Bi-Amping explained

Discussion in 'AV Receivers' started by Mario0511, Jan 1, 2011.

  1. Mario0511

    Mario0511 Auditioning

    Jan 1, 2011
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    A little technical explanation. I've been electronics technician for over 22 years, involved in custom speaker, x-over designs. The purpose of x-over is to either allow or reject certain frequency (s) to the drivers. When allowed, (passed thru), the amplifier sees low impedance and therefore creates necessary wattage (AC sine wave). The lower the impedance it sees, the higher output will be delivered, unless too low, causing catastrophic short.

    Any frequency (s) rejected by x-over, has essentialy in perfect world, infinite impedance and physical resistance, and therefore your amp will not output those frequencies, even thou it's running in full range. Obviously if you quickly connect full range speaker to the circuit, it will show it's getting full range, and that's only because suddenly amp no longer sees any x-over's for that particular external driver.

    This is very easly tested with resistor.

    Hook up, say 4 ohm (10 watt) resistor to your speaker output and crank up the amp, resistor will heat up because amp sees somewhat of a 4 ohm load.

    Now take a 1 Mega (million) ohm (10 watt) resistor and perform same test. Resistor will be cold, and that's because amp sees such a huge load, amp thinks it's an open circuit (no driver), and therefore there is NO output from the amp. Unless it's a tube amp, but that's another animal, and I don't want to confuse anyone.

    And this is what x-overs do to the output of the amp.

    This is why bi-amping doesn't really give you more power, or loudness. But what it does give you is more headroom from your amp/receiver, thus delivering cleaner power.

    X-over inside a speaker box or active x-over inside amp unit is essentialy the same. The only difference is the slope 6-dB, -12dB and so on. The more aggresive slope, the lower rate of frequencies overlap, and thus less of impedance drop. Sometimes this might create un-natural if slope is too aggresive.

    So removing tabs in the back of your speakers does 2 things.

    1. Gets rid of imperfections in x-over frequencies thus limiting impedance drop (frequency overlap) between tweeters/mids and woofer(s).

    2. Limits, yes limits the ouput frequencies from the amp/receiver. X-overs will dictate to the output of the amp which frequencies are to be reproduced, and only those will be present at the output of the amp, NOT FULL RANGE. And because of that, your headroom will somewhat increase, thus during peak demands, amp can essentially deliver slightly higher output(s).

    So when bi-amping, does the receiver/amp send bass to the woofers and not high frequencies, and vise versa, high frequencies to mids/tweeter but not bass even thou has no internal x-over on it's own?

    Y E S, and thats's because internal speaker x-overs make sure of that. It all comes down to impedance.

    And if you still won't believe me, hook up say, piezzo tweeters to your amp. If anyone knows how these guys work, basicaly, as the frequency increases, piezzo's impedance decreases thus allowing the frequency to go thru. The only reason you would use x-over on these little guys, is to control the intensity/loudness of these tweeters, not to prevent from burning out. If you were to measure physical resistance of piezzo, it would read infinate(open), and it almost behaves like a capacitor. Below operating frequency of piezzo, amp sees infinite impedance and therefore will not produce any output, unless there is another driver present in the circuit which will demand that part of spectrum.

    Unless individual has understanding of OHM's law and have understanding how resonant circuits work and behave in AC applications which that what your speakers see when operating, there is no further need for discussion. There is tons of physics behind sound reproduction.

    Food for thought: When drivers operate, they create negative voltage which is send back to the output of the amp/receiver. Impedance can drop to 0.5 ohms and shoot to over hundreds of ohms of resistance. This is why different speakers, sound different, besides having different cone material, voice coils, etc, their electrical impedance allows or rejects certain frequencies more or less than other speaker, thus having different sound.

    And since the negative voltage is send back to amp, amp/receiver has to have good enough Damping Factor to deal with that unwanted voltage. Like the name says DAMping Factor, it's a meassure how well the amp can control the movement of the cone and stop the unwanted voltages from introducing false frequencies thus distortion.

    Modern amp's use negative feedback from output to deal with that, but sometimes if too high, could create unwanted noise in the circuitry.

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