The SUPER high current Adcom GFA-535 II

This was a fun project!

Adcom had a sensational success with the GFA-535 in the 80s and 90s. A great bargain! For 300 bucks, you got a true dual-mono amplifier designed by Nelson Pass. (Separate power supplies for both channels.) Conservatively rated at 60WPC, with a high-current power supply, it was a lot of amp for the money. Lots of people read this glowing review at Stereophile magazine and bought one.

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Building off this success, they came out with a 4-channel version of the 535, the GFA-2535. It had a double-size power supply to supply four channels. No longer truly dual-mono, but still a beefy supply with two huge transformers.

So naturally, I thought… Why not use this big power supply to make a SUPER high-current 2-channel GFA-535? Something that won’t even flinch under load. So, I gutted the amp! The power supply was re-built with new bridge diodes and 15,000uF Panasonic filter capacitors.

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There are normally two pairs of output transistors in the 535, but the boards actually have accommodations for three pairs. (The more powerful GFA-545 uses the same boards.) I installed three pairs of On Semi MJL21193/94, and drivers MJE15032/33.

New bank of outputs, On Semiconductor MJL21194/93
New bank of outputs, On Semiconductor MJL21194/93

Of course, I did all my usual modifications. Nichicon, WIMA and Panasonic capacitor upgrades, matched transistors, new bias pots, Dale emitter resistors, better wire, etc.

This thing is all power supply.
This thing is all power supply.

The result? 95WPC just before clipping into 8 ohms, and 145WPC into 4 ohms! It cranked the hell out my studio monitors! Great sound. So now I am using this as the mid-range amplifier in my 4-way active system. Overkill? Certainly.

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