Bench amplifier
An old car radio with compact cassette player makes a fine bench amplifier
or PC speaker amplifier after a few modifications.
Step 1:
Identify the wires to the 'cassette loaded' switch. If you also want to use the
radio part these wires should be connected to a new switch mounted on the front panel of the radio. Otherwise short these wires.
Step 2:
Remove the cassette hardware. Now you have enough room to add whatever you
want.
Step 3:
Identify the wires to the cassette head. These wires will lead you to the head
pre-amplifier. This amplifier is frequency compensated looking like
one of the schematics below:
![[bench_amp.png]](bench_amp.png)
Expect values in the order of: R1: 100-300 ohm, R2: 5k-10k, R3: 100k-150k,
C1: 100u-200u and C2: 10n-20n.
Remove C1 and C2. This turns the pre-amplifier into a follower without
disturbing the DC path.
Warning 1: Sometimes the head is not connected to the radio ground, but
to some bias voltage. If this is the case, add the capacitor (100u)
and resistor Ra (47K) in the dotted box.
Step 4: Mount your favorite connector(s) on the front or back and
connect the head wires.
Warning 2: Some cassette head pre-amplifiers (LA3161, TA7784P) have a
maximum output voltage of around 1Vrms. If the input is driven with more
than this voltage, e.g. the output of some PC sound cards, severe distorsion
will occur. An attenuator (Rb or potmeter) will cure this problem.
Ra = Rb = 39K gave good results.
Archive containing the Xcircuit file (encapsulated postscript).
