Why good amplifier control of low frequency cones is essential in critical listening reference loudspeaker systems.

Or maybe; Why allowing the customer to choose whatever amplifier they want is a really bad idea in critical listening reference loudspeaker systems.

Thankfully more and more manufacturers of critical listening loudspeakers are incorporating the amplification into their systems, but some still sell passive box, amplify it yourself, systems. It is our opinion that selling a passive box is a very bad idea indeed, if you intend to sell a reference loudspeaker.

Firstly, passive crossovers almost completely decouple the drive units from the amplifier, the amplifier has a very soft drive of the loudspeaker and the loudspeaker itself is free to resonate to a large extent masking the drive signal time response with a time response of its own. Additionally, complex passive crossovers are reactive devices that can introduce unwanted properties into the signal path, sometimes even presenting impossible loads onto amplifiers causing non-linear operation, stress, or failure.

Secondly, many amplifiers performance can be load dependent, and many loudspeakers performance can be source impedance dependent. The same loudspeaker with a different amplifier can have a vastly different sound, just as one amplifier can perform very differently with different loudspeakers. Where is the reference in that?

Let us look at one of the supposedly critical Thiele-Small parameters; Free Air Resonance. It assumes that a loudspeaker in a box under drive resonates at a specifically measured frequency, and the system can be tuned based upon that. In a case of a reasonably high impedance source playing through a passive crossover where the loudspeaker has little electrical braking force acting upon it, these assumptions are true. However, if we take a modern drive unit with a strong motor structure (Large magnet), short thick loudspeaker cables, and a high power low impedance amplifier we find that the cone has comparatively no resonance at all. The amplifier has locked the cone in position and will not allow any free movement of that cone.

At Electromotive Laboratories we utilise this property to enable us to drive loudspeakers way below frequencies they would normally be comfortable working at. Our self-amplified systems have immense control over the cone movement, some of them even extending the amplifier feedback loop as far as the drive unit itself.

Below we can show an example of how a tightly controlled cone is able to exhibit immensely better impulse response and a far shorter decay time than the same cone with no load blocking it, or similarly driven through higher resistance loudspeaker cables and passive crossover networks.

It is worth noting that both graphs are “full scale” referenced and that the second graph would actually be lower in level if the reference was absolute dB reference. The actual difference can be seen in the three signal spectrum graph below where the peak if the second waterfall would be 10dB lower than the first.

Here we see the Energy time graph of the two situations. The graph on the right being the cone connected to the low impedance source.

Below we see a spectrum analysis of the frequency content (not the time response) of three situations.

The cone was mechanically driven with an impact.

In pink we have the cone loaded with a powerful low impedance source.

In green we have the cone loaded with a less powerful higher impedance source.

In light blue we see the cone free to move with no load.

The drive force was always equal, the greater low frequency content shows that the analyser is integrating the extended time of the signal as extra low frequency energy

A mechanical stimulus analysis of cone properties.

We can conclude from this that a loudspeaker driven by a closely coupled, strong, low impedance source will in almost all circumstances exhibit a far better time response than one with a higher impedance source. A loudspeaker driven by a weak higher impedance source will exhibit a longer resonance.

Additionally, a loudspeaker driven from a strong low impedance source will not significantly resonate at all in comparison to a higher impedance source.

In terms of cabinet tuning, and piston motion control, this changes everything.

Conventional cabinet tuning calculations are all out the window. We have to look at different ways to tune systems, and such new approaches have allowed us to extend dive units with great time responses down way below what conventional tuning would have us believe was sensible.

It still requires a specific type of drive unit, but it is all very good solid acoustic theory. We are not using complex trickery, nor excessive time damaging EQ. We certainly are not using resonant systems, quite the opposite.  Fix the time domain problems and we find we can do far more in the frequency domain.