Bi-wiring and Bi-Amplification
One of the overlooked issues in designing audio systems is the crossover hidden inside the loudspeaker box. The crossover has the job of splitting the signal into the low and high frequency components (e.g. in a 2-way system). It is deceptively simple, consisting of a handful of passive components. Yet, it has the exceedingly difficult job of carrying high current and high voltage signals -- about 1,000 times higher current and 20 times higher voltage compared to the line-level stage.
The signal path through the crossover is fraught with problems that can generate more distortion than the rest of the amplification chain, preventing the audio system from performing at its full potential. There already exist several approaches that can improve the fidelity of this interface. In this article, we describe the progression from the traditional approach to active bi-amplification, with progressively improving fidelity.
Traditional Approach with Passive Crossover
The traditional approach with passive crossovers suffers from serious limitations, some of which are:
- The crossover presents a complex impedance between the amplifier and the speaker drivers, making it difficult for the amplifier to control the drivers directly.
- Especially outside its passband, the crossover impedance rises significantly, undermining the amplifier's ability to control out-of-band resonances in the drivers.
- The inductors used in the crossover usually have ferromagnetic cores, which can generate high levels of distortion, typically 0.1-0.2% even at low levels. Often, this is the largest source of nonlinearity in the entire electrical chain leading up to the loudspeaker drivers.
In spite of its inherent limitations, the passive crossover is predominantly being used, mostly because it requires only one amplifier to drive a loudspeaker. However, with the cost of high-quality amplifiers falling, it makes more sense to consider bi-amplification, using two or more amplifiers per channel.
The first step toward improving this interface is bi-wiring, referring to the use of two cables to drive a loudspeaker, one for the low frequency section and another for the high frequency section. Most high-end loudspeakers offer bi-wiring terminals (two pairs of terminals per loudspeaker). The low and high frequency crossovers are separated, and the signal current flows through two separate cables. Even though both paths see the same voltage, the currents are now split into low and high frequency paths. One cable carries only the low frequency currents, and the other only the high frequency currents. With this separation, the interaction between the low and high frequency currents are reduced, resulting in lower intermodulation distortion.
In our experience, the audible improvements from bi-wiring are subtle. Some experts, however, believe that the effects are not so subtle. According to Vandersteen, "The improvements are large enough that a bi-wire set of moderately priced cable will usually sound better than a single run of more expensive cable." Jon Risch, through the use of multitone testing, also concluded that bi-wiring significantly reduced intermodulation distortion between high-frequency currents and low-frequency currents ("A New Class of In-Band Multitone Test Signals," AES Convention, August 1998).
We can go a step further by using two amplifiers to drive the low and high frequency paths separately. This configuration is usually called "passive" bi-amplification because it still retains the passive crossover. Notice that the two paths are now completely isolated from each other, and any potential interaction between the two paths is eliminated. The only residual interaction would be through the electromagnetic coupling between the two cables. Moreover, the impedance of each path rises outside its passband, significantly reducing the output current requirement of each power amplifier.
Passive bi-amplification makes subtle but discernable improvements beyond that of bi-wiring. The sound stage appears more sharply focused and detailed and the dynamic contrast increases, enabling music to be more engaging. Even in loud, complex passages, music seems to breathe more freely.
The greatest gain in the quality of sound comes from eliminating the passive crossover completely and replacing it with an active crossover before the amplifiers. This raises the definition, clarity, and dynamic contrast to another level.
The advantages of active bi-amplification are too numerous to list here (see Loudspeakers by Philip Newell & Keith Holland chapter 5 for more comprehensive discussions). In addition to solving all the problems mentioned above, it reduces the power requirement of amplifiers drastically. Since the signal is divided into low and high frequency components by the active crossover, each amplifier needs to handle only a limited range of frequencies. Moreover, active bi-amplification offers a much more direct control of loudspeaker drivers across the entire audio spectrum and provides adequate damping over all audio frequencies. It also eliminates the nonlinearities associated with passive crossovers, reducing intermodulation distortion and improving clarity and definition.
Considering all the advantages of active bi-amplification, we can no longer be enthusiastic about a single high-power, high-current amplifier driving a loudspeaker. No matter how much current it is capable of pouring into the loudspeaker, it still can do nothing about the bottleneck.
With the advances in digital crossovers, we believe that active bi-amplification will become increasingly more accepted in high-end audio. The digital crossover offers exceptional flexibility in customizing not only the amplitude response but also the phase response and delay. As with digital technology, the performance-to-price ratio of digital crossovers will continue to improve, and it is only a matter of time before they will become more accessible.
Unfortunately, at the moment, most loudspeakers are not designed with the active crossover in mind, and it involves removing the factory-installed passive crossover. Many audiophiles may not want to go this extent.
For most people, the best compromise between performance and practicality might be passive bi-amplification, using a pair of amplifiers to drive the existing loudspeaker. A pair of amplifiers with moderate current capacity, when used in passive bi-amplification, is sonically superior to a single amplifier with massive current capacity.
Benefit of Anedio’s Passive Bi-amp Implementation
Anedio's implementation of passive bi-amplification offers an important benefit: immunity to ground noise when two amplifier channels are tied to a single input.
A pitfall in bi-amplification is the potential ground loop introduced when the two inputs of the stereo channels are tied together. Depending on how the amplifier grounding system is designed, the additional ground noise can be quite high. That is why, in some systems, passive bi-amping actually degrades the sonic quality.
In the A1 Amp, the bi-amplification feature was conceived at the very beginning of the design stage, not as an afterthought and is optimized for the lowest ground noise. Even when the two amplifier channels are configured for bi-amplification, the noise level remains extraordinarily low. Moreover, the bi-amplification mode is activated with a built-in switch, and there is no need for an external Y cable.
Until high-performance digital crossovers become easily accessible, passive bi-amplification can serve an intermediate step that is sonically superior to a single amplifier with massive current capacity. The A1 Amp provides a robust way that enables the listener hear another layer of sonic information from existing loudspeakers.