Adcom GFA555 4 ohm stable?

[seeclear]

In bridged mode? I was thinking of looking for one for a sonosub build, but it may need to be, depending on the driver I get. Thanks in advance.

[george daniel]

ask hoosier,, he should know.

[dudeinaroom]

www.adcom.com, look under legacy products.

[seeclear]

www.adcom.com, look under legacy products.

thanks, but...

OK, all I get there is the manual, and it says "Bridged mode power of 600 watts into an 8 ohm load..." , so since it doesn't mention a 4 ohm load in bridged mode does that mean it isn't stable? That is what I would like to know. Because it says right before that that it is capable of driving very low impedance loads. Hence my confusion.

[Face]

If it's not clear about it, I wouldn't.

[curved]

I believe the manual says not to bridge it when running at a 4ohm load.....I've heard others that did with success.

Why don't you just run a DVC sub?

[hoosier21]

the 555 is not rated for 4 ohms when bridged (it will, but will run hot and pop fuses if it is driven hard)

just build two subs and use both channels of the 555


easy

[dorokusai]

Per the manual....NO. You typically lose that 4ohm capability when you bridge an amplifier.

[ben62670]

The GFA-555 is rated at 850watts bridged 4ohms. It is capable of 2ohm loads stereo. These Adcom's were built like tanks. I used my 555 bridged to drive a Titanic III sonosub no problems. Let it breath, and you are fine;) This info is per the Adcom service manuals, and can be also be seen under the GFA-555II at http://www.adcom.com/manuals.aspx
The Mark II has a slightly better transformer, and a claimed improved input stage, but shares the same identical output stage as the original.

[dorokusai]

Aren't we talking about a GFA-555, not a MkII? WHy would the service manual state something other than the owners manual? Just asking. I've owned enough Adcom to know they CAN do things they aren't rated for but I'd never suggest something other than paperwork.

[mmadden28]

The Mark II manual said it will drive a 4 ohm nominal load in bridged mode, but you should not try with anything that will dip below 4 ohms or risk going into protection. I would imagine this would mean you should not use this amp in bridged mode with something like an LSI9, right?

[ben62670]

The original 555 manual is lacking pretty bad with specs. The 555II has the same output stage consisting of the same Toshiba 2SD424, and 2SB554 output transistors that are rated at 16 amps each. There are 8 per channel so there is 16 in bridged mode. There is a theoretical 256 amp capacity of the output transistors. Many 4ohm stable amps that use the same trannies use half the number of outputs.

[ben62670]

Do you own a MkII?

I have owned 2 of the original 555's, 2 565's, a 7500, a 5400, 2 545's, a 585, and a 5800. The 555, 585, and 565's are capable of driving 2 ohms speakers with proper ventilation. Sub duty is fairly easy considering that it is mostly just huge peaks, and not long term power drains. My 555 bridged at 4ohms tore the cone loose from the voice coil former on a Titanic III. Those are rated at 800wrms.

[seeclear]

I believe the manual says not to bridge it when running at a 4ohm load.....I've heard others that did with success.

Why don't you just run a DVC sub?

That is a good idea, and one that I am considering, in fact, two 8 ohm VC in parallel for a 4 ohm load vs running each off a separate channel of the amp.


Thanks to all for your input.

[seeclear]

Straight from the horse's mouth, response to the email I sent on the Adcom website--->

"""Hi Tom,

Yes, it is stable into 4 ohms, bridged.



Best Regards,

Stan Brown
ASI Adcom Sales Engineer


Advanced Sound & Image, LLC
8541 E. Anderson Dr.
Suite 101
Scottsdale, AZ 85255"""

So I guess that pretty much settles it.

[anonymouse]

I ran my GFA 555 II bridged yesterday on a Polk LSi9 which is 4 ohms and it worked just fine. The sound was effortless and the heatsinks barely got warm. I was not cranking it though.

[shenlonco]

The guy's at adcom said that it could drive speakers that drop way down to 2 ohms or lower....I asked about this when i had my old Infinity Kappa 9 speakers...and there noted as AMP KILLERS!

[dorokusai]

That's great to hear and nice to know something new.

[mmadden28]

The guy's at adcom said that it could drive speakers that drop way down to 2 ohms or lower....I asked about this when i had my old Infinity Kappa 9 speakers...and there noted as AMP KILLERS!

In bridged mode? The LSI9's can drop below 2

[shenlonco]

In bridged mode? The LSI9's can drop below 2

I owned the lsi9 my onkyo 595 ran them with ease if you where to run the infinity Kappa 9's with the onkyo 959 as soon as you started turning up the volume and a deep base note played your reciever would click right off and shut down.... the kappa 9 is probably 10 times more demanding on power than those little lsi 9's!
And my 60 watt adcom amp ran those polk Lsi9's with ease!

A lsi 9 is not even considered a hard speaker to run you must not have owned a speaker that really demands power to run properly.

[mmadden28]

The LSi9 put my Emotiva XPA-2 into protection mode with a low freq signal-The LSi9's drops to below 2 ohms in certain low freqs.

Which Adcom model was the rep referring to? The 555? or the 555 Mk II?
Did the rep state that the amp could drive speakers that drop below 2 ohms in Bridged mode???

I'm just trying to be clear on what was said.


Disclosure: there is the possibility that there is something wrong with my XPA-2, just haven't sent it in yet to be checked out.

[Face]

Disclosure: there is the possibility that there is something wrong with my XPA-2, just haven't sent it in yet to be checked out.

Didn't we say that a long time ago? What are you waiting for?

[mmadden28]

Didn't we say that a long time ago? What are you waiting for?

I'm a notorious procrastinator with no cure in sight :o.

[shenlonco]

You guys with the LSi's a adcom 555 could probably run a few pairs on each side!
You think they suck as much power as these?
4 12in woofers 6 ribon tweeters 2 mids and 2 mid base drivers!
that's 14 drivers in all! and the crossover in it is almost 3/4 as big as a lsi9!

[shenlonco]

The LSi9 put my Emotiva XPA-2 into protection mode with a low freq signal-The LSi9's drops to below 2 ohms in certain low freqs.

Which Adcom model was the rep referring to? The 555? or the 555 Mk II?
Did the rep state that the amp could drive speakers that drop below 2 ohms in Bridged mode???

I'm just trying to be clear on what was said.


Disclosure: there is the possibility that there is something wrong with my XPA-2, just haven't sent it in yet to be checked out.

Do you mean using 2 adcom amps bridged in mono one for each speaker?
the 555's put out 850 watts in 4 ohms bridged..... I had Lsi 9's there is no way they would shut off a adcom 555 actually the adcom is so powerfull one 555 i feel could blow your LSi9's out and send them to speaker heaven...
Either your Lsi9's are damaged or the XPA-2 is broken or is not that good of a amp.

What's the XPA-2 damping factor?? the higher the better it can run low ohm loads more current draw...the 555 adcom is 600 damping factor ...if your amps specs do not show this it must be bad...as the guy's at adcom told me the damping factor is the amps ability to control your speakers cones and they also say watts don't mean nothing it's all about the damping factor if it's low you can't run low ohm loads.

[mmadden28]

Do you mean using 2 adcom amps bridged in mono one for each speaker?
...

Yes, That is what I mean. Does 'bridged' have another meaning with respect to an amp?

The 555 II manual said it will drive a 4 ohm nominal load in bridged mode, but that you should not try with anything that will dip below 4 ohms or risk going into protection.

When you said:

The guy's at adcom said that it could drive speakers that drop way down to 2 ohms or lower...

What I want to know is:
Which Adcom model was the rep referring to? The 555? or the 555 Mk II? Or both.
And did the rep specifically state that the amp could drive speakers that drop below 2 ohms while in Bridged Mono mode???

[BlueFox]

You guys with the LSi's a adcom 555 could probably run a few pairs on each side!
You think they suck as much power as these?
4 12in woofers 6 ribon tweeters 2 mids and 2 mid base drivers!
that's 14 drivers in all! and the crossover in it is almost 3/4 as big as a lsi9!

Is that your living room?

It looks pretty nice, but I think a bigger TV might be appropriate. :)

[okeeteekid]

there is something wrong with your speakers or your amp is no good enough for them, i used to own a pair of the infinity kappa 9's and the original adcom 555 i had used to run them just fine at normal listening levels, the kappa 9's earned the nick name AMP KILLERS as they would dip below 1 ohm and fry most amps, shenlonco is right check what your amps damping factor is, the 555 is 600 if your amps specs does not even list the damping factor then it is probably very low, the higher the better to to run low impediance speakers and to control the woofers when played loud.
okeeteekid

[shenlonco]

Yes, That is what I mean. Does 'bridged' have another meaning with respect to an amp?

The 555 II manual said it will drive a 4 ohm nominal load in bridged mode, but that you should not try with anything that will dip below 4 ohms or risk going into protection.

When you said:
What I want to know is:
Which Adcom model was the rep referring to? The 555? or the 555 Mk II? Or both.
And did the rep specifically state that the amp could drive speakers that drop below 2 ohms while in Bridged Mono mode???

Both models will run your LSi9s ...what are you worring about you will see the little lights start to blink on the adcom amps before anything shuts down.... You can actully run the adcom amp even though if sometimes the little lights flicker on once in a while...but trust me you will be deaf already if you play those 555 that loud to see those leds start to come on and flicker!

and i feel you would never see them come on with your LSi9 speakers....unless you play them so loud and distorted as if you where trying to blow them out.

[ben62670]

Both models will run your LSi9s ...what are you worring about you will see the little lights start to blink on the adcom amps before anything shuts down.... You can actully run the adcom amp even though if sometimes the little lights flicker on once in a while...but trust me you will be deaf already if you play those 555 that loud to see those leds start to come on and flicker!

and i feel you would never see them come on with your LSi9 speakers....unless you play them so loud and distorted as if you where trying to blow them out.

Yep.