Thursday, April 9, 2015

Adcom GFA-555 Power Amplifier

GFA-555 Power Amplifier

I wanted to take a moment and look at a Adcom GFA-555 Power Amplifier. This unit came in from a customer who stated after playing for an hour or two at high volumes it started to emit a lot of white smoke...yep smoke.

 One thing in particular I like about the 555 is cosmetically its a knock-out. Which might give you ad idea on my spartan decor' but I really like the plain-jane clean look, the front has this nice extruded type plate that envelopes the chassis in a U shape. That makes disassembly pretty quick.  The 555 has some impressive performance specs as well, with reported stability in the 4 ohm variant; rated up to 325 Watts and 200 at 8ohms respectively, the 555 has an easy switch bridged mode as well. 

Driver Assembly

I really like seeing the use of silver-mica capacitors employed vs cheaper mylar variants you typically come across. The silver mica's really have a great track record for reliability and sonic attributes. The metal film was replaced with a better Polypropylene Panasonic ECW film capacitor. The electrolytics were replaced with audio grade Nichicon KZ Muse and Nichicon FG with voltages bumped to 100vdc. 

The differential pairs Q01/02 were replaced with matched KSC1845 transistors. The Biasing network utilizes an interesting array of associated resistors in conjecture with the driver transistors to create a constant voltage bias, removing driver abnormalities and thermal influences on the circuit. Also updated were the original diodes to Vishay standard 1N4148's.

One channels NPN bank of outputs has shorted every single emitter resistor at the TO-3's, along with a few shorted resistors in the driver assembly, these were replace with MOX flame proof KOA 2W resistors and thermal compound and new MICA insulators supplied. 

One thing to note is the filter cap PCB's seem to suffer from some high heat situations, all the 3.9K resistors were again replace with MOX flame-proof KOA 2W resistors, along with a .1uf  Polypropylene Panasonic ECW film capacitor. One impressive note though is the application of X4 15,000uf capacitors used in the rectification filtering. What I don't like is a lack of robust dc protection.  This particular model has a host of modifications published online in regards to the MKII GFA-555 as well.

Currently I've got the GFA-55 hooked up to the Adcom GTP-500 Pre-Amplifier Control, which frankly…isn't anything to write home about, in fact its about as bland as Mayonnaise…I prefer Miracle Whip.

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