Adcom GFA-5802 Nudie Pic

I cleaned out my two 5802 amps a few days ago and snapped a few pics. I thought you guys might enjoy a look.

Rob

Look at all those output transistors and not a single chip type to be seen!

Yeah, that's the sight of a 450W power-amp. Impressive!
A big toroidal power transformer and separate secondary windings and independent ground connections and off course mosfet circuitry. Also a "clean" and geometric design. It's a delight for my eyes.
Tanx for sharing.

I love Adcom

awesome looking

Personally I believe that one of the reasons the GFA-5802 Amp, GFP-750 Pre, and GCD-750 CD player were so surprisingly good is that they all use discrete components. No op-amps anywhere.

Not only because of that but also the 100,000 micro-farads of power supply filter capacitance, the independent ground connections in each channel and i'd count 16 output devices per channel which eliminate any potential of a bottleneck in the power output.
An awesome example of a Nelson Pass electronic design.

Nelson did that? Hopefully without the Stasis circuitry.

I don't think the 5802 was directly a Nelson Pass design. He did the GFP-750 preamp, and I believe the GFA-5800 power amp. The 5802 was then based off of the circuits of the 5800, but I could be wrong about that.

If it doesn't run hot enough to roast chickens and doesn't suck all the current out of the house, it doesn't have Stasis. A good thing, IMO.

Well the 5802 definitely runs pretty warm. Part of that is the MOSFETs, which I understand to run hotter than bi-polars, and part of it is the fact that the amp is biased significantly into Class A. I am not sure to exactly how many watts of class. The highest number I have heard is up to the first 100 watts, but I remember also reading it being in the 20-50 watt range. At idle it draws 540 watts of power, which lead me to believe it is definitely more than the typical 1/2 to 1 watt of most class A/B designs.

That sure is pretty! I love nudies.

Jim

Well the 5802 definitely runs pretty warm. Part of that is the MOSFETs, which I understand to run hotter than bi-polars, and part of it is the fact that the amp is biased significantly into Class A. I am not sure to exactly how many watts of class. The highest number I have heard is up to the first 100 watts, but I remember also reading it being in the 20-50 watt range. At idle it draws 540 watts of power, which lead me to believe it is definitely more than the typical 1/2 to 1 watt of most class A/B designs.

540 watts at idle is consistent with what the Stasis equipped Nakamichi PA-7 draws. The PA-7 draws over 600 watts on start up and the settles down into the high 400 watt range, both figures for idle. This is spectacularly inefficient for what is only a 200 wpc amp. (In contrast, the 240 wpc Yamaha PC2002M draws around 70 watts at idle but the bias is exceptionally low.) There has to be some benefit to accepting high power consumption and heat in the 5802. Elimination of negative feedback might be one but at least in the PA-7, the sonic results do not vindicate the design.

Nelson did that? Hopefully without the Stasis circuitry.

Here's a quote from Nelson himself in another forum about the Adcom 5xxx series:

"I designed the 555. I believe the Mk II added a couple
of what were considered improvements, and it seemed
to me that it included triple darlington outputs instead
of 2 follower. I might have the schematic around, but
all told it was a simple bipolar design with a current
sourced input diff pair followed by a common-emitter
gain device that was also current sourced. This drove
the followers.

It was typical of all the 5XXX amps that I did the first one
and then they improved it until it had too many parts,
and then I would design the next in the series "

What can i say about Stasis topology...?! That Threshold amps with that technology were (are) mind blowing amps...?! With single or dual channel, non-phase inverting audio power amplifier employing circuitry to suppress voltage and current variation and enable performance to be dominated by a system maintained in a stasis condition of constant voltage/constant current, linear state operation. No overall negative feedback is employed around the amplifier system. No output protection circuits are utilized. Active constant current sourcing is employed at every gain stage.Power is sourced through a one kilowatt transformer and greater than a 70k microfarads capicitance..... That's the basics of the Stasis topolgy.

Here's an interesting article from 6 moons about Nelson Pass' Distortion & Negative Feedback:
http://www.6moons.com/industryfeatures/dis...distortion.html

I have heard the Stasis equipped PA-7 and to my ears, it is dull and ponderous. Definitely not worth the extra electricity and the inexcusable heat.

Real Thresholds may be "mind blowing" but since I haven't heard one yet, I will reserve judgement. The specs on this one looks promising:

http://www.threshold-audio.com/products/stasis8.0.html

The DF is 160, which is reasonably accurate and much faster than the PA-7, which can only manage a sluggish 60 rating.

I definitely wouldn't mind having a Threshold for comparos against my traditional AB amp that has negative feedback. I can tell you the PA-7 lost in a comparo by a landslide.

This post has been edited by socal sam on May 28, 2009 01:17 pm