Measurements of an Adcom GFA-5800

[mjarve]

It's been some time since I've posted here. But I think this is the right question for this audience. I'm just curious if anyone knows of a measurement work up for the Adcom GFA-5800- the sort of thing you would see Stereophile's J.A. doing in the tan boxes in the equipment review. I have the Adcom supplied Audio magazine review, but it unhelpfully omits any details.

Basically wondering what the measured power output into common loads was, and what is the lowest practical impedance it can play into.

Thanks much!

/Mike

[anatech]

Hi Mike,
That amp is designed to run 4 ohm loads, but be stable in case the impedance drops to 2 ohms. It is not designed to run a 2 ohm load at any volume.

Output power can be figured out by its supply voltage. That is perhaps the least important measurement because a 10 watt difference will be a fraction of a dB (= not audible).

250 WPC into 8 ohms, 400 WPC into 4 ohms at less than 0.18 % THD. AT 1 KHz the THD spec is 0.02%. 2.1 dB "dynamic headroom", pretty good. Damping factor is supposed to be 1,100 minimum. Silly high. S/N ratio is over 100 dB, not bad.

I was authorized warranty for these amps. They are well made and reliable as long as you don't get silly with them.

-Chris

[infinia]

Originally posted by mjarve

Basically wondering what the measured power output into common loads was, and what is the lowest practical impedance it can play into.

Hi
Not sure if stereophile did an in-depth write-up but should be able to glean some info for you from the Adcom specs.
see http://www.adcom.com/data/manuals/gfa5800manual.pdf

Rated continuous power into 4 ohms 400W both channels driven.
Dynamic headroom into 4 ohms = 2.1 dB ( ie ~ 650W) Distortion does not change appreciably going from 8 to 4 ohms. So s/b able to drive most 2 ohm speakers (with stability knowing Adcom) but not exceeding max transformer or heat sinking capability ( ie ~ 1800VA).
But I would think it practical to stick to 4 ohm speaker ratings, not knowing exactly the load impedance over the full freq. BW.
Why what are you thinking of loading on this amp?

[anatech]

Hi Infinia,

So s/b able to drive most 2 ohm speakers

No!

You didn't read my post, did you? At least our numbers matched.

-Chris

[infinia]

Originally posted by infinia

But I would think it practical to stick to 4 ohm speaker ratings, not knowing exactly the load impedance over the full freq. BW.

Hi Chris
Yer faster than me I think. I was calculating/typing same time you hit send.

At the era of the GFA-555 amps the Adcoms were tested with the worst load speakers (Apogee) <2 ohm.


edit> I'm sure this Adcom would be stable into 2 ohms over a reasonable phase angle. PS also given my other advice.

[mjarve]

Thanks indeed. That's just what I was looking for. I'm in the process or designing/building a set of speakers just for this amp, and needed to get some simple boundaries. I learned my lesson on another supposedly "bullet-proof" amp.

[sakis]

i will never in my life forget the first few pcs of Adcom 5800 side imported in greece .....it was fun to deal with.....

at the time adcom used to produce these amps with matched transitors that have been selected by hand by someone and you could actually see the same hand writing on each and everyone of them ....

two of these amps sealed from the box delivered to costumers 600.000 drahma at the time then few days later costumer complain about over heating even at idle ....

unit comes back in to look at and the first thing we see that there is no "hand writing " on the transistors ...also suported that with checking balast resistors that had weird voltage drop on them ....

informed adcom about this and they said that "a matched set of outputs is on the way " ...packet arrived including a full set of outs and insulators and screws but still no hand writing on them....they werent matched ....

informed adcom about this then a new set arrives fully matched and hand writed with a green marker .......

i still have a bunch of these laying arround ha ha ha ha ha

proof that even big and reliable company can make a mistake some times .... nice people otherwise .....

[anatech]

Hi Sakis,
I always had to hand match outputs for Adcom. That was normal service procedure. Even when I get matched sets of output transistors from anywhere, I always check the match. I've seen too many "matched" sets that weren't even close. At least they were honest about it at Adcom.

Hi infinia,

I'm sure this Adcom would be stable into 2 ohms over a reasonable phase angle.

That is a bad assumption, although they are stable, but by no means intended to operate at those low impedances.

Hi mjarve,
There is one universal truth about amplifiers and loads. The lower the load impedance is, the worse the performance from the amp will be. Once you start reaching into the 2 ohm areas, you start losing power in connections and cables, plus PCB traces heat up.

Another thing to consider. Hot speaker voice coils throw the specs out on the driver. You want to avoid high power operation as much as you can. Therefore, an 8 ohm higher efficiency speaker system will get more from your amplifier in terms of sound quality and dynamics. That is one misdirection that there is in the audio industry. "my speakers can handle 200 watts" sounds more impressive than "my speakers can produce 135 dB maximum SPL" If you can do that with 30 watts, you are further ahead than needing 300 watts to achieve the same thing.

Your focus for a speaker design should be on sound quality and efficiency. Make your speaker easy to drive.

-Chris

[Nelson Pass]

As the original designer of the GFA555 and GFA5800...

Too bad about the matching, which was a strict design requirement
for the Mosfets. The variation of voltage across Source resistors will
tell the tale in an existing unit.

As to the current capacity into low impedances, the 5800 is quite
good given the large output stage and what should be substantial
bias current.

I am talking about the complementary follower version, and there is
a later model version on which I don't have information.

[anatech]

Hi Nelson,

As to the current capacity into low impedances, the 5800 is quite good given the large output stage and what should be substantial bias current.

I agree with you from my own bench experience with these units. However, the long term heat buildup is more the problem area. At least there shouldn't be any driver current problems with mosfet outputs. I wonder how the diode bridges would hold up.

Matching outputs in high power amplifiers is something I've come to expect as required. How tight depends on source / emitter resistances. You can really squeeze that last bit of performance from most designs by ensuring the outputs are matched.

Guys,
Anyway, for long term use, don't go below 4 ohms. Keep in mind that the impedance will dip below the rated impedance at some frequencies, so if you are saying 2 ohm speakers are okay, what you are really saying is that 1.25 ~ 1.5 ohm loads are fine. Speaker wire and connection resistances are now beginning to help us out some.

-Chris