The GFA-565 boards keep getting better

Hi everyone,

I’ve just built a batch of eight GFA-565 boards, and they turned out amazing! I wanted to share the improvements I’ve made to the boards, and my build process.

Hoppe's Brain GFA-565 Input Board
Nicest one yet.

Perhaps the biggest improvement to my build quality, is the switch to a different type of solder! Like many people, I have an affinity for good-old Kester 44. It just works awesome and is a joy to use. One nice thing about Kester 44 is that you don’t actually have to clean it off. It’s considered a “No-Clean” solder, meaning you can leave it on there, and it will harden to an amber-like substance that resists moisture. (Technically, Kester considers 44 to be an “activated rosin” type solder, meaning it is corrosive and should be cleaned off. However for most practical purposes, it will be fine for decades.)

Well, I use an ultrasonic cleaner, so I don’t actually want hardened flux all over the board. It takes forever to come off in the ultrasonic bath, and it still leaves behind traces that I have to scrub off.

Kester 44 is great stuff, but it’s just not what I should be using if I am going to use an ultrasonic cleaner.

So I wondered if there is type of solder with a flux that stays soft and washes off easily.

Yes of course there is! The flux in Kester 331 is an “Activated Rosin Flux” type solder, which is actually quite a bit more aggressive than the flux in Kester 44. Kester 331 contains 1.2% Chlorides and Bromides, versus 0.44% in Kester 44. (Do not breathe the fumes, and absolutely use a fume extractor.) It should be cleaned off within 48 hours or it will start to dull the finish on solder joints. The extra chloride and bromide makes the solder flow even better than 44!

The cool part is, it does not harden, and is water soluble. It just wipes right off. Amazing! I thought these things were meant to be a struggle! Kester recommends ultrasonic cleaning in de-ionized water at 55C. I use ethanol instead, in case any liquid gets trapped anywhere. Ten minutes in the ultrasonic, blow off with compressed air, and the boards are gleaming.

Absolutely no residue.

A way better result, and way easier for me!

Also in this photo, you can see how I am thermally bonding the transistors that make up the input stage. These transistors are carefully matched, and should be kept at the same temperature, so that their change in beta versus temperature tracks to some degree. I stick them together with thermal epoxy, wrap with copper foil tape, and shrink-wrap. The balance of the input section thus drifts around less, and gives the DC servo less to do.

Also, I am now fitting heat-sinks to four of the TO-92 transistors that make up the current source circuit.

These transistors run at about 65C, sitting out in the open at room temperature, and hotter inside a running amp. That’s not super alarming or anything. I haven’t heard of or seen any failures due to this heat, but things last longer when they’re not hot, so I’m including the heatsinks now. They are made by Aavid, part # 575200B00000G, available at Digikey, Mouser and others. Attach with thermal epoxy heatsink compound, or good-old JB-Weld. (The slow-cure, original formula handles high continuous heat.)

And I discovered they still make the small white 3-pin and 2-pin JST board headers! Now you can keep your original cables instead of building new wiring harnesses. Apparently these connectors are popular in the RC model world, for connecting battery packs, servo motors and the like. They have a reputation for high reliability, and are so common that the term “JST” is often used generically to refer to any snap-in board or cable header.

An original Adcom cable fits into a new JST connector, which is still made 25 years later.

Also, I’ve made a ton of updates to the build documentation, and GFA-565 parts list.

Buy a board just like the one in these photos here.

Update March 7, 2019. I’ve sold all but one completed board. I’m building more now and should have them done next week sometime.

Thanks everyone for your support!

P.S. Here’s a little something I’m working on.


3 thoughts on “The GFA-565 boards keep getting better

  1. Those are incredible and, dare I say, beautiful!! Curious if you have seen any mods you have undertaken in the past on the new Adcom amplifiers?

    1. I haven’t actually seen the new Adcoms! I don’t know if they are using a similar circuit to the original GFA-565, but I would hope so! It’s a Walt Jung/Victor Campos collaboration AFAIK, and the circuit is the very picture of beauty in symmetry.

  2. Do I spy with my little eyes some GFA 555 mk 1 boards! SWEET!
    I have two 555’s to rebuild after I go through my GFA 535. Thanks for all of the hard work keeping our Adcoms chugging along and sounding sweet!


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