I wonder if anyone knows of high efficiency (95 dB/W/m or better) 16ohm 15" drivers (under $200 each) similar to these Delta 15B but with higher Xmax. The Xmech is about 11 but Xmax is only 1.6. Qts needs to be at least 0.5 or maybe 0.4 if fs is 35Hz or lower.
https://www.parts-express.com/Eminence-Delta-15B-15-Driver-16-Ohm-290-419
https://www.parts-express.com/Eminence-Delta-15B-15-Driver-16-Ohm-290-419
Faital Pro looks like the ticket here. Great efficiency, good Xmax, low Fs. Might be over your budget though but worth it.
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The $305 15FH500 specs like a high power '60s era 16 ohm Altec 803, so a ~ $105 for an extra ~8 mm Xmax seems a great bargain to me!
Expensive - maybe that's partly down to the neo magnet.
BUT - beware Faitalpro's Xmax figure, it's very optimistic compared to more usual assessment. They quote Xmax= [(winding depth - magnetic gap depth)/2] + (magnetic gap depth/3.
For this particular driver, the more usual figure would actually be 4.75mm (peak, or 9.5mm p-p), as opposed to "8.75mm (peak)".
That brings up a couple of questions like, Why would the 16ohm versions of the same driver have 40% less Xmax than the 8 ohm? Is there something about the geometry that has to change to get the extra wire in the VC? Also I wonder how much the distortion increases when exceeding Xmax but staying with in mechanical limits since in this case Xmech is almost 7 times Xmax. Has anyone ever graphed increase in distortion (and spectrum thereof) when exceeding Xmax? Might be an interesting study. It might be interesting to know if it is a linear increase or perhaps a second order function (K*(X-Xmax^2)).
A) The 16ohm version probably has a shorter voice coil height than the 8ohm.
B) Thinner voice coil wire..
C)Yes, Data Bass has many examples:
https://data-bass.com/#/?_k=3v6efr
From the Eminence website:
"Although suspensions can create non-linearity in output, the point at which the number of turns in the gap (see BL) begins to decrease is when distortion starts to increase. Eminence has historically been very conservative with this measurement and indicated only the voice coil overhang (Xmax: Voice coil height minus top plate thickness, divided by 2). The Xmax figures on this website are expressed as the greater of the result of the formula above or the excursion point of the woofer where THD reaches 10%. This method results in a more real world expression of the usable excursion limit for the transducer. Xlim is expressed by Eminence as the lowest of four potential failure condition measurements: spider crashing on top plate; Voice coil bottoming on back plate; Voice coil coming out of gap above core; or the physical limitation of cone. A transducer exceeding the Xlim is certain to fail from one of these conditions. High pass filters, limiters, and enclosure modeling software programs are valuable tools in protecting your woofers from mechanical failure."
What application are you planning to use 16ohm drivers for?
Art
That's interesting - we have another manufacturer with another way of defining their Xmax figures. Fair enough really, as a long voicecoil and large magnetic gap depth will make for a more gradual increase in distortion as the voicecoil starts to excurse beyond the magnetic field.
It's a pity there doesn't seem to be a standard for this - it would make comparisons easier. But never mind, since many manufacturers do publish gap depth and voicecoil length measurements, maybe these should be used for comparisons, rather than the Xmax figure alone...
As an example, a driver with twice the gap depth and twice the voicecoil length (but the same overhang) will have half the rate of increase in distortion as the overhang is exceeded (I would guess).
It's a pity there doesn't seem to be a standard for this - it would make comparisons easier. But never mind, since many manufacturers do publish gap depth and voicecoil length measurements, maybe these should be used for comparisons, rather than the Xmax figure alone...
As an example, a driver with twice the gap depth and twice the voicecoil length (but the same overhang) will have half the rate of increase in distortion as the overhang is exceeded (I would guess).
I am looking at options for a high efficiency build where I am trying to get as close to 100dB/W/m as possible. A pair of high efficiency 15s is one approach and I would prefer to have a system Z of around 8 ohms rather than 4. A single 15 may get to 96dB or so which may be what I have to settle for in the end but right now I am just surveying the landscape.
One 18" may be another possible option of course and I haven't completely ruled out a 4 ohm system as the only real issue would be the need to invest in more expensive output transformers but that may well be less of an issue than the cost to reach the 8 ohm goal. Of course the midrange has it's own issues because the list of direct radiating mid/full range drivers running near 100dB is also kind of short. In the end anything from 95dB and up should work fine. Even single ended 6EM7 should fill a modestly sized room nicely at that level.
One 18" may be another possible option of course and I haven't completely ruled out a 4 ohm system as the only real issue would be the need to invest in more expensive output transformers but that may well be less of an issue than the cost to reach the 8 ohm goal. Of course the midrange has it's own issues because the list of direct radiating mid/full range drivers running near 100dB is also kind of short. In the end anything from 95dB and up should work fine. Even single ended 6EM7 should fill a modestly sized room nicely at that level.
Why dont you think about the Faital 18" 16ohm woofer? High SPL and high impedance looks made for tubes?
Yes, you are right, that's just a guess. Often the suspension of a low-frequency driver is more a limiting factor distortion wise than the mathematical xmax.
For example:
"The 18S450’s displacement limiting numbers were XBl at 70% (Bl decreasing to 70% of its maximum value) was greater than 14.3 mm. The crossover (XC) at 50% (compliance decreasing to 50% of its maximum value) was 10.5 mm, which was fairly close to the 12-mm physical XMAX. For the 18S450, compliance is the most limiting factor for the 20% prescribed distortion level."
https://audioxpress.com/article/Test-Bench-BMS-18S450-18-Pro-Sound-Subwoofer
Yes, the motor system design is really just down to juggling between sensitivity, SPL/frequency and magnet total flux. The thing that really makes for a point of difference is the linearity of the suspension compliance - but mainly in the context of relatively small cone area (and therefore large excursions).
I could but that would reduce sensitivity rather than increasing it. At least I think it would. It gets a little confusing since the manufacturers insist on specifying sensitivity at the same voltage level regardless of impedance. The 2.83V would be divided between the two drivers. So they would not reach their specified output.
The target is 8ohms but if I end up having to get new OPTs anyway that would be an option. Of course a lot of newer commercial tube amps don't include 16 ohm taps so that would limit options somewhat.
Use of a pair of drivers gives you +3dB (owing to the better acoustic coupling afforded by having twice the cone area), and lower distortion (owing to lower excursion) - compared to a single driver running at the same overall power as the pair.
Since we're comparing 4 and 8 ohm versions of the same driver, you should find that the 4ohm version is specified as having 3dB higher sensitivity than the 8 ohm driver at 2.83V. When you connect them in series, they each receive half power, but overall power is the same as for a single 8 ohm driver. But you get the 3dB gain mentioned above.
A similar (but opposite) situation exists for the pair of parallel-connected 16 ohm versions - total power remains the same, but you get the 3dB acoustic gain.
Correct impedance matching at the transformer secondary gives you the voltage appropriate for the given power supplied to the speaker - unlike the situation with a solid-state amplifier.
2 x 4 ohm drivers in series=8 ohms (at some frequency..) 2 x 16 ohm drivers in parallel=8 ohms, 2.83 volts into 8 ohms=1watt. Presented with 2.83volts, the drivers each get 1/2 watt in either case.
If you are planning to use a single ended 6EM7 (about 2 watts output) there is no worry of reaching 1.6mm excursion.
Just to sum up - you can get greater sensitivity by doubling up on drivers, but it's limited to a 3dB gain (by virtue of better acoustic coupling). And that, for a given (single) driver, power conversion from electrical to acoustic energy will be the same, regardless of voicecoil impedance*. The amplifier's output transformer provides the correct voltage for a given power level, assuming correct impedance matching.
*Some drivers may be unusual, in that (power) sensitivity varies with the voicecoil impedances on offer. That's up to the manufacturer, of course.
*Some drivers may be unusual, in that (power) sensitivity varies with the voicecoil impedances on offer. That's up to the manufacturer, of course.
Keith and Welter; Thanks for the explanation. Very helpful. Finding appropriate 4 ohm drivers is probably an easier task.