The Adcom GFP-750 preamp was designed as a companion piece to the GFA-5802, with matching aesthetics. It’s a MOSFET based, single-ended Class-A preamp designed by Nelson Pass. Conceptually, it’s similar to the Pass Aleph P and the ‘Bride of Son of Zen’ DIY project.

It’s a beautiful circuit, both conceptually and visually. Look at that minimalist symmetry! Just a differential pair with current sources and cascodes. No global negative feedback. One half of the circuit is the hot balanced signal, and the other the cold.

Being single-ended class-A, it requires coupling capacitors on the inputs and outputs to remove the DC offset. These capacitors are an obvious target for upgrades. The originals are generic electrolytics bypassed with polyester films.

There are four amplifier circuits on board, two for each channel in opposite phases comprising a balanced signal.

Each amplifier section has only one gain stage, there is no output buffer, and the output of the preamp is taken straight from the wiper of the volume pot, which is a 4-gang unit to accommodate the balanced signal. The output of the amp circuit has has to drive a this 4-ganged, 2KOhm volume pot directly. This is an unusually low impedance to drive for a preamp stage, so the output coupling capacitor has to be large to maintain bass performance; 100uF. This means electrolytics are unavoidable. A 100uF film cap would be the size of a Red Bull can! On the output, the stock arrangement is a generic 100uF electrolytic paralleled with 2.2uF polyester film cap. This was upgraded to Nichicon FG, and paralleled with a Panasonic 4.7uF polypropylene film cap. There’s not a whole lot of physical space to install capacitor upgrades in this unit, so the Panasonic was chosen for its compactness. It’s the largest value polypropylene cap I could find that would fit reasonably well.

On the input, there is a 10uF electrolytic paralleled with 1uF polyester. The electrolytic capacitor can be eliminated entirely and replaced with a single 10uF polyester. Unfortunately a polypropylene will not fit. It would be about the size of a C battery, and would be more prone to pick up interference. A polyester 10uF is only about 30x10x20 and will fit snugly between the components on the board. Polyester is not considered the absolute best of input capacitor types, but they are compact, and will out-perform any electrolytic cap.

Incidentally, it says something about the strength of the design, that this preamp sounds great, and gets rave reviews, even with generic caps inside!

Here’s one amp section completed on the left, next to stock sections on the right.

And here it is all done.

Besides the coupling capacitor upgrades as described above, there are Kemet polypropylenes for the power supply bypasses (green boxes), and 2x470uF Nichicon FG for the balance circuit, which are also bypassed with 1uF Kemet polypropylenes mounted beneath the board.

There’s a spot on the board where they left out a 3mh common-mode choke that’s supposed to go on the AC power input. It’s in the schematic, but jumpered out in the unit itself. Since this is a class-A unit that draws constant current, I don’t see how it could possibly hurt, and it may improve the unit’s RF rejection, and lower noise. I found a really nice one to use from Schurter. Drops right in, though it fits a little crooked due to the way the fuse holder crowds it.

Power supply caps were also changed, just so they are new again. I used Kemet ELG and Panasonic FC.

The sound! Amazing. Really quite distinct from my GFP-565. In the bass, it’s got that warm, wooly MOSFET sound, but not so much that it loses accuracy. In the midrange, it’s really smooth and laid-back, and makes the 565 sound just a bit nasal in comparison. Highs are super clean and clear, and not the slightest bit harsh. Imaging is all over the place. Just stunning sound! It’s got me thinking of getting one for myself. These are my own biased impressions using my own system, so your mileage may vary.

Parts:

Common-mode AC choke – Schurter DFKH-22-0003
Power supply filter caps – Kemet ELG 1000uF/100V ELG108M100AR2AA
Output coupling  – Nichicon FG 100uF UFG2A101MHM
Output coupling bypass – Panasonic polypropylene 4.7uF ECW-FD2W475J
Input coupling – Panasonic polyester 10uF ECQ-E1106KF
Power supply bypass – Kemet F461 1uF F461BS105K250A
Balance caps – Nichicon FG 470uF UFG1C471MPM1TD
Balance Bypasss – Kemet F461 1uF F461BS105K250A

Thanks for looking!
Chris

UPDATE: 08-01-2017, and 02-27-2021

Replace that balance control with a single resistor, or install plug-in gain modules!

I had a customer whose GFP-750 had a physically broken balance control potentiometer. Finding a replacement is difficult. It’s a dual 1K linear pot, and not a normal balance control, which give zero ohms at the center position.

But who needs a balance control anyway?

One of the many unusual design aspects of the GFA-750, is that the balance control actually controls the gain of the amplifier circuit itself. It’s not a voltage divider. Turning the control actually increases the gain in one channel while decreasing it in the other.

So, instead of a balance control, gain can be set with a resistor. I use gold-plated DIP sockets to build plug-in gain modules. There are several advantages to this arrangement.

• Gain can be customized from 3db to 11db to match your amplifier gain and sources output levels.
• Channel balance between left and right is much more precise, due to the matched resistors. The stock Alps Blue RK27 is +/- 20%! Any remaining channel imbalance is mostly due to the volume control.
• The signal path is shortened. The signal traces to and from the balance control are long, and they travel through this twisted-pair of wires printed into the PCB. It’s a repeating X-pattern made with vias and traces. It’s kinda cool, but I don’t love it.
  
• Four electrolytic capacitors are eliminated from the signal path.
• The signal is run through nice Dale precision resistors, instead of a potentiometer.
• The sound of the preamp changes subtly with different gain settings. You can experiment to find the best balance for your system between gain and sound character.

What values of resistor to use? You’ll have to experiment. The stock arrangement is a 332 ohm resistor in series with the balance control, which is 1K, so it’s 500 ohms set to the middle balance control position. That’s around 800 ohms total, which gives stock gain of around 9db. This varies from unit to unit! Since there is no negative feedback, the gain of the preamp is entirely dependent on the voltage gain of the MOSFETs. They’re matched at the factory, so balance between channels is pretty good. If you notice more than 1db imbalance between channels, you can trim these resistor values.

If you want to make pluggable modules like I did, I used Molex Nanofit board headers, plugs and pins.
Plug housing – 1053071204
Board header – 1053091304
Pins – 1053001200