[sdiy] Small amplifier circuit using AC power supply

Jon Darby jdarby at lplizard.com
Thu Jun 20 00:46:45 CEST 2002

Yes, I completely forgot about that diode on the board possibly rectifying
the AC to DC, that was my confusion totally. I will be able to isolate the
rectifying parts from the amplifier circuit and diagnose it easily now,
thank you for jogging my memory!

Jon Darby

----- Original Message -----
From: "Takuya KATAYAMA" <cxh02175 at nifty.com>
To: <jdarby at lplizard.com>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, June 19, 2002 6:25 PM
Subject: Re: [sdiy] Small amplifier circuit using AC power supply

> Hello Jon and the Lists!
> Jon, you're misunderstanding the circuit.
> It works with DC SUPPLY, NOT AC.
> Yes, an AC to DC conversion circuit is on the board.
> Look at the board carefully, from AC 12V input to a 4-terminal device
> (called bridge diodes) is connected, isn't it?
> Or, instead of 4-terminal device, 1 or 4 diode(s).
> It converts AC to DC.
> I think the problem is a big capacitor rating 25V 470 - 2200 uF
> following the diode(s) is out of order.
> Replace the diodes and the capacitor with similer one, maybe the board
> works again.
> =======================================================
> Takuya "PATA" Katayama at Muse Music Synthesizer Lab. Japan
> jdarby at lplizard.com wrote:
> >Howdy!
> >   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
> >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
> >drives the record player's motor and the 12v side drives the amplifer)
> >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
> >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
> >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
> >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
> >with transistors to replace the ones in the circuit with my Radio Shack
> >purchased transistors (not certain of capatibilty even after I figure out
> >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
> >circuits I would appreciate it! I would like to apply it later to adding
> >on-board amplifier to an AC theremin circuit. Thank you very much!
> >
> >Jon Darby
> >
> >
> >
> >
> >

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