[sdiy] Small amplifier circuit using AC power supply

Jon Darby jdarby at lplizard.com
Wed Jun 19 23:34:21 CEST 2002

   I am searching all around for information on how to design a small
transistor amplifier using a 12 volt AC power supply so I can repay my
friend for a favor by repairing her early 70s Welcome Back Kotter record
player. Finding information about DC transistor amplifiers has been
overwhelmingly easy, every one of my electronics books describes using a DC
power supply to amplify an AC signal, which would be fine, except I would
like to utilize the 12v AC power supply (A shared 120v/12v transformer that
drives the record player's motor and the 12v side drives the amplifer) and
potentiometer with attached on/off switch for simplicity's sake and not
having an additional 9-volt battery and switch. One side of the AC power on
the circuit board follows a similar routing to what you would expect a
ground trace would look like, worming its way all the way around the circuit
board with many components connected to it. I just can't understand the
circuit any further without some help. I thought replacing the 25-year old
caps and resistors would save me a hill of work since when I got it it
seemed to work for a few seconds until it warmed up and then faded away
scratchily, but the problem will not go away and I am not confident enough
with transistors to replace the ones in the circuit with my Radio Shack
purchased transistors (not certain of capatibilty even after I figure out if
the originals are NPN or PNP). So, if anyone can suggest maybe an online
resource I can brush up on using AC power supplies with transistor amplifier
circuits I would appreciate it! I would like to apply it later to adding an
on-board amplifier to an AC theremin circuit. Thank you very much!

Jon Darby

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