Re: volume control adapter?

[Rubywand <rubywand@swbell.net>]

Simon Biber writes ...
> 
> It would still be ok to put my circuit between the two lines to the speaker,
> forgetting about what I said with ground.
>

     Yes. That's a good way of putting it. What you do is deal just with the
two lines going to the speaker.

 
> You may be right about the size of pot to use, but the site I referenced
> used 470 ohms (on a pc speaker which I'd say is similar).
> 
 ....

     The circuit used on the site is okay. And, as you say, there is probably
no significant dissference between the usual PC speaker and a speaker used in
Apple II's. Here is the circuit:

[A]
__________________________
_____________             |
             |            | 
             | Rp     Rs  |
             |\/\/\/\/\/\/   470 Ohms max
             |     ^      
             |     |      
             |     |________   /
             |______________[ < )  Speaker
                               \



     My chief criticism of the 'voltage divider' style circuit used is that,
with a 470 Ohm pot and an 8 Ohm speaker, it really works no better than a
circuit which is a lot less bother to wire:


[B]
____________________________________   /
_____________  <snipped>   _________[ < )  Speaker
             |            |            \
             |        Rs  |
             |\/\/\/\/\/\/
             |     ^      
             |_____|      

             470 Ohms max


In both cases, the reduction in volume is going to come as a result of
inserting a large series resistance (Rs).

     There is no real 'divider' action in Circuit A because the resistance in
parallel with the speaker (Rp) is too large to matter for most of the control
range. By the time the parallel resistance is small enough to split off a
noticeable portion of the signal voltage, the signal voltage is so close to
zero due to the high series resistance (Rs) that it makes no noticeable
difference.


     For the speakers you are likely to encounter in an Apple II computer, you
would want to use something like a 25 Ohm pot in Circuit A.

__________________________
_____________             |
             |            | 
             | Rp     Rs  |
             |\/\/\/\/\/\/   25 Ohms max (3-5 watts, wirewound)
             |     ^      
             |     |      
             |     |________   /
             |______________[ < )  Speaker
                               \


This pays for the bother of the extra wiring by delivering a moderately smooth
control of output volume over most of the pot's range.



Rubywand