Preamp and impedance question

@georgehifi Please excuse my limited knowledge, I don’t fully understand the phase angle and stuff.  But to clarify, my X5 has one 12” Eminence driver and one 12” powered Eminence subwoofer by Hypex Fusion NCore plate amp.  

The cross over frequency is 90Hz. AMT - 1 kHz and up
12” mid range - 90Hz to 1 kHz
12” powered subwoofer - 90Hz and below
But that 90Hz to 1 kHz is exactly where the impedance dips to 7 ohm.

actually, I can do a test on my current amp which supports 4 ohm.  And I have been connecting my X5 to 4 ohm.  I did a brief test earlier and I remember there is no difference on my X5.  But I remember there is an improvement to use the 4 ohm tap on my Zu Omen Def.  So, it seems this really depends on the speaker.  Maybe the phase angle is less relevant in a open baffle speaker?  
But I may change speaker or I may sell this.  Definitely having a 4 ohm tap is more future proof, but I am not sure if I want to spend 20% more for the unknown future.  If I happened to buy a new speaker, maybe I should just buy another better matching amp .... decisions,  decisions .... 

The phase element, if Georgehifi is relating it to EPDR is not relevant. It relates to thermal dissipation in output devices which tubes don't have an issue with.

gte357s OP
@georgehifi Please excuse my limited knowledge, I don’t fully understand the phase angle and stuff.

EPDR is the -phase angle (expressed in -degrees) combined with the low impedance measured, and represents a "far more severe loading" to the amplifier than just the impedance itself, don’t listen to any idiots that says otherwise.

Here is the Wilson Alexia, it's one of the worst bass loads I’ve seen, and so thinks HNRR and Stereophile, it has an EPDR of 0.9ohm!!!!. And the reason why you should get the 4ohm transformer tap as well as 8ohm.

https://ibb.co/stQ44Ry

https://ibb.co/C8d2Zny

Cheers George

This "idiot" obviously knows a heck of a lot more about amplifier design than you do. Do you even know what EPDR means? It is a measure of the voltage delivered / current delivered at a given frequency. Why does that matter? .... It matters because that comes into play for power dissipation in an amplifier in the linear region. Keep in mind, we are talking real music here, not continuous pure tones, so low EPDR only increases the dissipation a bit dependent on music material. EPDR raises the power dissipation at the device level, and for BJT devices, that can lead to BJT secondary breakdown. This is from a Toshiba app note.

Secondary breakdown is a failure mode in bipolar transistors in which negative resistance (current concentration) occurs under high-voltage and high-current conditions. Current concentration causes local heating, resulting in a small hotspot. The impedance of the hotspot decreases, causing further current concentration. This cycle called thermal runaway leads to device degradation and destruction.

MOSFETs and Tubes do have secondary breakdown effects, but their safe operating areas and secondary breakdown mechanisms are different and hence they don’t experience failure in the same way a BJT amplifier will from short term thermal events such as those that could be induced by low EPDR. Amplifier power dissipation at a given power output will go up with phase angle (assuming not pure class-A), but you don’t get the catastrophic failure of a BJT amplifier.

EPDR does not mean your amplifier has to delivery higher peak currents. An EPDR of 2 ohms, does not mean the current drawn is higher than an impedance minimum of say 3 ohms. What it means is the amplifier power dissipation is a maximum at the EPDR minimum. It would be rare that it would be at the impedance minimum (if at all), so the impedance minimum does represent the maximum current delivered by the amplifier.

Having a 4 ohm tap isn’t going to make your amplifier more robust to a lower EPDR than the impedance peak. It is quite meaningless within the framework of impedance matching of the amplifier, or the output impedance of an amplifier. EPDR just tells you for a given speaker what the potential maximum power dissipation will be in your amplifier output devices, assuming a particular amplifier architecture.