DIY Amplifiers -- Adcom GFP-565 "Upgrade"
The Adcom GFP-565 Preamplifier was subject to a comprehensive upgrade in four issues of AudioXpress from November 2003 to February 2004. 

Here are some graphs of the performance characteristics before surgery:

Below:  Adcom GFP-565 THD+N% 1V output into 100K

 
Below:  FFT of 1V Output Signal

Analog Display of Noise -- CD Inputs Shorted

FFT of Noise -- CD Inputs Shorted, Volume Control at Maximum Setting

RIAA Accuracy (very good indeed!)

RIAA THD+N% -- (very good as well):

 

Comparison of a stock GFP-565 to a modified unit. The transformer, diodes and one level of filtration are moved to a small metal housing.

The mains feedthrough from 60Hz is -98dB, about 6dB better than the stock unit.  Importantly, the other harmonics (except the 4th) are very substantially reduced as is the chatter which appears at 60Hz intervals!

 

Connection of 10 ohm "Sense" resistor to power rail adjacent to Line Amplifier

 

Connection from PCB to Super Regulators
 

Connection from Super Regulators to PCB

New Line Amplifier with Texas Instruments OPA134 and National Semiconductor LME49600

 

Schematic for Adcom GFP-565 Preamplifier

 
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