frequency emphasis. One must note that a very good quality in recording can be achieved by proper setting of the tone control section.
The third section is a recording amplifier. I’he gain of the amplifier is around 85, which is more than sufficient. I’he gain is determined by the ratio (R7 + R9)/R9.
VR3 is a dual potentiometer which controls overall recording signal to be supplied to the recording amplifier in stereo applications. Using two separate potentiometers for VR4, individual channel signal strength can be control led independently. The recording signal can also be tapped out from the volume control of the power amplifier section of any tape recorder set.
The final section is of a bias oscillator. Bias signal is applied to the recording head from a high frequency oscillator. The bias places the recording signal into the linear portion of the tape’s magnetisation curve which brings about improved linearity, lowered distortion etc. An AC bias (sinusoidal) is preferred over a steady DC bias.
The frequency of the bias signal must be above audibility, i.e. in the range of 75 kHz 100 kHz. The bias frequency can be set by adjusting the core depth of the oscillator coil or IFT. A detector IFT with a capacitor can be used as an oscillator coil. The oscillations must be ensured and set properly using a CRO. It is highly impossible to record the signals without proper bias voltage. (Ref. Fig. 3.) The bias must be above 18 volts.
Finally, the tape recorder must have a provision for erasing previous recordings. Here also AC erasing is preferred over DC erasing. The output of a bias oscillator can be used to feed the erase head
Final recording quality, sharpness, channel separation etc depend upon one’s skill in adjusting Master and Slave heads and amplitude of the recording signal. More and more signal amplitude leads to saturation of the magnetic particles on the tape which increases the distortion during playback. A head similar to the playback head does the function of recording in the recording set.
Some important tips
1. Good and smooth running mechanism solves most of the problems concerning the project. Mechanisms should be properly grounded.
2. Both mechanisms must be operated in play mode and the heads should be aligned properly for sharp pick-up using any standard pre-recorded cassette.
3- The bias oscillator works with detector IFT (Philips 30044, 30043 brown) or Nelco detector IPT (black) and delivers a peak-to-peak output voltage nearly twice the input.
4. All signal connections must be done with shielded wires.
5. Frequency booster and preamplifier circuits must be less noisy. Any tone control circuit built around transistor may also be used.
6. If the buffer stage is not used, the playback signal to the power amplifier will be reduced significantly.
Construct a provision such that the buffer stage gets connected to the preamplifier only when the record button is pressed.
For a stereo construction, the above PCB must again be duplicated except bias oscillator.
The bias oscillator can be checked as follows without a CRO. Connect the output of bias oscillator to an AC erase head and play a pre-recorded cassette in the recording deck.
The recording will be erased, only if the oscillator is giving the oscillations and working well. I n response to several letters received from EFY readers regarding my construction project published in Jan.*92 issue, I’m pleased to give below some clarifications:
1. The cassette copier is mean, to he a stereo copier, though only one channel was shown for simplicity. An identical channel has to be made for stereo.
The bi as oscillator is however common to both the channels. This oscillator delivers a frequency of about 80 kHz when the IFT core is at the top position and about 65 kHz when the core is at the bottom. The oscillator output is directly fed to the recording head through the capacitor.
2. The value of C13 should have been 4.7 kpF and of C14 3.3 kpF. Transistors such as BC549C or BC149 can also be used in place of TL
3. IC 741 is a bit noisy only if used to amplify low-level signals of less than 100 mV. Usually the output signal amplitude from a preamplifier is 350 mV to 600 mV. This recording circuit does not give any unwanted noise other than preamp noise. One can use low noise operational amplifiers if necessary.
Please note, our intention is to have a recording circuit with independent controls for both the channels. Therefore an automatic controller has not been discussed.
Potentiometer VR3 is a dual control and common to both the amplifiers for simultaneous control on the balanced channel. Each channel can be balanced by using two individual potentiometers (VR4) for the two channels, if required, One can mark the ‘normal’ position of level controls by recording the signals from a standard pre-recorded cassette of a reputed make. Some trial and error may be required in doing so.
5. No special noise limiters are required. Hissing noise will increase if the ‘treble’ is boosted highly,
£ A level VU meter can be added to the output point A of the recording amplifier as follows.
7. The circuit should be kept away from the transformer. A separate transformer may be required for the motor supply. Try to isolate the power supplies of motor and recording circuits. Keep the circuits at least 3 cms above the cabinet’s base. Ground the bodies of all the potentiometers.
To get better bass effect, connect a lkpF or 4.7kpF capacitor in place of 220pF.