Polk SDA1 (1st Gen) w Adcom GFA555 Common Ground

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le91163
le91163 Posts: 3
edited December 2021 in Vintage Speakers
Hello, I have gone old school and bought a 40 year old pair of polk SDA1A W sL1000 Tweeters and from what I can tell these are first generation of this model type with silver tweeters, if that makes a difference.

I also got the interconnect cable as well.

I also have an Adcom GFA 555 that I bought in 1988 that I think would work well with these speakers.

The back of the polk says I must use an amp with a "common ground. "

Question I have is this.. does my Adcom have a common ground? Can anyone in this group verify or steer me towards finding and answer?

Thanks in advance.

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  • pitdogg2
    pitdogg2 Posts: 24,559
    edited December 2021
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    The Adcom you have IIRC is common ground. From my understanding the 4 number series like 5500, 5800 for example are not common ground.
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  • pitdogg2
    pitdogg2 Posts: 24,559
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    Here is a way to double check. Remember Amp off, no cables hooked up.
    Schurkey wrote: »
    What ohm reading do you get when testing left black speaker post to right black speaker post?

    I don't know of a better test than a simple ohmmeter connected across the black posts. Less than one ohm resistance, you're set. Of course, this will require the amp to be in stereo mode, bridged/balanced amps are not SDA-compatible without an isolation transformer.

    I have heard of manufacturers who don't friggin' know whether their product is common-ground or not, and give bad advice to SDA owners. Adcom, for example has told at least two folks on this forum that a certain product is common-ground when it was not.
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  • boston1450
    boston1450 Posts: 7,453
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    Yes. 555 I used on my SDA2b & with a good preamp it will sound very good. IIRC the 555 can be modified/upgraded also. Being it's age. Nice amplifier for SDA'S
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    Randy/Maine
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  • Schurkey
    Schurkey Posts: 2,100
    edited December 2021
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    The 555 should be common ground. Others have used them successfully.

    Be sure you check DC offset before you connect them to your speakers. The offset is not adjustable, it's controlled by a pair of MATCHED transistors; and they get un-matched or outright fail over time.

    That amp is old enough that you'd do well to have it properly serviced. It's a fine amp, has lots of potential...but it's old and may need some lovin'.

    Some folks are weirded-out by the lack of protection circuitry; but I don't think that's a particular problem provided the amp is in otherwise good condition. Again...that amp is old.
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  • TennMan
    TennMan Posts: 1,264
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    The GFA-555 is definitely common ground. I've been running one for several years with my SDA TL-2Bs without problems. I'm very happy with it. I think it would have plenty of power and be a good match for the SDA 1As.

    There are several mods that can be made to the 555. Upgrades have been made to my 555 that are similar to the LX Mod made by Musical Concepts to the GFA-555. A speaker protection board was added at the same time that shuts the power off to the speakers if a problem is detected. The board also delays sending power to the speakers for a few seconds when the amp is first turned on. That eliminates the "pop" or "thump" that sometimes can be heard in the speakers when you turn a 555 on. You can see the board mounted to the right side of the cabinet in the photo below.

    hhv4evauponz.jpg
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  • Schurkey
    Schurkey Posts: 2,100
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    I just love topless photos.
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  • vader540is
    vader540is Posts: 178
    edited January 2022
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    TennMan wrote: »
    The GFA-555 is definitely common ground. I've been running one for several years with my SDA TL-2Bs without problems. I'm very happy with it. I think it would have plenty of power and be a good match for the SDA 1As.

    There are several mods that can be made to the 555. Upgrades have been made to my 555 that are similar to the LX Mod made by Musical Concepts to the GFA-555. A speaker protection board was added at the same time that shuts the power off to the speakers if a problem is detected. The board also delays sending power to the speakers for a few seconds when the amp is first turned on. That eliminates the "pop" or "thump" that sometimes can be heard in the speakers when you turn a 555 on. You can see the board mounted to the right side of the cabinet in the photo below.

    hhv4evauponz.jpg

    Beautiful. I might do this mod with my GFA555.

    I also recently acquired a NAD 2200 and I'll be comparing both of those amplifiers back to back.
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  • woodsman
    woodsman Posts: 5
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    Have had the 555, never an NAD. I would really be interested in the A / B of those two amp.
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  • vader540is
    vader540is Posts: 178
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    woodsman wrote: »
    Have had the 555, never an NAD. I would really be interested in the A / B of those two amp.

    In a few weeks I'll be going over a buddies house and we will A/B the 555 and the NAD 2200.

    We will be using the Philharmonic BMR

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