current vs. voltage amp output

Discussion in 'AV Receivers' started by John Meringolo, Aug 28, 2004.

  1. John Meringolo

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    could one of the "engineer" types out there help me to understand what the difference is b/w an amp with a current and a voltage source on the output stage, what it actually means and how it effects speaker performance? I am referring to the Sunfire Stereo Signature amp. It is being used to push 2 subs; JL Audio 12W3's which are 3 ohm min impedance Yes, I measured the min impedance not DC resistance).
    Thank you
    Dr John
     
  2. alan halvorson

    alan halvorson Cinematographer

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    Well, I ain't no engineer, but I do know a touch about what Sunfire is doing. The difference between the "Voltage" and "Current" outputs is that the the "Current" output has a 1 ohm resister in series. This is an easy way to mimic a tube amplifiers output impedance. An amplifiers output impedance becomes part of the speaker load; therefore, it is desireable to have as low an output impedance as possible so that the speakers frequency response is not modified. It's doubtful that the amplifier designer is also the speaker designer; I'd sure hate spending big bucks on speakers designed to sound one way, only to insert an amplifier whose effect is to make it sound another.

    Use the "Voltage" output; the "Current" output is a bad joke.
     
  3. Dave Milne

    Dave Milne Supporting Actor

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    A true voltage source provides a fixed output voltage regardless of load impedance. For example, if the amplifier puts out 40vrms, that will be 40^2/8 or 200 watts (which also happens to be 5 amps - more about that later). If the load impedance is then halved to 4 ohms, the amplifier will put out 40^2/4 or 400 watts (and 10A). Put another way, this amplifier has zero output impedance. This is considered the "ideal" amplifier. The term "damping factor" is just the reciprocal of output impedance. So, an amplifier with a damping factor of 500 has an output impedance of 1/500 or 0.002 ohms.

    A true "current source" amplifier would be rated in "amps" and would put out, say, 10A max. With an 8 ohm load, this would be 10^2*8 or 800 watts. With a 2 ohm load, it would be 10^2*2 or 200 watts. This represents infinite output impedance or zero damping factor.

    So with low impedance loads, you really want a voltage source type of amp. Bob Carver's Sunfire amps, while switchable from "voltage source" mode to "current source" mode, aren't really capable of behaving as a true current source (not that anyone would really want this with real world speakers). They just switch from very low output impedance in "voltage mode" to just "low" output impedance in "current mode". One clue to this is the Sunfire spec sheet that says operation into very low impedance loads is on a "time limited basis". If these were true current sources, the amp would prefer lower impedance loads - it would produce zero power into a zero ohm load!

    I will have to say that Bob Carver is a master of marketing hype, similar to Amar Bose in this regard. Nevertheless, his products are solid performers. I owned one of the first Phase Linear 400 amps, a Phase Linear 700, a Phase Linear 1000 preamp (with "autocorrelator" and "downward expander"), A Carver C4000 premp (with "sonic holography") a TX-11b tuner (with "asymmetrical charge-coupled detection") and a CD player (with "digital time lens").

    Don't get too worked up about voltage source vs current source. Pick the setting on your Sunfire that sounds the best and doesn't drive the amp into thermal shutdown.
     
  4. David Judah

    David Judah Screenwriter

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    Which is better will depend on your speakers and your personal tastes. I preferred the current source output going to the high frequency posts of my B&Ws with a Sunfire Cinema Grand.

    Try it both ways to see which works out best for you.

    DJ
     

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