Many audiophiles often wonder: "Are these speakers difficult to drive?" For most speakers with an efficiency higher than 85dB, they should generally be manageable, but there are definitely some speakers that are exceptionally challenging. These speakers are sometimes referred to as "hard-to-drive" or "demanding" speakers. Some of these difficult-to-drive speakers are inefficient and expensive bookshelf models (often known as demanding speakers). They have high demands on amplifiers, requiring not only a large output power but also sufficient output current and excellent damping characteristics. Without these, the results are often less impressive compared to average speakers. This is something everyone should be fully aware of. Sometimes, to properly handle these speakers, the cost of the amplifier can be several times that of the speaker itself. Some people might just switch to a different speaker, while others, who are passionate about their unique sound, invest more money to find the right amplifier. A prime example is the Rogers' LS3/5A. Fortunately, due to advancements in technology, there are now many bookshelf speakers that offer great sound quality without being overly demanding.
One common complaint among enthusiasts is that many speakers with excellent sound quality don't perform well when paired with a general amplifier. This is often perceived as the speaker being difficult to drive. The difficulty in driving a speaker is closely tied to the trend of the impedance curve, sensitivity, phase angle shift, and the strength of the counter electromotive force.
First, let’s talk about the impedance curve. In the specifications of a speaker, we often see the nominal impedance listed as 8 ohms or 4 ohms. These numbers are just approximations because no speaker maintains a constant impedance across the entire audio frequency range of 20Hz to 20kHz. Impedance values can fluctuate dramatically, sometimes reaching as high as tens of ohms or as low as 1 ohm.
How does the changing impedance curve of a speaker affect the amplifier's performance? Remember that the power output of the amplifier is determined by the load impedance of the speaker. If a rear stage amplifier claims to produce 100 watts at 8 ohms, the output could drop to 50 watts at 16 ohms, and even less under other conditions. Conversely, when the impedance drops to 4 ohms, the output might surge to 200 watts, and at 2 ohms, it could go as high as 400 watts. As the speaker's impedance increases, the output of the amplifier diminishes. However, when the impedance drops, the amplifier's output doesn’t necessarily increase proportionally. If the amplifier's power supply cannot meet these demands, it won't be able to achieve 200 watts at 4 ohms, let alone 400 watts at 2 ohms. Even if the power supply has sufficient capacity, another issue arises: can the power transistors withstand such high voltages or currents?
Although 4-ohm speakers require lower voltage compared to 8-ohm speakers, they demand significantly more current. For instance, at 4W output, an 8-ohm speaker requires 0.7A, whereas a 4-ohm speaker needs 1A. This is why people often say that low-impedance speakers are harder to drive—they “consume†more current, necessitating a powerful amplifier capable of delivering large currents. The lower the impedance of the speaker, the more the amplifier's output power increases.
The impedance curve is one of the key factors determining whether a speaker can be driven effectively. Dynaudio speakers, for example, are notoriously difficult to drive due to their aluminum voice coils, which cause significant fluctuations in the speaker unit's impedance (ranging from 3 to 30 ohms). Therefore, an amplifier lacking both high voltage and high current output (essentially a requirement for high-powered monsters) will struggle to deliver a balanced sound. Using an underpowered amplifier can lead to a thin sound, poor bass extension and definition, a narrow soundstage, and a lack of depth. However, with a sufficiently powerful amplifier, all aspects of the sound improve dramatically.
Next, let’s consider the sensitivity of the speaker. On the surface, a 90dB-sensitive speaker might seem superior to an 86dB-sensitive one. However, sensitivity tests measure the total sound pressure emitted by the entire speaker system, not the individual contributions of the high, midrange, and low-frequency drivers. So, when 100 watts of power are fed into the high, mid, and low-frequency drivers of a three-way speaker, the crossover network consumes some of this power before distributing the remainder to the drivers. At this point, the efficiency and impedance curves of the three drivers result in varying responses to the input power, leading to differences in the volume levels of the high, mid, and low frequencies. Often, if we notice a lack of low-frequency presence, we conclude that the speaker is difficult to drive. Even if the specifications claim high efficiency, it can still be challenging to achieve a balanced sound. This type of difficult-to-drive speaker often comes with another issue: the tweeter tends to be easier to drive. With the woofer struggling and the tweeter performing well, you can imagine the frustration—weak bass and harsh highs.
Low sensitivity means the speaker requires ample power to perform well, such as the renowned LS3/5A speaker. With an impedance ranging from 11 to 15 ohms and an efficiency of only 82dB, this combination of high impedance and low efficiency is one of the main reasons why the LS3/5A is challenging to drive. Some people try using high-powered amplifiers, but the LS3/5A struggles to handle excessive power. Too much power can easily overdrive the bass, making the KEF woofer lose its dynamics.
Another factor is phase angle shift, which involves the complex interaction of the speaker's capacitive reactance, inductive reactance, and impedance. Since speakers are influenced not only by electronic interactions (like passive crossovers) but also by mechanical ones (such as cabinet design), along with air volume, these elements interact in complex ways. This results in the amplifier constantly battling against the speaker's resistance, impedance, and inductive reactance, contributing to the difficulty in driving the speaker.
Fourthly, there is the counter electromotive force (EMF). Think of the speaker driver as a generator with coils and magnets. When the amplifier's current drives the diaphragm to move, the speaker generates an induced current, which flows back into the amplifier. In Class AB amplifiers, this phenomenon is known as a counter EMF. The stronger the back EMF, the harder it is to drive the speaker. Due to its direct connection to the speaker, the amplifier is particularly vulnerable to this back EMF.
Lastly, some speakers have complex crossover circuits with large capacitors, resistors, and inductors, consuming significant energy. To ensure a detailed sound across all frequencies, more power is required.
There are also issues with the speaker's diaphragm support structure. A softer structure can lead to uncontrolled free vibrations, resulting in a booming, unsupported bass, poor control, and long decay times. A speaker with a high damping factor should be used to suppress these unwanted vibrations effectively.
Conversely, a stiffer diaphragm structure can make a speaker feel like it lacks low frequencies when driven by a low-power amplifier. The sound tends to lean towards mid and high frequencies, sounding dry and harsh. Such speakers need an amplifier with high dynamics and large peak output current to introduce a sense of low-frequency and balance the overall sound. We call this type of speaker "dynamic current hungry."
Some speakers combine multiple challenging characteristics, making them particularly difficult to drive. For example, a soft diaphragm support structure coupled with low sensitivity makes these speakers truly difficult to manage.
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