[AMRadio] D-104 modifications

Paul Christensen w9ac at arrl.net
Thu Aug 26 20:35:09 EDT 2010

> He said " The preamp is to make the terrible new crystal elements sound 
> good"

Astatic's 2-stage pre-amp has several design deficiencies, the most 
problematic being the first stage where the input Z calculates to 
approximately 470K-ohm -- at least in the version I have from the late '70s. 
One well-known mod is to use the existing pre-amp but insert a 1 meg 
resistor between the crystal mic element and the pre-amp.  But the addition 
of that resistor has detrimental impact on the SNR of the circuit. 
Increasing the R value even more continues to degrade stage SNR.

It's tough to beat the simplicity and performance of the D104 when the 
element is coupled onto a tube's grid, or the gate of an n-channel JFET when 
used with more modern pre-amps.

Paul, W9AC 

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