[TenTec] Fw: question Digital matching-- rig to sound card

[Jim Brown K9YC]

On Fri, 21 Dec 2007 18:51:07 -0600, Stuart Rohre wrote:

>Barry, thanks for remembering the other small Radio Shack 1:1 transformer 
>for 600/ 900 ohm isolation.

>I was thinking of the speaker output side of sound cards being around 8 
>ohms, and the other side can be a variety of values, but many rig mike 
>inputs are closer to 600 ohms today.

>For ham audio applications exact impedance matching is not imperative.  
Most 
>of the circuits can be thought of as voltage applications.

Virtually ALL audio circuits have low impedance outputs and high impedance 
inputs. They are INTENDED to be used without any form of "impedance 
matching" or loading. The concept of "impedance matching" and 600 ohms for 
audio has been obsolete for at least 40 years. In fact, the only folks 
talking about 600 ohms for audio are those who know nothing about audio -- 
the people who make video gear and those who make ham gear. :)

Virtually ANY computer sound card can drive any rig without the need for a 
transformer. What often IS required is a simple pad (also known as a 
passive attenuator or voltage divider), because the sound card puts out a 
lot more VOLTAGE than the mic input is designed to accept. That pad can be 
as simple as a series resistor and parallel load resistor, and a divider 
ratio of 10:1 is suitable for most such applications.

You can find several tutorials on this topic on my website, including one 
on "impedance matching" that I wrote around 1976, pointing out then that it 
was an obsolete concept. 

http://audiosystemsgroup.com/publish

73,

Jim Brown K9YC