Description

New product! (December 2022)

Improved power supply for the Adcom GFA-565.

• Reduced power supply impedance through the elimination of 20 inches of 14ga power supply wire.
  • As original, the GFA-565’s fuse-holders are mounted to the back panel. This makes it easy to change fuses, with no need to open the case, but it makes for lots of loooong power supply wires. Wires from the filter capacitor bank are routed to the fuse-holders on the back panel, and from there sent to the output modules and input board.
    The Hoppe’s Brain BFA-565 moves the fuseholders to the tops of the capacitors, which eliminates this back-and-forth trip to the back panel, and shortens the supply wires to the output modules and input board.
  • As original, the GFA-565’s bridge rectifier is mounted to a heat sink, which is in turn mounted to the floor of the amplifier for additional heat-sinking. Thermally speaking, this works very well, but as the bridge rectifier is located on the opposite side of the amp from the filter capacitors, long wires are needed to connect it to the filter capacitors.
    The Hoppe’s Brain BFA-565 moves the bridge rectifier to the top of the capacitor bank, eliminating these wires and replacing them with short PCB traces.
    The original Adcom layout:
    
    And the Hoppe’s Brain BFA-565 layout:
    
    Which results in the elimination of a fist-full of wire! On the left is all the power wires needed to connect the BFA-565, and on the right, as OEM.
    
  • Optionally, the back-panel fuseholders may be retained as original. A pair of wire pads is provided to bypass the on-board fuseholders. Wiring is then done as original.
    
  • Hole plugs and a label are provided to cover the spot where the old fuseholders lived.
    
• All connections to the board are made with WAGO cage-clamp terminals.
  
  These spring-loaded clamps grip the wire very, very firmly, and make an oxygen-tight connection that will never come loose. (Screw-terminal blocks can come loose over time as the copper squishes.)

  

  The board can be removed for troubleshooting or repair in just a few minutes.

• Wide diode selection: The bridge rectifier is made of four discrete TO-220 diodes, and is compatible with any standard 2-pin TO-220 diode, of which there are many to choose from! You may have your favorite; Silicon, Schottky, Silicon Carbide, etc. Diodes must be rated at least 30A average forward current, and at least 100V peak repetitive reverse voltage. Board is currently supplied with Vishay VS-ETH3106FP-N3 Hyperfast recovery Fred Pt diodes. This is subject to change, as this is a rapidly moving segment in semiconductor technology, with short product cycle lifetimes.

• Heatsinking is slightly improved. This is actually a tall order, as the original heatsink design is really good already. It was not a design goal to improve the heatsinking, but it needed to be at least as good, if not better. And it’s a little better! Read more about how I validated this heatsink’s performance here.
• For electrical safety, a diode clamp, made from a bridge rectifier, is added to the stock earth-loop breaker circuit. The earth loop breaker circuit reduces noise—caused by ground loops flowing from the safety earth connection and the patch cords connected to the rest of the audio system—by inserting a buffering impedance between the earthed amplifier chassis and the signal ground of the amplifier. This makes any ground loop currents small so they are amplified less. The parallel capacitor conducts RF at frequencies where the resistor becomes too inductive.
  
  The diode clamp does nothing in normal operation, as there is 0VDC across it. In case of a fault, such as a DC power rail coming in contact with the chassis, or AC power being shorted into the signal ground, the voltage across the diodes will rise above 0.6V, it will clamp, and the fuse should blow.
• Low impedance: Board is made from double-thick 2oz copper, (expensive) and all power connections are made of large, low-impedance polygons. Nearly the the entire top layer is used for the ground plane.
• Bypassing consists of a 100uF Electrolytic capacitor, in parallel with a 0.1uF polypropylene capacitor.
• For the lowest possible system noise, correct star-grounding is built into the board design. Charging pulses do not share conductors with signal ground references.
• Each rectifier diode is bypassed with a 47 ohm metal oxide resistor in series with a 47nF polypropylene capacitor, and the AC input to the bridge has a 0.1uF polypropylene capacitor in parallel. (Thus in parallel with the transformer secondary, providing an AC current return path while the rectifiers are switched off.) These snubber values were chosen as a good starting point, may not be optimal, and the user is encouraged to experiment! I have not made meaningful measurements as to the performance of these R-C values. It’s going to be different for different types of diodes. If you own a Quasimodo test jig, or have some other methodology to determine optimal R-C values for snubbers, please experiment and let me know what you find, I would be very interested!

Supplied as a kit with all parts. Power supply capacitors not included. If you want to replace the power supply capacitors, I recommend Kemet ALS70A363MF100, available at Mouser, Digikey and others.

Installation documentation here.