LA3161 Playback Pre Amplifier for Cassette Deck

In this Instructables I am going to make a playback amplifier circuit for a cassette player. This circuit is necessary if you want to make a DIY cassette player, this amplifier circuit amplifies the low ac voltage induced in the windings of the playback head. You can't connect the output of the playback head directly to audio power amplifier. This is due to because the induced voltage in the windings of playback head is very low (about 1 micro volt). Thus in this situation we use a playback amplifier circuit with a proper equalization characteristics. Here I am going to use LA3161 IC which is very popular in cassette decks for playback. This IC is manufactured by Sanyo Corporation, Japan. It is available in SIP8 (Single In line package 8 leads).

Step 1: LA3161 IC

To keep everything as simple as possible, I am using LA3161 IC (integrated Circuit). This IC consists of two pre amp stages for left and right channel. Features of LA3161 are listed below.

Features:

On-chip 2 preamplifiers.
Good ripple rejection owing to on-chip voltage regulator.
Minimum number of external parts required.
Low noise.
8-pin SIP package facilitating easy mounting.
Pin-compatible with LA3160

Maximum Ratings:

maximum Supply Voltage VCC = 18V
Operating Temperature = -20 to +75 Degree C

Recommended Operating Conditions at Ta = 25˚C

Supply Voltage Vcc = 9V

Load Resistance RL = 10K

Function of External Parts:

C2, C4 are input coupling capacitors. In NAB equalizer amplifier, the gain at low frequencies is high and 1/f noise inside the IC is emphasized as output noise. Therefore, if the reactance of capacitor at low frequencies is increased, the dependence of 1/f noise on the signal source resistance causes the output noise voltage to deteriorate, and the value of reactance must be made small enough as compared with the signal source resistance. C2, C4 also influence the operation start time and the adequate value of these capacitors is 10µF. (Since C2, C4 of less than 4.7µF make the operation start time longer, use C2, C4, of 4.7µF or more). C5, C11 are NF capacitors. The lower cut-off frequency depends on the value of these capacitors. If the lower cut-off frequency is taken as fL

C5 (C11) = 1/2π · fL · R2 (R7)

If the value of this capacitor is made larger, the operation start time of amplifier is more delayed. The adequate value of capacitor is 47µF. The frequency characteristic of the equalizer amplifier depends on C6 and R4, R3 (C10 and R5, R6). The time constants to obtain the standard NAB characteristic are as shown below.

Tape Speed

9.5 ips 4.75 ips

C8 is bias capacitor for the power line. C8 of 47µF is inserted at a point as close to the power supply pin (pin 4) as possible. C1, C3 are for preventing radio interference in the strong electric field, interference attributable to engine noise, and blocking oscillation at the time of large amplitude operation. The adequate value of C1, C3 is approximately 1000pF. C7, C9 are output coupling capacitors. The adequate value of C7, C9 is 10µF

NAB element and determination of gain:

Since the DC feedback is provided by R1, R2 of NAB element, which brings about DC output potential at pins 3, 6, it is impossible to change the value of R1, R2 of NAB element greatly. Therefore, when determining the gain, change RNF with R1, R2, C1 (NAB element) kept constant