[TenTec] ASTATIC D-104 ON OMNI6+

[Robert & Linda McGraw K4TAX]

As a general rule, the D-104 element likes to see 10 megohms as an input 
impedance, typical of tube circuits.  The input impedance of the Omni and 
Paragon series has a 1 meg input shunted by a 0.01 uF cap at the MIC input 
and again at the AUD input on the rear panel.  Based on the combined network 
in the first stage, the true input Z is somewhere near 50K ohms. 
Traditionally, the D-104 is loaded by the lower Z input.  With the 
introduction of solid state audio stages some years ago, Astatic produced 
the amplified G stand.  This is really more of a impedance converter to 
allow the D-104 element to see something like 10 meg while being able to 
drive lower impedance inputs.  Heavy loading of the D-104 element has been 
shown to cause excessive HF loss and a generally unpleasant or muffled 
sound.

As to transformers, unless carefully used and shielded, they typically are a 
source of humm being introduced into the audio if they are in the proximity 
of power transformers.  Good quality well shielded transformers are 
expensive.  On the other hand, several have reported good results with those 
"in-line" units sold by RS.

73
Bob, K4TAX