Can I mono two Adcom GFA 555's together w my SDA's?

Dabutcher
Dabutcher Posts: 2,596
edited January 2013 in Vintage Speakers
I have been using a GFA 555 with my twin towers and they sound the best sound I have ever experienced. My idea without putting out a lot of more money is two buy a second 555 and run each as monos . Can I just link the grounds or do I have to build or buy a A1 Dreadnaught ? As always thanks for all the help. Darryl. P.S. Apple posted record fourth quarter profits and sold 50 million iPhones over the holidays but today the stock is tanking? Is it a good time to buy?
MIT Magnum MH-750, Monster HTS 5100MKII, Sony 77" Class - A80CJ Series - 4K UHD OLED,PS4, Def Tech 15” sub,LSIM 706c, Sunfire Signature Grand 425 x 4,Parasound hca 120, LSiM 702 x 4, Oppo 103D, SDA SRS 1.2, Pioneer Elite SC63 , Pioneer Elite BDP-05 “Why did you get married if you wanted big speakers?”
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  • ZLTFUL
    ZLTFUL Posts: 5,648
    edited January 2013
    GFA-555 or GFA-555II?
    One is common ground (GFA-555), one is not (GFA-555II).
    I believe (but I am no expert) that the AI-1 is required for non-common ground amps.

    That would be the determining factor. Mono or Stereo operation doesn't matter.

    Although, if you have been running them and they sound good, I would assume you have the common ground version.
    "Some people find it easier to be conceited rather than correct."

    "Unwad those panties and have a good time man. We're all here to help each other, no matter how it might appear." DSkip
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  • nwohlford
    nwohlford Posts: 700
    edited January 2013
    When they are bridged, you don't have a ground. You have positive and negative, so there is no ground to link. You would need AI-1. Given that the amp sees half the impedance when bridged and your SDAs are not 8 ohms to begin with, I would not try running them bridged anyway.

    Both GFA-555s are common ground when used in stereo (the GFA-5500 is non-common ground).
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  • pitdogg2
    pitdogg2 Posts: 25,443
    edited January 2013
    nwohlford wrote: »
    When they are bridged, you don't have a ground. You have positive and negative, so there is no ground to link. You would need AI-1. Given that the amp sees half the impedance when bridged and your SDAs are not 8 ohms to begin with, I would not try running them bridged anyway.

    Both GFA-555s are common ground when used in stereo (the GFA-5500 is non-common ground).

    correct so both amps will see a 2ohm load. Most amps when bridged also have more distortion.
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  • Schurkey
    Schurkey Posts: 2,101
    edited January 2013
    pitdogg2 wrote: »
    correct so both amps will see a 2ohm load.
    These are 1.2s...right? What's the impedance of the 1.2? I thought they were "8-ohm compatible" (which might be a way of saying "not under 6 ohms for much of the frequency spectrum".) I'd hope the '555s could handle that while bridged; although some research in the amplifier owner's manual might be wise.
    pitdogg2 wrote: »
    Most amps when bridged also have more distortion.
    More of some forms of distortion, less of other forms. 'Course, it does depend on the design of the amp that's been bridged, including the condition of the specific amp itself. Those '555s are getting along in years.
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  • ZLTFUL
    ZLTFUL Posts: 5,648
    edited January 2013
    nwohlford wrote: »
    Both GFA-555s are common ground when used in stereo (the GFA-5500 is non-common ground).
    Odd...I got that info from Adcom directly (555 vs 555II).
    Not saying you're wrong or anything just weird that Adcom wouldn't have correct info on their own gear.
    "Some people find it easier to be conceited rather than correct."

    "Unwad those panties and have a good time man. We're all here to help each other, no matter how it might appear." DSkip
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  • gdb
    gdb Posts: 6,012
    edited January 2013
    AFAIK, two amps require either tying the amps speaker terminal grounds together or, an AI-1 or Dreadnaught transformer set-up.
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  • pitdogg2
    pitdogg2 Posts: 25,443
    edited January 2013
    My bad I was thinking that we were talking 4ohm speaker.
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  • pitdogg2
    pitdogg2 Posts: 25,443
    edited January 2013
    ZLTFUL wrote: »
    Odd...I got that info from Adcom directly (555 vs 555II).
    Not saying you're wrong or anything just weird that Adcom wouldn't have correct info on their own gear.

    Both the 555 and 555II are common ground(in stereo) the 5500 nwohlford refered to is not common ground.
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  • Dawgfish
    Dawgfish Posts: 2,554
    edited January 2013
    Most amps do not sound that great bridged. If you want to bring in a second 555, I would try passive horizontal bi-amping. You won't need an AI-1 or a dreadnaught in this configuration. By this I mean hook the left and right speaker cables from one 555 to the left and right speaker low frequency terminals. Hook the left and right speaker terminals from the other 555 to the left and right high frequency terminals. It helps if your pre has two sets of outputs. If not you can use a Y IC adaptor. One other thing to keep in mind when doing this is it is ideal to use the same type and length of ICs and speaker cables on both amps, so you have to also factor in the cost of another set of ICs and speaker cables if you don't already have them.
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  • DaveHo
    DaveHo Posts: 3,526
    edited January 2013
    Both the 555 & 555II are rated @ 850WPC bridged into 4 ohms. So no problem there if they are.
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  • Dabutcher
    Dabutcher Posts: 2,596
    edited January 2013
    GFA 555 is what I have. And the SDA's are from 1988 1.2. So I guess they are 6 ohm speakers. I guess I will hold off for now? I am more confused than ever? Thanks for all the help.. D
    MIT Magnum MH-750, Monster HTS 5100MKII, Sony 77" Class - A80CJ Series - 4K UHD OLED,PS4, Def Tech 15” sub,LSIM 706c, Sunfire Signature Grand 425 x 4,Parasound hca 120, LSiM 702 x 4, Oppo 103D, SDA SRS 1.2, Pioneer Elite SC63 , Pioneer Elite BDP-05 “Why did you get married if you wanted big speakers?”
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  • deronb1
    deronb1 Posts: 5,021
    edited January 2013
    Dabutcher wrote: »
    GFA 555 is what I have. And the SDA's are from 1988 1.2. So I guess they are 6 ohm speakers. I guess I will hold off for now? I am more confused than ever? Thanks for all the help.. D

    Unless you are totally sold on Adcom, why not sell the GFA-555, take the money you would have used for a second one and look for a more powerful amp. Bridging does increase distortion dramatically and who wants that?
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  • stubby
    stubby Posts: 723
    edited January 2013
    deronb1 wrote: »
    Unless you are totally sold on Adcom, why not sell the GFA-555, take the money you would have used for a second one and look for a more powerful amp. Bridging does increase distortion dramatically and who wants that?
    +1 If you really want mono blocks and like the Adcom sound, get a pair of GFA-565's and a common ground interface of your choice. Of course, that will require a little more room in the budget.
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  • motorstereo
    motorstereo Posts: 2,133
    edited January 2013
    Having owned a pair of 555's my thoughts would be to skip the bridged idea. Sure you get lots of power but to me those bridged adcoms amounted to the worst sounding amps that I've ever heard. Those 800 wpc quickly became meaningless.
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  • george daniel
    george daniel Posts: 12,096
    edited January 2013
    Bridged Adcom 555's are Painful-- been there,,done that. I sold mine and bought a Belles 350A(used),,huge difference,,just my .02
    JC approves....he told me so. (F-1 nut)
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  • Anthony Hinton
    Anthony Hinton Posts: 107
    edited January 2013
    Geez, I was just considering a pair of 555's. I think I will pass now, good info here
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  • sda2mike
    sda2mike Posts: 3,131
    edited January 2013
    bridging also reduces your damping factor...who needs that shiite??
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  • Drenis
    Drenis Posts: 2,871
    edited January 2013
    sda2mike wrote: »
    bridging also reduces your damping factor...who needs that shiite??

    There is a member here who swears by his bridged Cambridge amps. Personally it all confuses me. I'd rather run mono's because I can.
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  • FTGV
    FTGV Posts: 3,649
    edited January 2013
    It depends upon the specific design of the amplifier.One with very tight gain matching between its two channels can actually have reduced THD levels when operated due to differential common mode reduction.A well designed amplifier should retain it's sonic character in bridged mode as long as the minimum impedance loading is heeded.Some of the very best sounding hi powered amp's are (internally permanently wired)bridged output designs.
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