Speaker impedance, amps and sound quality

[eriksquires]

Hi Everyone,

Sorry this is going to be a little long, but I've detected something and I'm wondering if you speaker builders have thought about this, or can refute me.

A long time ago I fully analyzed a Focal Profile 2.5 way floor stander. One oddity I discovered was the number of high power resistors in the low pass woofer section. It seemed to be designed to waste heat.

So I did some analysis with Xsim and managed to come up with a number of improvements which left the electrical transfer function in tact, while raising the minimum impedance from 3 ohms to about 5. (i'm a little foggy now).

Based on this, I concluded that the crossover was deliberately designed to dip in impedance, and I surmised it may have been so that it would appear more discerning. That is, it would require a bigger amp to sound fuller, and therefore appear to the audiophile to be a better speaker.

So my first question is, have you seen this elsewhere in commercial speakers?

Also, in a couple of cases I have seen where small changes in impedance, say from 3.5 Ohms to 3 Ohms really makes a significant difference in output, which implies that solid state amps are more sensitive than we think, based on damping factor, to speaker impedance when it's low.

Sadly I lack the ability to test anymore, or to do a lot of explorations. I just wanted to share this with you all and ask if you had similar experiences.

Thank you,

Erik

Specifically, the impedance dipped around 100Hz.

[denibeni]

If you check other Focal speakers, you can see an usual frequency bump around 100Hz on the cheaper floorstanders.

This is a byproduct of a passive low pass filter with low cross point. Manufacturers and designers can leave this alone for cost cutting, if the impedance dip is not too severe and if the sound is fine to them.

But often the impadence correction circuit is there to avoid the frequency bump can cause the impedance dip. In this case you need huge resistors to withstand the current flowing through the low impedance circuit

Can you post the original and the modified schematic?

[Lojzek]

Hey Erik, was it the schematic showing 2 large caps(200 uF, 400uF) each in series with 4 resistors in parallel and 2 inductors (5mH, 12mH)? I wonder sometimes what they were thinking and if they were thinking at all.

[Speaker Dude]

Here are a couple of reasons for high power resistors in the low pass section that I have seen. 1. Impedance compensation for the low pass part of the crossover. 2. Impedance compensation for the woofer resonance to make the high pass part of the crossover work correctly, especially for 2.5 way crossovers.

[weltersys]

Originally Posted by eriksquires

Also, in a couple of cases I have seen where small changes in impedance, say from 3.5 Ohms to 3 Ohms really makes a significant difference in output, which implies that solid state amps are more sensitive than we think, based on damping factor, to speaker impedance when it's low.

Erik,

More likely the "significant difference in output" between the small changes in impedance were due to the amplifier's current limiting than the difference in damping factor.

Art