Selecting an 8 vs. 4 ohm woofer… potential sound quality effects

tajanes

(assuming having a stable amp that’s rated 2x at 4ohm vs. 8ohm)

What do you guys see as the +’s and –‘s to selecting 8 or 4 ohm woofer, for example in building a two-way, 2nd order LR speaker?

Regarding the woofer low-pass filter going from 8 to 4 ohms doubles the cap and cuts in half the inductor, so is there a sound preference to selecting either set of components (ultimately to consider in driving the woofer selection?)

Regarding the amp, even if rated stable at 4 ohms (and less) is it more advantageous to go with the 8ohm woofer?  i.e. run cooler, less distortion, less stress either way?

thanks

 

Wolf

Copper can be more costly than caps, so that does matter, as well as reduced DCR for the system. Generally, higher voltage sensitivity in the 4 ohm driver, lower Le, so maybe better freq response as well. It also changes other T/S, so modelling one over the other can be advantageous depending on the application.

dcibel

Le is "lower" in that it is proportional to the nominal impedance, so don't expect any real performance enhancements as a result. Amps will be able to produce more power as a result of the lower impedance, but generally also produce higher distortion into lower impedance loads, but probably not enough of a difference that people can discern. Cables and inductors will produce higher insertion losses when used on a lower impedance circuit.

A good comparison of drivers can be made by looking at Wavecor specs, since they generally have 4 and 8 ohm versions of their drivers and list them side by side on the spec sheets.

http://www.wavecor.com/html/wf120bd03_04.html

http://www.wavecor.com/html/wf152bd03_04.html

http://www.wavecor.com/html/wf182bd03_04.html

http://www.wavecor.com/html/tw030wa11_12.html

Overall, it really doesn't matter much. Using a single woofer, 4 ohm may be a good idea to maximize available amp power if you're limited there. Using pairs, choose 8 ohm and run them in parallel. 4 ohm tweeter may be a good option for sensitivity reasons only, otherwise I don't think it really matters much.

tajanes

thx for feedback, as always

I was also wondering/thinking say if caps (or inductors?) cause more sound issues/distortion than inductors (or is it visa-versa caps worse than inductors?) then, do smaller caps (values) cause less issues than larger values? (Given the same quality of cap). If so, that may be an additional reason to go for 8ohm woofer?  

rjj45

I don't have golden ears or a MS, but inductors have hysteresis, and caps do not. So arguably caps introduce less artifacts. 

jr@mac

Caps exhibit microphonic behavior, though. I measured it but admittedly it is way down on my concern scale. 

The drivers are always going to be the biggest contributor to distortion.

Ed_Perkins

Some quotes from Rod Elliott's passive crossover section (http://sound.whsites.net/lr-passive.htm ): "It is worth pointing out that inductors are, in general, the worst passive component imaginable. This is particularly true for use in crossover networks." and "By comparison to inductors, capacitors are positively benign."

See section 4.3 in the link for his rationale. 

tajanes

thanks for this link !

so, Q:  does size matter (sorry 'bout that)  i.e. does it follow that using a smaller value inductor (by selecting to go with a 4ohm woofer vs 8ohm) cause less problems?  or stated differently; is there a sonic advantage to using less of a bad thing?

dynamo

I don’t know much in the scope of things but two things come to my mind:

I’ve heard amplifier damping factor is higher into higher impedance.

Lower impedance allows smaller inductors making large gauge air core inductors more obtainable instead of needing to resort potentially to a ferrous core which may saturate/distort.

These are pros for both high and low impedances. Thoughts?

kennyk

I base impedance choices on the final product. High load for amp ease, low for spl/efficiency and the desired driver only comes in a single impedance.

tajanes

FYI, I rec'd an email back from Rod Elliott, summed as: 

"In reality, there will be little difference.  Although the inductance is smaller for a 4 ohm woofer, the wire has to be thicker because of higher current... 

Air cored inductors are at least linear, and their self-capacitance is usually small enough that there isn't much effect at audio frequencies... Resistance is more of a problem, as that causes losses across the full frequency range...

Everything is a compromise, but using an active crossover means that you don't need to use inductors at all (but you do need two amplifiers)."