This post demonstrates the significance of the electrical interaction between an amplifier and its connected loudspeaker. Drivers, crossovers, and even cabinets all effect the impedence curve (and therefore output) of a speaker in ways that we cannot predict by simply looking at the characteristics of any one of these components in isolation.

Ultimately this illustrates why passive crossover design is very, very hard, and why different combinations of amplifiers and drivers - that otherwise generally measure flat - can and likely will sound different.

All this uncertainty is one of the big reasons why so much of the studio monitor market has moved to self-contained, bi-amped designs. Its impossible to predict how a customer’s amplifier is going to react to a passive monitor or vice-versa.

All measurements are taken at .145w, as this is the loudest I care to listen to sine wave sweeps for now.

To demonstrate this effect, here is the frequency response and phase rotation of a 60w 6L6 class AB amplifier into an 8ohm non-inductive load:

As you can see, this amp has a very nice flat output from 20hz-20khz.

Now here it is again being played into a Tekton Lore Reference, with our scope probes tapped directly into the wires at the speaker’s terminals:

What happened! Its a mess!

What we’re seeing here is the effect of impedence spikes caused by crossover, woofer, and cabinet resonances.

Those lower lumps have peaks at 25hz and 63hz. Maybe cone resonance and cabinet resonance?

Its important to remember that this is not necessarily equivalent to literal audio output - we’re only looking at voltage here and we don’t know the current. This demonstration does however illustrate the significance of crossover design - one of our biggest ’lumps’ in both voltage and phase rotation is right in the middle of our most sensitive hearing range. Its clear looking at this graph that the crossover center is somewhere around 2500hz-3500hz.

This should prove to be fairly concrete evidence that its not all audiophile nonsense when you hear someone talking about their preferences between different pairings of amplifiers and drivers, all else being linear.

The guitar community has come to know this as well, hence the emergence of “reactive loads” on the market that emulate the impedence curves of various classic drivers.

It is true that solid-state amplifiers with high damping factors will exhibit less of this effect - I hope to update this post when I get my hands on a suitable class AB ss representitive.