ADCOM GFA-585 Improvements Wanted

[DCPreamp]

Suggestions requested for modifying/improving an ADCOM GFA-585. I just purchased a non-functional ADCOM GFA-585 from eBay for $200. I also picked up the service manual and schematics for $15 delivered. It took forty-seconds to remove the cover and twenty-seconds to locate the shorted output device. The unit is very clean, solid, and looks to be a typical, high quality ADCOM design. Albeit a design for mass-production convenience with the obvious wire tangles, multi-purpose AC wiring block for international voltage compliance, and single-sided circuit bards. Otherwise a very nice project base to begin with.

My initial improvement thoughts are the following: 1) Replace the output devices with MJ15022/23s. 2) Improve the grounding topology. 3) Clean up and solder the AC primary wiring. 4) Add additional bypass capacitors to the main filter caps. 5) Clean up the wiring in the signal path for a neater appearance. 6) Add WIMA caps in the signal path where practical.

My primary goals are sound quality, dependability, low-noise, good dynamics, and the sort. I’m not trying to squeeze 2KW out of it, switch it to Class A, or drive one-ohm bridged loads. I’m also not trying to do those woo-woo mods such as eliminating feedback, coating the output devices with fairy dust, or add $5,000 power cords. I just want to do some good quality, real-world, repeatable work that is normally a little out-of-range for a mass-produced unit.

Anybody have any suggestions? Are there other obvious upgrades I’m missing? I welcome any comments, experience in having already done this, suggestions, words-of-wisdom, “gotcha’” warnings, etc.

Thanks

[jrdmedford]

People seem to talk bonkers of Class A bipolar amps... i.e.
Hiraga, JLH, Death of Zen. On the commercial front, look at the
love affairs people have with Krell 50's and Bedini 25s.

The 585 has the heatsink space and PSU components to
do up any number of nice 20W designs. The $200 you paid
easily covers the heatsinks, sheet metal and power transformer
plus the convenience of assembly.

What do you think of the ThermalTrak devices from On Semi ??

-- Jim

[anatech]

Hi DCPreamp,
Adcom did most things right with this amplifier. For output devices, I would use the MJ21195G and MJ21196G. Match them for beta!

Replace and bypass the electrolytic caps on the driver boards. Replace any ceramic caps you find with mica or film types.

When you set the amplifier up, use the proper procedure and bias levels that Adcom recommends. There is no sonic benefit to over biasing with this amp. Matching the drivers and predrivers will yield far more improvements than bias levels ever will.

The input ground is carefully designed (done right). Do not mess with it.

Jim,
The On Semi ThermalTrak devices are the wrong package to start with and the diodes will not be of any use in this design. So much for that.

-Chris

[DCPreamp]

Hi anatech,

Great feedback. I appreciate your comments.

The GFA-585 schematics look pretty well planned and of a very good dual-diff, dual class'A' driver topology.

By grounding, I questioned the metal strap connecting each channels' star ground. It doesn't look bad, but it seemed a high-current path between the 'stars' was not (fully) needed thanks to the transformer's isolated secondaries. I suppose it doesn't hurt.

I'm not familiar with the MJ21195G/96G outputs you mentioned, but I will certainly research them. I just happen to have a handful of new MJ15022/023s in my parts bin, so I thought this was a good place to use them. I fully intend on hand-matching beta and vBE. I will also check all pre-drivers and see what upgrades are possible there too.

Here's a question I thought of this weekend while snooping around the amp: With the transformer's multi-primary, universal input voltage capability, would there be advantages to setting it for 100VAC input instead of 115VAC? It would push the rail voltages up another 10% or so. I don't see any real ill effects other than bias current increase and stress on some power components, but it could add some decent headroom. I haven't checked the float-voltage on the main filter caps, but Iwill probably update them too. Any warnings against it? I can't imagine a gross transformer failure, and so long as all high-voltage and power components are up to it, it should drive even more power <<grinning>>.

I would be more inclined to use the ON Semi ThermalTrak devices on new designs; they wouldn't lend themselves to replacing TO-3 devices.

After looking, I will probably also relocate the four rail fuses from under the signal board to somewhere neat the power caps. I don't like that much current flying around under the inputs - other than Vout.

Thanks again for the feedback, and I'm preparing to make my first cut. . .

[anatech]

Hi DCPreamp,
Sorry, didn't get notification on your response.

MJ15022 and MJ15023 are fine parts. The MJ2119X types have higher SOA and better high frequency and gain linearity. You have them, use them. I used to use the same parts as you are thinking for these, I just stocked the MJ15024 and MJ15025 to reduce the skews I had to carry. These will work well.

Moving the fuse holders is a great idea. Just mount them safely. They were put there for convenience. Go for it!!

Don't increase the supply voltages. You will run the transformer and everything else hotter for basically nothing. A fraction of a dB. Not worth the shorter life. Out mains voltage is creeping up year by year anyway. I once saw a pair of nice NEC mono blocks burned up. 100V primaries used in Canada at 118 V. Thermals went open too. They were not mine.

The thermaltrack devices may be a good idea. I have some samples but will need to design the circuit to take advantage of these. The standard vBE multiplier works well. What excites me are the MJW types. Still, the TO-3 package is more robust.

-Chris