Re: Apple speaker

[Michael Mahon <mahon@cup.hp.com>]

Rubywand wrote:
> 
> TomMage writes ...
> >
> > Hello, I tried it, bought a cable with an RCA plug at one end and two > bare wires on the other end. Tried hooking it up by splicing into the > speaker wires and also disconnecting the speaker wires at the post and > hooking up directly to the posts. Neither method seems to work, do you > have any idea why??
> 
>  ....
> 
>      Yes. The speaker is in the Collector circuit of the output
> transistor-- one end of the spkr goes to a resistor and capacitor
> connected to the output transistor Collector and the other goes to +5V
> (not ground).  To get an audio output signal, the spkr must be in place;
> or, you can substitute a 1 watt resistor-- something in the 22-39 Ohm
> range.
> 
>      The output should come from the side of the spkr (or 1 watt
> resistor) going to the resistor & capacitor connected to the
> transistor-- i.e. the side which is _not_ the +5V side. The output goes
> through a coupling capacitor to the center lead of your RCA plug. The
> Ground side of the RCA plug goes to ground.
> 
>     "Ground" is DC ground = any motherboard trace area which is
> connected to the ground side of the power supply. For example, the
> outside "shell" of the composite video output is soldered to ground.
> 
>      Here is a rough picture ...
> 
>      +5V side
>         |
>         |
>         []< Speaker or 27 Ohm resistor
>         |
>         |---------------------|(--)|------------------------ Output
>         |                   10uF  10uF                    to Amplifier
>         |       2 caps connected as bi-polar capacitor
>         |
>       Transistor                           _________________ Ground
>        side                                |
>                                         DC Ground
> 
>      You can get a bi-polar coupling capacitor at Radio Shack (2uF-5uF
> is fine) or make one by connecting two 10uF caps neg end to neg end.
> (The value is not critical, two 5uF caps connected back to back is
> fine.)

AOK, right up to here...  A non-polarized capacitor is not required
in this circuit, precisely _because_ the transistor side of the
capacitor never goes below ground.  A 4.7 uF capacitor, with the
(+) side toward the transistor collector and the (-) side toward
the amplifier will do the job nicely.

-michael

===============================================================
This is my personal opinion, and does not in any way represent
the position of my employer, etc...