Bi-Wiring and Bi-Amping Loudspeakers
By Chuck Hawks
One of the recent trends in home stereo (two channel) music reproduction favored by the nuttier audiophiles is bi-wiring loudspeakers. Somewhat similar in concept is "passive bi-amping." Then, there is true (active) bi-amping. In this article, we will examine all three and attempt to explain their benefits, if any.
Bi-wiring requires speakers with two pairs of binding posts for speaker wire. Many upscale loudspeakers now come with this configuration, including a removable shorting bar that connects the two "+" and the two "-" terminals for use with normal, single pair, speaker wire. One set of binding posts connects to the high frequency side of the passive crossover inside the speaker box and the other set of binding posts connects to the low frequency side of the crossover. The shorting bar (when left in place) insures that both sides of the crossover receive the same input signal.
Remove the shorting bar and the speaker is ready for bi-wiring. This procedure runs two lengths of two-conductor speaker wire in parallel from the power amplifier's output to the dual binding posts on the loudspeaker. Note that these dual speaker wires are shorted together at the amplifier end; thus, both speaker cables are carrying the same signal from the amplifier to the loudspeaker. One speaker wire is attached to the high frequency side of the speaker's crossover binding posts and the other is attached to the low frequency side. It doesn't matter which wire pair goes to which side of the crossover, since both are identical. The result is both sides of the crossover getting the same, identical signal, just as it would if the shorting bar were in place and both sides of the crossover were fed by one speaker cable.
A reasonable question at this point would be, "Since, either way, the speaker is receiving the same signal, how is this supposed to improve the sound?" This is a logical question and, as long as the speaker wires are identical in length, type and gauge, there should be NO effect on the sound of the loudspeaker. Bi-wiring with identical speaker cables accomplishes nothing, at least from the music listener's perspective.
However, it does accomplish something from the cable manufacturers and retailers standpoint: it doubles their profit on cables, which are already the most profitable item in the store. This last point is sweetened by the fact that it is the most extreme audiophiles, often with more money than electronics knowledge, who are likely to bi-wire and buy the most expensive speaker cables. For example, a pair of 10 foot Audioquest Flat Rock Series K2 speaker cables suitable for bi-wiring (two "+" and two "-" speaker connectors per cable) carries a 2010 MSRP of $10,450.
Folks with scant technical knowledge and lots of blind faith report sonic improvements with bi-wiring. This is known as the placebo effect in medicine. Empirical testing would reveal that bi-wiring is a waste of effort and money.
Bi-wiring could be used to alter the sound of a loudspeaker by using different speaker cables. Use 10 feet of 12 AWG line cord to connect the amplifier to the low frequency side of the speaker and 100 feet of 24 gauge zip cord to connect the amp to the mid/high frequency side and the result should be decreased mid/high output, due to the increased wire resistance. This would be a crude way to "correct" overly bright loudspeakers, but it would be easier, tidier, cheaper and more effective to use the tone controls provided on most control amplifiers.
Passive bi-amping (also known as "fools bi-amping") requires loudspeakers with the dual binding posts used for bi-wiring, again with the shorting bar removed. The difference (and profit advantage for the retailer) is that it requires two stereo power amplifiers, instead of one, and an additional pair of speaker cables.
In passive bi-amping, the output from the pre-amplifier is fed to a pair of identical stereo power amps (or, even better for the retailer, four monoblock power amps). All four amplifier channels are fed the same, full range, signal from the pre-amp. That is important, so take note of it. The output of the power amps is fed to the loudspeaker, the left/right output of one amp to the speaker's high frequency binding posts and the left/right output of the other amp to the speaker's low frequency binding posts. We now have the same, full range signal everywhere.
From the listener's perspective, if all goes well, the sound quality should remain the same, but the placebo effect insures that most folks who passively bi-amp their music systems report a sonic improvement. The system's total amplifier power has been doubled. However, if increased power is the goal, it would be cheaper to buy one 200 watt stereo amplifier than two 100 watt stereo amps of the same quality.
From the amplifier's perspective, the picture is somewhat murky. Each amplifier, driving only part of the speaker system, is looking at less total load (maybe half) than if it were driving the whole speaker system. If the speaker system was a nominal 8 or 16 ohms, the amplifiers are probably safe. If the loudspeaker system was a nominal four ohms to begin with and either the low frequency or the high frequency side of the bi-amped passive crossover dips below about two ohms, the amplifier may see this reduced load as a short and fry itself. This "benefit" of passive bi-amping is seldom mentioned! Now you understand why it is called fool's bi-amping.
What is now called "active bi-amping," to differentiate it from passive or fool's bi-amping, used to be just "bi-amping." Unlike bi-wiring and passive bi-amping, active bi-amping has definite and provable benefits. Unlike the sham bi's, real bi-amping sends one signal to the woofer and a different signal to the tweeter in a two-way loudspeaker system. This requires two stereo power amps (or four monoblock amplifiers). Perhaps I should interject that bi-amping is for two-way loudspeakers (woofer and tweeter). A three-way loudspeaker system (woofer, midrange, tweeter) would require tri-amping (three stereo power amps).
Here is how bi-amping works, using one stereo pre-amp and two stereo power amps. The full range, left and right signals from the pre-amplifier are fed to an outboard (also called "active") crossover. This crossover splits the low and high frequency information for the two stereo channels. The L/R high frequency output is fed to one stereo power amplifier and the L/R low frequency output is fed to another stereo power amplifier.
Note that the active crossover is between the pre-amp and the power amps, not inside the speaker box. The active crossover has two (left and right channel) inputs and four signal outputs (left channel high frequency, right channel high frequency, left low frequency and right low frequency). Also, notice that the two stereo power amplifiers are being fed, and will amplify, different signals representing only part of the original (full range) signal.
The stereo power amp handling the right and left high frequency signals is connected directly to the right and left tweeters, NOT to the speaker systems' internal passive crossovers. The power amp handling the right and left low frequency signal is connected directly to the right and left woofers. The loudspeakers' internal passive crossovers have been bypassed and are no longer connected to the system's drivers. The passive crossovers may be physically removed from the speaker cabinets, if desired.
Active bi-amping has several advantages. Since each power amp is driving only part of the frequency spectrum, intermodulation distortion is virtually eliminated. The inevitable power loss associated with a passive crossover is eliminated. Since the low frequency driver (woofer) requires the bulk of the power in any speaker system, a less powerful amplifier may be used to drive the tweeters in a bi-amped system. (Of course, there must be level controls on the active crossover to balance the system.) The power amps are not threatened by a very low impedance load, since each channel is powering an individual eight ohm (or whatever it is rated at) driver. The power amps are not working as hard to drive a bi-amped speaker system to any given volume, so harmonic distortion is reduced and clipping at high volume is unlikely. The stereo system's amplifier power is effectively doubled. The benefits of active bi-amping are real (not subjective) and can be measured with test equipment.
In the real world, there is no free lunch and this remains true for active bi-amping. The most obvious drawback is increased expense, since two stereo power amps and an outboard crossover are required, not to mention four sets of speaker wires. System complexity is increased and there are more interconnects (RCA patch cords and speaker cables). In addition, the active crossover must match the characteristics of the speaker system to be bi-amplified and must often be custom built. To set-up a bi-amped loudspeaker, test equipment is required, along with the knowledge to use it correctly. At a minimum, this means a sound pressure level (SPL) meter and a test CD with a sine wave test tone at the crossover frequency. Much better is a sine wave test tone generator, and these are not cheap. If you are considering bi-amping to take advantage of the increased system power from dual power amps, let me point out that it would be cheaper and much simpler to buy a single stereo power amp with twice the power and use it conventionally.
I have considerable experience with active bi-amping. For many years, my stereo loudspeakers used 12-inch Altec-Lansing 419-8B biflex woofers and Altec 3000H horns in seven cubic foot (internal volume), custom built, bass reflex enclosures. There was no internal passive crossover included, as these speakers were designed from the outset to be bi-amped. (This was in the 1970's.) An electrical engineer friend designed and built the outboard 3000 Hz crossover with a 12 db slope required for active bi-amping. This crossover had individual level adjustments for each of its four outputs. I used a Realistic (Radio Shack) SPL meter and a Techtronics sine wave test tone generator to balance the outputs of the various drivers. If I remember correctly, I placed the SPL meter three feet from and on axis with each driver, individually, when I adjusted the crossover. I didn't have access to an anechoic chamber and the whole system was too big and complex to move outside, but the result was good.
Over the years, I used various stereo power amplifiers with these speakers. The initial set-up used a 40 watt-per-channel Dynaco Stereo 80 for the woofers and a 10 watt-per-channel Kenwood for the horns. The Altecs were highly efficient drivers and this worked fine. Later, I moved the Stereo 80 to the horns and used a 75 watt-per-channel amp on the woofers. It is hard to say for sure, but there may have been a slight improvement at very high listening levels. These were impressive loudspeakers and I used them for almost two decades. I eventually gave them and their active crossover to a friend, when I had a chance to replace them with a pair of Altec-Lansing A7-500 Voice of the Theater speakers. By then I had a Marantz 250M power amp, which had plenty of power to drive the ultra-efficient A7-500's without bi-amping. High power amplifiers with very low distortion are available these days and, despite its real advantages, I have never felt the need to return to bi-amping.
Copyright 2011, 2012 by Chuck Hawks. All rights reserved.