Adcom GFA555 4 ohm stable?

mmadden28

mmadden28 wrote: »

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???

shenlonco wrote: »

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.

I'm not trying to figure out my XPA-2/LSi9 problem here. The OP was specifically asking about 4 ohm loads in Bridged mode. My questions were geared to helping to that end and I mentioned LSi9's as an example since they do dip below 4 ohm. This could apply to any speaker like that.

My questions, however, were very specifically about clarifying what the adcom rep said to you since it might counter what the Adcom manual stated. However, it appears that what H9 said works just as well as an answer for the OP.

ben62670

The Triple Darlington is the input stage not output. Both the 555, and 555II have the same output stage. The 555II has a better tranny, and has more/deeper cooling fins. All this means is the 555 will run out of gas sooner than the 555II. I was able to run a 555 out of gas bridged at 4ohms, but it was an unrealistic test using a sine wave, and continuous output. Bottom line there is no issue running either amp bridged at 4ohms.

Systems

ben62670 wrote: »

The Triple Darlington is the input stage not output. Both the 555, and 555II have the same output stage.

According to the manual it is the driver and output stages that are darlington configuated.The input stasge is in all likelihood a differential config.

Other than some increased headroom on top of what is already a very powerful amp I don't see the need to bridge.


As for damping factor mentioned in a previous post the actual DF will be considerably smaller when the resistance of the speaker cable is added into the mix.It is not a reliable spec to guage amp quality by.

heiney9

ben62670 wrote: »

The Triple Darlington is the input stage not output. Both the 555, and 555II have the same output stage. The 555II has a better tranny, and has more/deeper cooling fins. All this means is the 555 will run out of gas sooner than the 555II. I was able to run a 555 out of gas bridged at 4ohms, but it was an unrealistic test using a sine wave, and continuous output. Bottom line there is no issue running either amp bridged at 4ohms.

Sorry Ben, that's not what I've read or been told. All the series II Adcoms have the Triple Darlington Output stage. That's one of the reasons I prefer the original series and I don;t think the Darlington outputs are quite as transparent and the series I.

Nelson Pass surmised they used the Darlington outputs just so it could be stable into a 2 ohm load (stereo). Also remember the DC offset was servo controlled in the series II amps. IMO, there was no reason to change the output stage. But you are correct the series II had a beefier tranny as well as better cooling fins and the nicer Teflon connectors.

In the end we are probably splitting hairs

H9

P.s. Ben read page 1 of the 555 II owners manual. You are partially correct as the 555 II has Triple Darlington config for the input and output stage.

ben62670

Input not output

Systems

heiney9 wrote: »

Ben read page 1 of the 555 II owners manual. You are partially correct as the 555 II has Triple Darlington config for the input and output stage.

The manual mentions the driver and output trannies in darlington config not the input.It would be highly unusual to have a darlington input stage let alone a triple darlington,most are differential configs.Darlingtons are used when increased current output is wanted.The down side with the triple is added complexity.

heiney9

mmadden28 wrote: »

hould not try with anything that will dip below 4 ohms or risk going into protection.

Adcoms have no current limiting circuits so anything other than thermal "protection" is the DC rail fuse. Those can be popped, but anything other than a short would have to be an extreme load.

It can be done but I'd bet even LSi 9's would have a very difficult time presenting a load low enough to pop a rail fuse. I had 9's with my 545 and it barely broke a sweat at elevated listening levels.

I have a medium sized room and sit about 6 feet away so I didn't need to fill a cathedral. At higher listening levels I think my SDA's have a more complex load than the LSi's ever did...................but that's just a guess. SDA's put a heavy load on the negative side of an amplifier because of the dimensional drivers.

H9

heiney9

ben62670 wrote: »

Input not output

If I could "copy" from a pdf file I'd highlight the exact wording from Adcom.

2nd paragraph under the warning box on page 1. It couldn't be more clear

H9

P.s I also agree with GV27 no Triple Darlington on the input stage........they are differential. The driver stage and the output stage are Triple Darlington.

Systems

Assuming its a 4 stage amp,it would be input ,followed by the pre driver,then the driver, then output stage.The last 2 stages are in darlington config.

debussyj

Hi, Just an FYI. I have two original GFA-555's that I bought back when they were released in the 1980's. I recall not eating for a period of time in order to pay for these monsters! LOL! Over the years, they have had the normal service from the folks at Adcom. I've used them (bridged) and still do, to drive my Acoustat 2+2 electrostatic speakers. These speakers are 8' feet tall by roughly 2 1/2' feet wide and aren't the most efficient. The Adcom's have easily driven these babies for many years. Believe me, they've got "the right stuff" to keep my stats happy and are hardly anemic. Good luck with your setup. DJ

Bass_Pedal

This may seem like a dumb question but I'm sure a few polkies reading this thread might be wondering... Why is it more taxing on an amp to run one 4ohm speaker in mono configuration that it is running two 4ohm speakers in stereo configuration? I was considering getting a second 555 and using them bridged for my 9’s until I saw this thread.

Bass_Pedal

Never mind, I think I figured it out. Bridging an amp halves the impedance, doubling the load. Turning a 4 ohm speaker load into a 2 ohm load, making it much more difficult for the amp to drive without overloading. Which I think has been eluded to earlier in the thread. I really could not justify risking possible damaged equipment for the minimal gain this would provide.

BlueFox

Bass_Pedal wrote: »

This may seem like a dumb question but I'm sure a few polkies reading this thread might be wondering... Why is it more taxing on an amp to run one 4ohm speaker in mono configuration that it is running two 4ohm speakers in stereo configuration? I was considering getting a second 555 and using them bridged for my 9’s until I saw this thread.

I am not sure that it is more taxing for an amp to drive one speaker in mono mode than it is to drive two speakers in stereo mode. I guess it depends on how the amp was designed.

I just went through this exercise on Sunday when I added my second Cambridge-Audio 840W amp into my stereo along side of the original 840W. In stereo mode these amps are rated at 350W/ch into 4 ohms, and in mono mode at 800W/ch. The last adjective I would use to describe mono mode for these amps is ‘taxing’. Rather, in mono mode, ‘loafing’ is a better choice.

I bet I picked up at least 10db (as measured by the pre-amp setting) in volume. Where before in stereo mode I listened mostly between -15 and -25, now it is -25 to -35. Additionally, I cannot believe how much better sounding it is now. More open, clearer, precise, detailed, use any complementary adjective.

I had a little buyer’s remorse until I hooked it up, but not now.