A Tour of a Hoppe’s Brain Adcom GFA-585 Restoration

hoppesbrain

8 years ago

Hi everyone,

This is perhaps the nicest amplifier I’ve ever restored! A GFA-585; The first of its kind with newly designed replacement input boards. (To fix the infamous leaking capacitor issue.) And I can’t believe I’m saying this, but…

It’s for sale. $1800. Update: SOLD to previous customer, Bill.

Also, I wanted to share what I know about restoring a GFA-585. I hope this is useful to others who are tweaking their GFA-585’s, or installing a set of my replacement circuit boards.

I needed to test my new Adcom GFA-585 replacement circuit boards, by actually installing them into an amplifier. I happened to have a super nice GFA-585 sitting on the shelf, waiting for this moment to arrive.

So I installed these boards…

Into this amplifier.

So the first step is to tear everything down to the bare chassis. I don’t have a picture of this, but you know what sheet metal looks like.

The boring part first; I restore the soft-start module.

The 4.7R 20W ceramic soft-start resistor often burns out, so I replace it with a 50W aluminum-cased version.
R501 runs hot, so gets an upgrade to 1/2W.
New electrolytic caps all around. C502 is under some stress, so a high-ripple cap is used. (Part numbers in this spreadsheet.)

The power supply is rebuilt.

A thick copper bus-bar is fabricated.
Hoppe’s Brain “Cap Topper” boards are installed with drain resistors and bypass caps attached.
A 0.1uF polypropylene X2 type capacitor is installed across the AC input to the bridge rectifier.
An auxiliary heatsink is attached to the other side of the bridge rectifiers.

The back panels are restored next.

Check out these custom binding post adapter plates! I got fed up with using washers and epoxy to mount binding posts. So I designed these boards to mount new-style binding posts—into the OEM holes—which are too big and the wrong shape. The plates hold the binding posts in perfect alignment, on traditional 3/4″ centers.

The adapter plates inside the amp have SMD component pads for the Zobel resistor and capacitor, as well as a solder pad to attach a feedback wire going back to the input board. This is a much more tidy arrangement than the stock zobel mounted on the input board.

None of this IEC jack rubbish! Neutrik Powercon is far superior IMO.

Next, the input boards are attached to the back panel.

We’re going to assemble as much as we can before installing “the head” into the amp chassis. Things fit pretty tight in the GFA-585, so I try to get everything right the first time.

Refurbishing the output modules:

The output stage is a triple-darlington arrangement. The original transistors that make up the first and second stages don’t have that much breakdown voltage headroom, so they are replaced with 250V transistors, MJE15032/33, and NJW3281/1302.
All components are checked. Sometimes the ceramic emitter resistors get broken from being bent against the board.
Panasonic ED for the bypass caps. These are super high-endurance types.
A polypropylene bypass cap is soldered across the local supply bypass caps.

Putting it together:

The “head” is installed in the amp chassis, and then the output modules are attached to it. I didn’t take enough photos of this stage. Don’t install your power transformer until the last step!

Performance!

Power at <0.1% THD+N
8 Ohm: 279 WPC
4 Ohm: 361 WPC

Awesome low distortion, which is hardly affected at 4 ohms.

Check out this beautiful square-wave response. This is 40V P-P into 8 ohms. Hardly any overshoot, no ringing. The smooth curved shape of the falling and rising response, (in the zoomed in, 2uS/div view) shows nothing but the expected band-limitation of the frequency compensation designed into the amplifier.