Impedance and tone

[leadfootdriver]

What would the sound differences be if I swapped out the same model of speakers between 8 ohms and 16 ohms in the same cab?

Is there a sound difference 8 and 16 ohm even if the impedance is matched?

[Steve Conner]

Hard to say. The cones should be the same, and the voice coils will have about the same weight of wire, it'll just be more turns of thinner wire, so you wouldn't expect a major difference. However:

Speakers vary between batches and also break in with age. If you bought some new speakers they would probably sound different even if they were the exact same model as the old ones.

Speakers in series sound different to the same speakers in parallel. When speakers are connected in series, the amp's output impedance can't damp the "odd" resonant modes, where one cone pulls in as the other pushes out, because the back EMFs for these modes cancel.

If the two speakers are identical and mounted in the same cabinet, and driven by a tube amp that can't provide much damping anyway, I wouldn't expect this to be a major issue. It could well mess things up in a hi-fi speaker though. I don't think I've ever seen a hi-fi design with drivers connected in series.

[leadfootdriver]

Thanks for the reply. So, slight variances in speaker tolerances aside, there shouldn't be much of a difference then.

But you bring up another tweet factor; series vs parallel.

What about parallel sounds? What happens to the damping when 4 speakers are in parallel? And as far as 4x12's go, is the traditional wiring series/parallel, or 2 pairs of speakers paralleled at the jack? People say there's a difference going 1 way or another, but can't agree on the definition of series/parallel! LOL

[Bruce / Mission Amps]

I have quite a few semi-pro customers who insist on using the max secondary output impedance with their rigs... swear up and down that their gear, as a whole, sounds better to them when running the entire secondary impedance.

[RudeBoy]

Speakers in series sound different to the same speakers in parallel. When speakers are connected in series, the amp's output impedance can't damp the "odd" resonant modes, where one cone pulls in as the other pushes out, because the back EMFs for these modes cancel.

Steve, would they not acoustically cancel?

[leadfootdriver]

I have quite a few semi-pro customers who insist on using the max secondary output impedance with their rigs... swear up and down that their gear, as a whole, sounds better to them when running the entire secondary impedance.

I did some searching on the subject, and the general opinion is that the amp compresses more.

[leadfootdriver]

This is from Steven Fryette:

"All impedance settings use all of the output transformer and deliver full power to the speakers. It's not a question of how much transformer is being used.

The difference in sound is attributed to:
1. The change in Damping Factor (google it) which increases as the impedance decreases. Net result is tighter, more focused sound at 4 ohms, looser and more open sounding at 16.
2. Difference in the way speakers interact with the amp and with the other speakers when all are connected in parallel(4 ohms) vs when all are connected in series/parallel(16 ohms). Speakers interact less when all are parallel, more in series/parallel."

[Steve Conner]

RudeBoy, not necessarily if they were at high enough frequencies that the path length differences became significant.

I think Fryette is confusing a few issues, perhaps deliberately.

The only kind of output transformer that uses "all of the secondary" for all impedances is the hi-fi type where the winding sections are reconfigured in series or parallel by a multipole impedance switch. Even this doesn't use the full secondary for every impedance setting. Switching between series and parallel gives you a factor of 4 impedance change, say either 4 ohms or 16. So another little winding section is added that only gets used for 8 ohms.

For the guitar amp type of OT with a tapped secondary, the highest impedance tap gives the highest coupling and the widest frequency response.

The biggest effect on damping factor is whether the speaker is connected to the same tap as the NFB. This gives the highest damping, and every other tap has a lower damping factor.

This assumes all other factors held equal, so when switching from the 16 ohm tap to 8, we also swap out our speaker for an 8 ohm one. If you drove a 16 ohm speaker off the 8 ohm tap, you would certainly get a higher damping factor.

[Enzo]

I think it also matters whether you are looking for night and day differences or the subtle nuance that a lot of this offers.

[Leo_Gnardo]

1. The change in Damping Factor (google it) which increases as the impedance decreases. Net result is tighter, more focused sound at 4 ohms, looser and more open sounding at 16.

Agree. Careful listening tests at Criteria Miami @ April 1986 brought this conclusion to me, also to some "golden ear" engineers & rock stars. Used Marshall 4x12's with Vintage 30 and G12-80.

Both are good, select wiring method for the result you seek.

[jazbo8]

1. The change in Damping Factor (google it) which increases as the impedance decreases. Net result is tighter, more focused sound at 4 ohms, looser and more open sounding at 16.

That's backwards, 4 ohms load has less damping than 16 ohms which suggests a looser low-end... How does it produce a tighter, more focused sound? Damping factor - Wikipedia, the free encyclopedia

[g1]

1. The change in Damping Factor (google it) which increases as the impedance decreases. Net result is tighter, more focused sound at 4 ohms, looser and more open sounding at 16.

That's backwards, 4 ohms load has less damping than 16 ohms which suggests a looser low-end... How does it produce a tighter, more focused sound? Damping factor - Wikipedia, the free encyclopedia

Damping factor is Zload/Zsource so, as jazbo stated, it is backwards. Damping factor increases as the SOURCE impedance decreases, not the speaker impedance. Or if the speaker impedance increases and the source impedance remains constant.
So if we are using impedance taps to match impedance, damping should remain the same. It would only change if we mismatch impedances (different speaker impedances, same OT tap).
On top of all that, better damping is opposite of bass response. From the same wikipedia entry: "A lower damping factor helps to greatly enhance the bass response of the loudspeaker". So tighter bass is not the same as "more" bass.

[Gingertube]

If you swap in a16 Ohm speaker for 4 Ohm and then change the OT tap in the amp to 16 Ohms, then the damping factor is unchanged. Those claiming to hear differences are deluding themselves.

There are some issues worth noting.
If you have a choice between wiring speakers in parallel rather than in series then that is always better. Example: for an 8 Ohm box choose 2 x 16 Ohm speakers in parallel rather than 2 x 4 Ohms in series. The speakers in parallel will help damp each other electrically, just as the output impedance of the amp does.

The HIFI boffins will talk about the Q of a speaker cabinet (Q being the inverse of the damping).
A speaker cabinet will have a Qts (I think of this as Q "total speaker" but it is more likely that it stands for Q "Thiely/Small" after the two (Aussie) boffins who developed the theory and the maths for this stuff).
Qts is the RMS value of the Qes and the Qms
- Qes is the electrical Q and is damped by the the output impedance of the amp and the impedance of any parallel speaker
- Qms is the mechanical Q and is dependent upon things like the stiffness of the speaker cone and spider, the volume of the enclosure, the enclosure being sealed or unsealed or ported etc. and is NOT influenced, in anyway, by the output impedance of the amplifier but MAY be influenced by a parallel speaker due to mechanical influences (for example: both speakers trying to drive in the same direction against a back pressure).

If the speaker cabinet Qts is dominated by the Qms term then the damping factor is of less importance (as will be the case in sealed boxes and ported boxes).
If the Qts of the speaker cabinet is dominated by the Qes term then damping factor becomes more important (as will be the case for combos and open back enclosures).

The output impedance of an amplifier will consist of the output tubes effective internal impedance reflected from primary to secondary, plus the DC resistance of the secondary winding. The DC resistance is typically about 0.1 to 0.2 Ohms and so It will have no significant effect on output impedance regardless of whether we are using the 4 Ohm or the 16 Ohm tap.

I see no direct correlation between impedance and tone however there may well be correlation between how that impedance is formed (in terms of series or parallel speaker connections) and tone.

Cheers,
Ian

[J M Fahey]

Two versions of same speaker, the higher impedance one has a lighter coil and higher BL product, both factors contribute to a "snappier" tone.
Akin to higher slew rate in an amplifier stage.
We are talking round wire voice coils, of course.

[Gingertube]

Two versions of same speaker, the higher impedance one has a lighter coil and higher BL product, both factors contribute to a "snappier" tone.
Akin to higher slew rate in an amplifier stage.
We are talking round wire voice coils, of course.

Juan,

Higher Impedance means more turns of lighter guage wire - so lighter coil...maybe / maybe not.

Higher BL product - more turns means more L (length) so absolutely right, that would change the Qes. so things are'nt as "black and white" as I suggested above.

Cheers,
Ian