Restoring Vintage Audio Equipment

Phoenix HiFi

As the Altec Model 19's become assimilated into my main system, it became apparent that I needed a line preamp to 1)  switch signal sources and 2) to buffer and balance the single ended inputs to drive the DEQX HDP Express via the XLR inputs and 3) provide a very clean and detailed sound with a low noise floor.  This because my high efficiency Altecs are very good at revealing detail.

A friend here in Phoenix, who is a knowledgeable recording engineer, suggested I look into the advantages of low noise JFETs manufactured by Linear Systems.  He has a mic preamp that uses them and his description of the sound of that preamp is essentially what I'm looking for in my system: detailed sound, quiet background and lots of dynamic headroom.  An app note written by Bob Cordell on the Linear Systems site shows how the LSK489 might be integrated with TI's LM4562 low noise opamp to make an RIAA phono preamp.  For my line amp, I decided to build the front end of his design for my gain stage using a single J105 JFET for the current source,  and then add an inverter to give me a balanced output and a DC servo to provide DC offset correction.   Here is a basic schematic of a functional breadboard.