Bill's Adcom GFA-555 Power Amplifiers Driver Rebuild

Two Adcom GFA-555 Power Amplifiers 

Today were showing a pair of Adcom GFA-555 Power Amplifiers that came in for driver board updating and testing.  The Adcom GFA-555 is still today one of the most popular, affordable power amplifiers on the new and used markets. Designed by the famous Nelson Pass with Adcom in the mid 1980’s which was later proceeded by the Adcom GFA-555 MkII Amplifier. The major differences in the MkI and MkII is the integrated of a DC servo circuit and a bit more beefier output/driver stage modifications of which Nelson Pass did not have a hand in.

The GFA-555 uses a pair of sturdy plastic 5-way binding posts with gold plated RCA inputs and a bridged/Stereo switch. Ive noted several complaints, as one of these unit exhibit of failing rocker switches for some reason.   The GFA uses a large Toroidal transformer and dual rectification stages with each 30,000 microfarad filter per side. 

I have always found the GFA-555 to be an incredibly well built and operating amplifier, very passive power design with no major characterization to add to the mix..exactly what we want in a power amplifier.  The only downside, which is quite major due to the fail types seen in these, is the lack of any protection. In-Line speaker fuses are highly recommended running these amplifiers. 

Driver Assembly

The  GFA uses a pair of NPN initial differential devices which were replaced with low noise Fairchild TO-92 transistors matched within 1% and thermally coupled for more stability, along with a constant current configuration for stabile Bias operation with a solid constant voltage bias eliminating thermal run-off and other drift issues. Pre-Driver BJT devices were replaced with modern low noise TO-126 Fairchild devices as well.

*thermally coupled 1% matched differentials 

The Adcom GFA-555 is well known to have faulty electrolytic used during construction. All of the electrolytics were replaced with new low impedance high temp (105C) Nichicon PW capacitor with an increase in operating voltages. The original metal 1MFD film I found to be of poor construction, this was replaced with a high grade MKP Polypropylene film capacitors along with the .1MFD mylar to high grade  .1MFD MKP Polypropylene films

BEFORE

AFTER

**Constant Current Notes GFA-555

The GFA-555 has such a stabile bias because the main output devices (Q13-Q20 NPN/PNP) in connection with 3rd stage Q11/12 emitters to help create a simulated circuit through R19/20 to Q9/10 which establishes a constant current at R21. This is achieve through matching the devices with Q11/12 creating an established Iref (Current Reference.)

Audio Notizen (Ser# 182/374)