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I have an Adcom GFA-555 MKII.

When I had the Axiom M22ti speakers, I used a Harman Kardon 3480. The amp never got warm enough to heat the plastic.

Now I have the Thiel CS3.6 speakers with that new adcom, and my dad has the Axioms as a birthday present. He loves them of course.

They're about 83 dB/1 watt/1 meter, and my multimeter measures them to be 3-3.5 ohms.

When I have music on at a louder volume and there's a loud bass riff, if the room is hot(goes up to 85-90F in the summer, is 80 now with all the computer stuff here), the heatsinks get burning to the touch.

It's not in a rack or a case, it's in the middle of the room. I read in the manual Adcom sold an active cooling piece for it, but this was almost 20 years ago. So they may not have them anymore.

Should I worry about this, or is this normal? The Thiels made the HK shut down at higher volumes, the timing was funny. I have a few songs with long bass transients that were made artificially that swoop down in frequency quickly and go up in amplitude, before it hit 35-40 hz the HK would blink off if I had it on too loud which was the reason to get this Adcom. When that swoop comes here the amp gets noticeably hotter, and stays like that until I turn it off for a while.

While playing music, it also stays on for about 20 seconds after I unplug it.. at the same volume. Is this normal for a large amp or does this mean the capacitors are in need of replacing?

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They're about 83 dB/1 watt/1 meter, and my multimeter measures them to be 3-3.5 ohms.

Holy mackerel.

So much for using the T-amp, huh?

If the amp is getting so hot that it will burn you if you touch it then that certainly is not good. The Thiel CS3.6 do seem to be a pretty tough load for any amp but the 555 should be ok, it was a heck of an amp in its day. Maybe the Adcom is getting a little long in the tooth and you should start to think about a replacement. You could also think about something like this to help keep it cool.

I can take the decimal point off and have the same thing just without the parasound name. Maybe I will make my own.

I bought this amp in august, it was used. It ran as hot then as it does now, it depends on the room temp and what is playing. Music with heavy bass like some Iced Earth CDs really burn up the amp, but when the room is cooler and shostakovich is playing, it is much cooler.

Then maybe all you need to do is use the set up in a refrigerated room

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They're about 83 dB/1 watt/1 meter, and my multimeter measures them to be 3-3.5 ohms.

A quick Google suggests that the Mk II gets hotter than the Mk I but I couldn't find any information on the amp circuit. DC measurement across the posts (and add a third because loudspeakers aren't a standard/passive load) confirms they are 4 ohm nominal. So no stray wires.

Looks like the Adcom just likes frying eggs when hooked up to speakers with low nominal resistance. And yes, it's hard on the transistors for them to get that hot (and other components, especially near the transformers & heat sinks)

Bren R.

T, sure it isn't as good for your amp as if it was just running warm. But with those speakers, playing that sort of program material and at those levels, apparently that's the way it's going to be. Keep in mind that from the numbers you give those speakers are drawing about five times as much power for a given sound level as, for example, the M80s, which are a relatively easy 4ohm load because of their higher sensitivity and flat impedance curve. So, it may not be good for the long term, but enjoy the moment.

When I did a search for the Thiel 3.6s it stated its sensitivity rating @ 86dB/W/m (2.8V) which would be within what would be a normal sensitivity rating. Their impendence however does drop off to a low 2.3 ohm and 3 ohm through most of their range. I would suspect this would be a difficult load for any amp.

Does the thermal overload led come on when you play the music loud ? If not I wouldn't be to worried about it but running some sort of fan ( Parasound or not ) would be a good idea imo.

I don't get 86 at 1 watt 1 meter.

Maybe I have a room null in the frequency range I used for testing when I got them. I don't trust specs, I like to measure myself.

I only have the multimeter now, the mic is long gone so I can't retest.

I borrowed my dad's axioms and tried the amp. I put my earplug headphones on so I could play them at stupidly loud levels and it never got more than slightly warm.. I guess it's a speaker thing. Then again I also measured these to do a solid 28 hz in this room before they even began rolling off in 1 dB increments, so the amp does have that burden.

I also tried the old HK again on these. It didn't have a chance to get hot.. it got warm, warmer, then shut off. What a wussy.

I have also put those earplug headphones on and tried playing far above the levels I'd ever reach without them on just for testing purposes. The overload LEDs never went on. I guarantee I did my best.

I'm sure the M80s would have been easier to power, they don't dip as low in impedance and even if they did are far more sensitive. But I can't give these speakers up now.. they don't sound amazing, they sound normal, until I listen to any other speaker side by side. It's worth the trouble.

I think I'm going to run a molex extension cord from my computer which is near the amp, and use two 120mm computer fans I have. I can tape the corners of the fan to the two back corners of the amp and it'll blow cool air through the fins. If I hook it to the fan controller, I can even control the volume. If I keep them low it won't even be noticeable over general room noise.

Thanks for all the responses.

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They're about 83 dB/1 watt/1 meter, and my multimeter measures them to be 3-3.5 ohms.

Holy mackerel.

So much for using the T-amp, huh?

I had one of those actually. Can you believe it didn't agree with the speakers?!

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I don't get 86 at 1 watt 1 meter

Just being curious. When it is stated 86dB/W/m (2.8V) does this mean it is tested @ 2.8 V ? If this is the case then wouldn't it require a doubling of watts @ 4 ohm to produce the 86 db rating and not 1 watt.

Rick, Thasp hasn't checked back in and didn't give full details of how he measured and calculated, but yes, what you're suggesting is correct. The applicable form of Ohm's Law is that power equals the square root of (Vsquared/resistance), so the standard measure is 2.83V input into 8ohms, which is equivalent to 1 watt. If 2.83V was instead input into 4ohms, the power would be 2 watts rather than 1 watt. To actually get a 4ohm number for 1 watt the input would be 2.0V rather than 2.83V(or the result would be adjusted down by 3dB to compensate for the lower impedance).

Another point is that the basic sensitivity measure is supposed to be taken under anechoic conditions, and as we discussed with Gena quite a while ago, measured in room at normal listening distances the reverberent sound field can be as strong or stronger than the direct field(so the 6dB loss per doubling of distance isn't the complete picture and less power is required than that measure would imply).

I measured the impedance myself, this is quite easy with a multimeter. I did not measure sensitivity myself, someone else did. I didn't ask him to but he was curious, since he also noticed the amp getting hot. He plays live music and has an SPL meter in his bag with a relatively nice mic compared to the radioshack crap, but I don't know how he got exactly 1 watt out of the amp.. or maybe he didn't. It happened after he noticed it was hot when he walked past the amp.

I still see a jaw drop to everyone I demo these to.

heatsinks will get much hotter than the outside of the amp, Depending on the design you can touch the heatsinks, my old NAD was like this, if you stuck your finger up the bottom vent and touched the heatsink directly after cranking it you would be greeted by a little too much heat.

The sound that keeps playing when you unplug the amp means that first of all you have got a decent amount of filter capacitance, and that you also dont have a speaker protection relay that will usually come on when you do something like unplug the amp. This is perfectly normal for amps without a relay.

Also keep in mind that you cannot measure impedance with a multimeter by just sticking the probes on the speaker terminals. Speaker Impedance is the average of the different resistance at different frequencies.

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if you stuck your finger up the bottom vent and touched the heatsink directly after cranking it

Brandon, you're making this way too easy. . .

Thanks John.

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I can tape the corners of the fan to the two back corners of the amp and it'll blow cool air through the fins.

Someone with more smarts than me can chime in, but I thought that you were supposed to be drawing the hot air out. Not trying to blow air in.

PC cooling designs always have the fan pointing down to bring cool air down to the heatsink. VGA coolers, active chipset coolers, hard drive coolers(if your PC has these, doubt it does unless you put them in yourself).

And amps and rackmount equipment take hot air out with filter elements on the intakes.

Slit throat, slit spine : same-same.

Bren R.

>>Someone with more smarts than me can chime in, but I thought that you were supposed to be drawing the hot air out. Not trying to blow air in

This isn't a cast-in-stone rule... it's just that you normally want the fan to be assisting natural convection, not fighting it. As others have said, though, sometimes there are good arguments for doing something different.