Message-ID: <37593FA3.1CBD2F05@regandesigns.com>
Date: Sat, 05 Jun 1999 08:17:55 -0700
From: Brent Regan <brent@regandesigns.com>
To: lancair.list@olsusa.com
Subject: Re: 8:600 ohm Impedence Matching Tranformer
References: <19990605041609.AAA29157@truman.olsusa.com>
Mime-Version: 1.0

          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>>
          <<  Lancair Builders' Mail List  >>
          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> 
>>
Bill:

Sounds like are trying to connect the 8 ohm speaker output of your cassette deck
to the microphone auxiliary input to your intercom. The 8 ohm speaker output is
designed to fill your car with loud music using lots of watts. The microphone
input to the intercom is designed to take the output from a microphone, micro
watts,  and amplify it into something useful. Connecting the two will give the
intercom a fire hose enema and produce sounds that will make a cat in heat sound
melodious.

I assume that you have already checked and found that there are not "Line Output"
and Line Input" connections available on the two devices (there should be).

What I don't understand is how a matching transformer will help. The only reason
to match impedance is to optimize power transfer but in this case we want to
attenuate the speaker output. Furthermore the turns ratio and voltage ratio are
proportional so the proposal to connect the two using a 500:8 (1000:8 center
tapped) transformer would turn a nominal 10 volt signal at the 8 turn side into a
625 volt signal at the intercom side.  When you yell "Clear" it will look like a
scene from ER with the intercom flopping about like a cardiac patient on a
defibrillator.

What you need to do is attenuate the signal first and then couple it to the input.
I would try building a variable voltage divider using a 1000 ohm 1/4 watt resistor
in series with a 100 ohm potentiometer across the output. Connect a 600 ohm
coupling transformer (600 ohm primary and 600 ohm secondary) between the pot wiper
and one of the pot ends. The transformer will block and DC bias voltage. Connect
the output of the coupling transformer to the input of the intercom. Set the pot
so the volume knob on the FM system works the way you want it, disconnect the
wiper and measure the two sides of the pot. Replace the pot with two resistors.
Fly happy.

If any of you electron heads out there have a better idea, lets hear it. I could
be wrong about this, it happens a lot ;)

Brent

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
LML homepage:   http://www.olsusa.com/Users/Mkaye/maillist.html