Speaker Cable Different for 8ohm vs 4ohm

As four ohm speakers go, Maggies usually have a fairly benign curve and are usually not that hard to drive.

All other things being equal, if you have a 4 ohm speaker and an 8 ohm speaker, you will always find the speaker cable to have a greater effect on the 4 ohm speaker.

About 25 years ago I saw a nomograph published by RCA (if I recall right) that showed the impedance of the speaker, the output impedance of the amp, DC resistance of the cable and the damping factor the amp is able to express (Crown publishes something similar in one of their amplifier manuals). What you see is that in 4 ohm speakers if the cable has DC resistance of more than a few 100ths of an ohm, that the effect on damping factor is rather profound- much more so than one would intuit.

Essentially with 4 ohm speakers the cable becomes critical and should never be long- in most cases less than 6 feet (bass impact and resolution can both suffer)! By contrast a cable for a 16 ohm speaker can by much longer and not be nearly as heavy with no worries. That's why a lot of us old-timers got away with using zip cord in the old days...

This page of Stereophile's review shows the impedance curves of the MG3.6/R.

Regards,
-- Al

Al,

Where can I find an impedance curve graph for Magnepan 3.6's? I 'Googled' this but could not locate any graphs.

Hi Nick,

Yes to both questions. However, the most significant issue is not the voltage loss in itself, but the fact that the amount of the voltage loss will vary as a function of frequency, and thus it can affect tonal balance. While the Maggies that Stickman is asking about have a relatively flat impedance curve, as you no doubt realize the impedance of many speakers (Z2 in the equation) will vary widely over the audible frequency range. So even if Z1 is constant (i.e., a pure resistance), if Z1 is not small enough frequency response flatness may be affected to a perceptible degree. And Z1 can also vary as a function of frequency at least slightly, due to the effects of its inductive component at high frequencies. Also, excess resistance can degrade bass damping, especially for speakers having cone-type woofers.

That dependency of the effects of the cable on the impedance vs. frequency characteristics of the speaker is one example of why cable effects tend to be system dependent.

Regards,
-- Al

Al, your voltage divider example does clear things up I think.

Let me just clarify one point, is the goal for Vin and Vout to be equal (ie in perfect world where the speaker cable would have no resistance or inductance) such that the voltage output by the amp (Vin) would be the same at driver (Vout)?

Thus in the real world as Z1 increases then for a given Vin the Z2/(Z1+Z2) ratio decreases and reduces the value of Vout.

Thanks

So that is what a dead short is! I always wondered.

I find that using shorter speaker cables lessons the ability of the cables to actually physically reach to the location of my speakers, resulting in a distortion-free, low noise, utter lack of any sound at all.

Thank you for excellent information. I am using Magnepan 3.6's at the moment with six foot runs of DH Labs Silver Sonic Q-10 Signature biwires. The Q-10's use one run of smaller wire for the tweeter/midrange and a heavier run(12gauge I think) for the woofer connection. I've used this cable for years with no complaints. I believe that it is solid copper with a very thin cover of silver.

I am looking at the new Magnepan 20.7's and if I go there I will have to change the speaker wire since they are not bi-wireable as a result of the new crossover design. Also, I will probably want to extend the speaker cable length to around eight feet. The six foot runs are a bit tight.

I may just go with DH Labs single runs but I am open to sugestions for a really top notch speaker wire that works well with the 20.7's.

What values of resistance and inductance put a speaker cable in the 'low' category. What measured value should you look for? Also, does that mean good 12 guage is better than a good 14 guage when connecting to a 4 ohm speaker?

12 gauge will have lower resistance, and so it is "better" in that sense. However, depending on the length it may be overkill.

Assuming you want the cable to be as neutral as possible, the basic ideas are that:

1)You want to limit the resistance of the cable to a very small fraction of the minimum impedance of the speaker across the audible frequency range.

2)You want to limit the inductive reactance of the cable to a small fraction of the impedance of the speaker at upper treble frequencies (inductance might be a significant factor at those frequencies, but will not be significant at low frequencies).

Resistance and inductance are both directly proportional to length, as I indicated earlier.

You can calculate resistance from a wire gauge table such as this one. For a 4 ohm speaker I would try to keep the cable resistance in the area of approximately 0.05 ohms or less. That would be the resistance of the cable length x 2, reflecting the total length of both conductors. 0.05 ohms corresponds to the round-trip resistance through both conductors of a 10 foot run of 14 gauge wire.

For a speaker having an impedance of 4 ohms at high frequencies, I would try to keep the inductive reactance of the cable roughly in the area of 0.5 ohms or less at 20kHz, as calculated by the formula 6.28 x 20,000Hz x L, where L is inductance in Henries (for the combined inductance of both conductors in the cable, which is sometimes referred to as the "loop inductance"). A ten foot run of most and perhaps nearly all decent quality speaker cables will meet that requirement. Inductance is more likely to be an issue for speakers such as electrostatics, whose impedance may drop to very low values at high frequencies, and/or for run lengths that are particularly long.

Once those requirements are met, IMO what remains are effects that are pretty much unpredictable and/or unexplainable, that have to be addressed by trial and error.

Regards,
-- Al

So, what values of resistance and inductance put a speaker cable in the 'low' category. What measured value should you look for? Also, does that mean good 12 guage is better than a good 14 guage when connecting to a 4 ohm speaker?

A p.s. to my last post.

See this Wikipedia writeup on the voltage divider effect, and in Figure 1 think of Z1 as being the combination of the cable's resistance and inductive reactance, and Z2 as being the impedance of the speaker. That may help clarify why it is the relation between the two that is important.

Regards,
-- Al

The magnitudes of whatever sonic effects the resistance and inductance of the cable may cause, if any, are dependent on the relation between those parameters and the impedance of the speaker.

The higher the resistance of the cable in relation to the impedance of the speaker, the greater the possibility that it will be sonically significant. The higher the inductive reactance of the cable (the inductive form of impedance, which is directly proportional to frequency) in relation to the impedance of the speaker at high frequencies, the greater the possibility that it will be sonically significant.

Therefore lower speaker impedance makes it more important that cable resistance and inductance be kept low.

Resistance and inductance (as well as capacitance, which is generally a less important parameter for speaker cables but may affect amplifier performance if it is extremely high) are all directly proportional to length.

Regards,
-- Al

The 4 ohm speakers may draw more current than 8 ohm ones so larger cable and/or shorter runs may help. I have never seen any difference myself with decent speaker cable but could make a difference in some circumstances.