Adcom GFA555II amplifiers

Mr. Bubbles

I had read on the old boards somewhere a couple years back that someone was using a couple of Adcom amplifiers with their grounds strapped together to create a common ground system while still using 1 amp per speaker. Does anyone know if this is possible with the GFA555II's. I would like to try using one 2 ch amp per speaker in a bi-amp configuration using 1 channel for low/mid and the other for highs on the same speaker channel and the same with the other amp and other speaker channel. The system i would like to try this on is the one with my SRS and not my 1.2TL's so the AI-1 is of no use in this case. i have sent e-mail to Adcom with this question but have not gotten an answer yet. Does anyone know if this is possible? or have info where I can find the old posts discussing this?

boston1450

Im not 100% sure But i do believe i read the 555ii is common ground amp as my 555 is also.....Edit: not sure im any assistance : i know nothing about bi-amping & I would check with a meter first to be sure

ZLTFUL

You're quite literally gaining nothing by doing what you propose.

The tweeters don't use 200 watts. They use a tiny fraction of the power provided by the amp. The real power is being used by the woofers.

Think of it as trying to fill a shot glass with a 55 gallon drum. Yeah...it will fill it but it sure as shoot ain't gonna take 55 gallons to do it.

From personal experience of bridging the 555s, expect a harsher, grainier sound than just running them in stereo. This with barely any audible difference in volume.

Mr. Bubbles

ZLTFUL wrote: »

You're quite literally gaining nothing by doing what you propose.

The tweeters don't use 200 watts. They use a tiny fraction of the power provided by the amp. The real power is being used by the woofers.

Think of it as trying to fill a shot glass with a 55 gallon drum. Yeah...it will fill it but it sure as shoot ain't gonna take 55 gallons to do it.

From personal experience of bridging the 555s, expect a harsher, grainier sound than just running them in stereo. This with barely any audible difference in volume.

I realize it is not like doubling the power and is not perfect as I wont be using an electronic crossover but should still see a significant gain as by using half the amp for low and half for highs the amp would see a fraction of the load it normally would. Not like using one amp for low and another for highs. The way i'm looking at it should take a lot of load off of each power supply which (theoretically anyway) should make a significant difference in headroom and detail.

I have done this before on other speakers (Vanys, VMPS) and there is a noticeable difference in dynamics and detail. I justy have not done it with the Polk SDA and am concerned about the common ground issue betwen the two amps. I know it works well iwith other speakers. I just don't want to risk my amps or my SRS's.

Mr. Bubbles

Oh yeah, I forgot. I'm not talking about bridgeing either of the amps. I am not a fan of brideging. I am only talking about passive bi-amping with one amp per speaker using one channel of one amp for highs and the other channel of the same amp for low/mid on the same speaker. Then doing the same on the other channel with the other amp and speaker. This would be a non issue on my 1.2TL's but thats not the system I would like to use this on.
This is actually a system that is going to more of a mutil purpose system. HT and 2 ch music with the SRS as mains, a custom center made from a pair of 5 jrs (still in progress) and another pair of monitor 5's as rear surround. I have 3 555II's and would like to use 2 to bi-amp the mains as described, one to biamp the center and the other for the rears. I also have 2 545IIs and if the 555II's are too much can use the 545II's for the center and the rears if necessary. There will also be 4 10 inch velodyne subs and a 12inch Polk spread through the room for HT use. I figure the HT is well covered but want to have the most out of the mains as far as SQ and ability so when i switch to 2ch it is still pretty sweet. Though i realize since it is not dedicated 2ch at this point there will be some resulting comparative degradation.

ZLTFUL

It's your baby and if you like the results before, run with it. But strapping the grounds of 2 separate amps together sounds like a disaster waiting to happen to me. Especially with the 555s which offer no speaker protection if something does fry.

Schurkey

There are two amplifier channels in each stereo GFA555. The amplifier channels driving the HF section do not need to be common-ground, the SDA signal does not include the HF drivers.

The amp channels driving the mid/woofers will have to be strapped to make them common-ground, since the SDA signal includes mid/woofer frequencies.

Mystery

You are better off using one amp for highs and another for lows.
555ii is common ground.

Schurkey

Mystery wrote: »

You are better off using one amp for highs and another for lows.

But then one amp has a very easy life, driving the HF of both speakers.

The other amp has to handle the much-more-demanding mid/bass frequencies for both channels.

Splitting the duty the other way allows each stereo '555 to use nearly the entire power supply for mid/bass, while leaving enough for the HF being driven from the other channel.

That wouldn't work on a dual-mono amp like my 8008BBs, but it would work on any stereo or multi-channel amp that shares the power supply among the channels.




The question here is whether the power supply of the '555 needs any "boost"; I'm guessing that the thing has enough snort that the second amp is superfluous--but I've never tested 'em.

Mystery

Schurkey wrote: »

Mystery wrote: »

You are better off using one amp for highs and another for lows.

But then one amp has a very easy life, driving the HF of both speakers.

The other amp has to handle the much-more-demanding mid/bass frequencies for both channels.

Splitting the duty the other way allows each stereo '555 to use nearly the entire power supply for mid/bass, while leaving enough for the HF being driven from the other channel.

That wouldn't work on a dual-mono amp like my 8008BBs, but it would work on any stereo or multi-channel amp that shares the power supply among the channels.




The question here is whether the power supply of the '555 needs any "boost"; I'm guessing that the thing has enough snort that the second amp is superfluous--but I've never tested 'em.

I'm confused what you saying here.
If the same amp is powering HF, MF and LF, how will it be doing anything more when it's only powering less drivers?
Bi-amping doesn't make amp work harder than what they were doing before plus 555ii doesn't mind working hard. I had 555ii running 4 ohm speakers and never broke a sweat.

When someone thinks of bi-amping HF and MF/LF separately, there needs to be a level/power matching as well.
Of course if you connect the tweeter directly to 555ii, it'll blow.
BUT, even if one is using amp separately to the tweeter, it still will go through HF crossover inside the speaker unless one is using external crossover.
Then, it needs to match the level/power/gain of the MF/LF that's powered by separate amp.

Yes, the tweeters don't need much power so better combo will be a 555ii and a 535.

Mr. Bubbles

DSkip wrote: »

The amps don't know what they are powering and will be sending a full signal to whatever it's connected to without a crossover in front of the amp. Your arguments are illogical.

This post is not directed at Dskip, just an extension of thought based on his post.

This is partly correct; the amps don't know if it's a woofer or a tweeter they are driving, but they do know load. Load is very much dependent on frequency though it may not be as direct as it is indirect.

fact is any 2ch amplifier that shares a common power supply between the channels will have it easier by loading one channel with high frequencies and one with low frequencies. Amps that utilize load sharing power supplies will benefit even more than those that do not. however simple physics dictates that straining the power supply less improves amplifier function. this is why the Super high end amps have such massive power supplies to begin with. running one of those in a scenario such as this would have very minimal if any effect.

Nightfall

Why not sell one and put the money towards a sweet sounding tube amp for the highs then use the other Adcom for the lows?

Mr. Bubbles

Not looking for an increase in power. Dynamics and detail is what I'm shooting for. But I agree completely there would be no more power capability. But dynamic presentation is another matter.

Mr. Bubbles

Load on the amplifier is not the same for an amp driving 2 woofers vs an amp driving 2 tweeters. The nominal driver impedance may be similar in rating but true load due to difference in frequency, phase, and actual impedance (not to mention the resistive, capacitive, and inductive differences) of the different types of drivers due to their physical build differences in their voice coils, diaphragms, and suspensions.
I do agree that with certain amplifiers it would be a complete waste of time and would be a moot point, but those amplifiers are few as compared to the many that should realize a benefit from a situation such as this.
Just need to know abut the strapping of grounds between the two amps. that's the big question.

tonyb

What the guys are trying to convey here is, by using an amp just for the tweets, your wasting power that won't be available to the speaker as a whole. For the same reason you wouldn't put an amp with 100 amperes of current on some Klipsch speakers, they simply can't use all that power.

Plus amps don't really strut their stuff unless presented with a load. I'd be more inclined to do as Skip suggests, sell them both and buy one better amp.

Schurkey

Mr. Bubbles wrote: »

Just need to know abut the strapping of grounds between the two amps. that's the big question.

Answered.

Schurkey wrote: »

There are two amplifier channels in each stereo GFA555. The amplifier channels driving the HF section do not need to be common-ground, the SDA signal does not include the HF drivers.

The amp channels driving the mid/woofers will have to be strapped to make them common-ground, since the SDA signal includes mid/woofer frequencies.

Strap the negative terminals of the two mid/bass amp channels.

motorstereo

Having owned a pair of 555 II's back in the day I agree with the sell them for something better comment. When they're bridged yes they do have egads of power but they are very unpleasant to listen to in that mode. I also tried running one for the lows and one for the highs on my 2.3s. I didn't gain a thing soundwise.