15 ohm speakers

[John Roman]

I've heard of 4,8 & 16. But 15 seems a bit odd. Please enlighten me.

[Grainger49]

Maybe the maker is indicating that the speaker is more often 15 ohms than 16 ohms in the impedance curve.  Since cone speakers and speaker systems are a different magnitude and angle at every frequency there is a "nominal" impedance.  With SET amps it is important to look at the whole chart to see that the magnitude doesn't get too far from your output impedance (secondary of the output transformer).

In my humble (maybe not) opinion a single number is next to useless.  At best it is a general guideline that needs further investigation.

[Wanderer]

The speaker rated as 15 ohms that spings to mind is the original Ls3/5A, later/current editons are called 11 ohm.

Few speaker systems measure as purely resitive across frequency and, as Grainger49 states, one needs to look at the impedance curve to decide the speakers usablity with a given amplifier.   

You can see in the measurements of an "11 ohm" LS3/5A from a 1989 Stereophile review the impedance can be anywhere from 8 ohms to over 20 ohms at certain freqs.   

http://www.stereophile.com/content/bbc-ls35a-loudspeaker-1989-measurements

Kevin R-M

[Grainger49]

Here are charts showing Magnitude only and one showing both magnitude and phase angle from 10 Hz to 20k Hz for a loudspeaker I have owned.  You can save the JPG and zoom in to see the details.  The speaker is resistive at 8 specific frequencies and 8 ohms, the "nominal" impedance, at only two frequencies.  This was a great sounding loudspeaker.  Yet it didn't have anything close to a purely resistive load nor a fixed impedance value.  

The X-Y graph shows reactance (angle) and resistance (magnitude).  It is easier to see the relationship than the Stereophile chart that doesn't integrate the two.

I think that planar magnetic speakers are the only drivers that are purely resistive.  But, the crossovers in them make them look as reactive as the chart below.

Edit: In the second plot resistance, positive and negative (generating), are on the X axis.  The reactive component is on the Y axis.  Inductive reactance is above the X axis, capacitive reactance is below the X axis.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi244.photobucket.com%2Falbums%2Fgg7%2FGrainger49%2F38DImpedance.jpg&hash=05c78ed99ae2e926d6a2a566e8313873856dd140)

[Paul Joppa]

There are standard impedances which are applied to speakers, to reduce confusion about which tap to chose. In the US those were 8 and 16 ohms in the Good Old Days. There was also a widespread rating of 3.2 ohms for cheap utility speakers (intercoms, cheap radios, cheap PA, etc). 4 ohms is a more recent rating that became common about the time that solid-state amplifiers appeared.

In England the older common ratings were 3 ohms (cheap) and 15 ohms (high fidelity).

In recent years, many other values are used - probably because solid-state amps do not need to have taps selected for optimal power transfer.

In addition to the wide variation of actual impedance of speakers, most output transformers have secondary turns ratios of 2:3:4 or 3:4:6 relative to the 4-ohm tap. This gives actual impedance of 4, 9, and 16 ohms or 4, 7, and 16 ohms respectively. They are all still labelled 4, 8, and 16 ohms.

[John Roman]

Thanks Guy's!

[Grainger49]

   .  .  .   In the US those were 8 and 16 ohms in the Good Old Days.  .  .  .   

Did the AR 3 not come out in the good old days?  Weren't they 4 ohms just to make them hard to drive, or some other good engineering reason?

[Paul Joppa]

...
Did the AR 3 not come out in the good old days?  Weren't they 4 ohms just to make them hard to drive, or some other good engineering reason?

Well, I think I'm older than you, so to me the AR-3 was the first of the new age speakers designed for solid state amplifiers. It was less efficient than the previous norm, and the low impedance allows it to draw more power from a solid-state amp.

[Grainger49]

I agree on both counts.  You are a few years older and the AR 3 was intended for a solid state amplifier.  It insulted many that were out there when I sold them (AR 3s and SS amps).  The AR amp did a pretty good job with 60 per channel.  Compact little thing too.