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  1. #1
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    4 Ohm Planar Speakers - Driven In Bridged Mode?

    I own a set of Martin Logan Ascent speakers. I had been driving them with a Pioneer Elite VSX-45TX receiver (which only goes down to 6 ohms.) The Receiver has died and I will replace with another Pioneer Elite Receiver (VSX-84TXi), but I bought some Adcom GFA 555 amplifiers as I know they's drive the 4 ohm load of the ML speakers (they produce 325 watts/channel) into 4 ohms. The manual says they can be set up in a bridged mode and used to drive an 8 ohm load at 600 watts per channel. (Unbridged, the amp delivers 200 watts per channel into 8 ohm loads.) However, the manual says nothing about running the amp bridged into a 4 ohm load. Conceivably, the bridged set up into a 4 ohm load could be 650 watts per side.

    Does anyone here know if this is too much power for the Martins? Does anyone here know if the Adcoms will not tolerate being placed into bridged mono mode for the 4 ohm application. According to the manual, the amp is equipped with a 700 watt transformer.

    I appreciate your input. Thanks again.

  2. #2
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    Adcom GFA 555 In Bridged Mode for 4 Ohm Planar Speakers?

    I own a set of Martin Logan Ascent speakers. I had been driving them with a Pioneer Elite VSX-45TX receiver (which only goes down to 6 ohms.) The Receiver has died and I will replace with another Pioneer Elite Receiver (VSX-84TXi), but I bought some Adcom GFA 555 amplifiers as I know they'll drive the 4 ohm load of the ML speakers (they produce 325 watts/channel) into 4 ohms. The manual says they can be set up in a bridged mode and used to drive an 8 ohm load at 600 watts per channel. (Unbridged, the amp delivers 200 watts per channel into 8 ohm loads.) However, the manual says nothing about running the amp bridged into a 4 ohm load. Conceivably, the bridged set up into a 4 ohm load could be 650 watts per side.

    Does anyone here know if this is too much power for the Martins? Does anyone here know if the Adcoms will not tolerate being placed into bridged mono mode for the 4 ohm application. According to the manual, the amp is equipped with a 700 watt transformer.

    I appreciate your input. Thanks again.

  3. #3
    Shostakovich fan Feanor's Avatar
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    Quote Originally Posted by bones327327
    I own a set of Martin Logan Ascent speakers. I had been driving them with a Pioneer Elite VSX-45TX receiver (which only goes down to 6 ohms.) The Receiver has died and I will replace with another Pioneer Elite Receiver (VSX-84TXi), but I bought some Adcom GFA 555 amplifiers as I know they's drive the 4 ohm load of the ML speakers (they produce 325 watts/channel) into 4 ohms. The manual says they can be set up in a bridged mode and used to drive an 8 ohm load at 600 watts per channel. (Unbridged, the amp delivers 200 watts per channel into 8 ohm loads.) However, the manual says nothing about running the amp bridged into a 4 ohm load. Conceivably, the bridged set up into a 4 ohm load could be 650 watts per side.

    Does anyone here know if this is too much power for the Martins? Does anyone here know if the Adcoms will not tolerate being placed into bridged mono mode for the 4 ohm application. According to the manual, the amp is equipped with a 700 watt transformer.

    I appreciate your input. Thanks again.
    Have you tried the 555 with the MLs? What makes you feel that 325 wpc isn't enough? Do you listen very loud in a large room?

    It's not clear that the Adcom GFA 555 will drive 4 ohms in bridged mode, however the GFA 555II will do so, (850 watts according specs: here). Definitely the latter is the preferred of the two, and I've seen them for $400 on eBay which is a heck of a bargain.

    In domestic use there is really no such thing as too powerful an amp.

  4. #4
    Shostakovich fan Feanor's Avatar
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    Bones, please don't double post. See my reply above under 'Planar Speakers'.

  5. #5
    Music Junkie E-Stat's Avatar
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    Quote Originally Posted by bones327327
    The manual says they can be set up in a bridged mode and used to drive an 8 ohm load at 600 watts per channel....However, the manual says nothing about running the amp bridged into a 4 ohm load.
    Bridging reduces the amp's the ability to drive low impedance loads. It is effectively halved. Driving a 4 ohm load bridged is comparable to driving the unbridged model at 2 ohms.

    Quote Originally Posted by bones327327
    Does anyone here know if this is too much power for the Martins? Does anyone here know if the Adcoms will not tolerate being placed into bridged mono mode for the 4 ohm application.
    While the speaker may present a nominal 4 ohm load - translation: only some of them time - most electrostats drop to extremely low values in the higher frequencies. Take a look at this impedance plot for another M-L flavor. Like my Sound Lab stats, it goes down to around 2 ohms up top and dips below 4 elsewhere. I would not recommend bridging the amp for driving an electrostat. In the manual Feanor linked, it cautioned against driving loads that are substantially below 4 ohms. That they are across a pretty wide spectrum.

    rw

  6. #6
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    Thanks for the input & sorry about the double post. I was not sure which of the fora my inquiry belonged in. I have not yet tried the amps as I don't have the pre/pro yet. I hopethe one amp is enough to drive the MLs the last pioneer did not have enough juice. I guess I have an extra GFA555 now.

  7. #7
    Music Junkie E-Stat's Avatar
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    Quote Originally Posted by bones327327
    I hopethe one amp is enough to drive the MLs the last pioneer did not have enough juice. I guess I have an extra GFA555 now.
    What the Pioneer didn't have is the drive capability. A 42 lb Class AB amp claiming to produce 700 watts must necessarily cut corners in many areas. It was designed to drive compact monitors, not electrostats. Most likely, it had a a pair of outputs per channel and was incapable of delivering sustained power into the challenging world of reactive loads. A Pass Labs amp with equivalent power specifically designed to handle such loads weighs 150 lbs. It requires lots of current and having a real power supply.

    rw

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