There was one question that could never escape the back of my mind since I have built my desktop version of LXmini—is it possible to fix the distortion issue with the full range driver? This issue was also the major negative point in the Erin's review of LXmini.
Because the full range driver used for LXmini—SEAS FU10RB—was released 15 years ago, I thought, perhaps there are any better alternatives on the market that have been developed with modern materials and better technologies, so they can provide more even (without the prominent bump between 1 and 2 kHz), and hopefully achieve a 10 dB or better improvement in the overall distortion level? I checked distortion measurements for various drivers available on Zaph|Audio and at Erin's Audio Corner, but could not find any small "full range" drivers that would come up as obviously superior to SEAS FU10RB in terms of distortion. One notable exception are drivers made by Purifi, however, with the current global trade situation getting them in the USA is very costly.
The Candidates
Scanning through the available stock of MadiSound and Parts Express, I have came up with a very scarce list of possible candidates for high fidelity full range 3"/4" drivers:
- SEAS MU10RB-SL—this is the "midrange driver" of the LX521 speaker (I guess, the "SL" suffix are the initials of S. Linkwitz). But the fun fact is that is has the same specs as FU10RB, except for the properties of the suspension. The suspension in the "midrange" version is stiffer, which limits the cone excursion, and as a result the driver has poorer bass. But the bass is not an issue for its use in LXmini—there is a woofer for that, and I was thinking that perhaps the difference in suspension makes a positive effect on reducing distortion (spoiler: not so much!). I haven't found any reputable measurements for this driver so decided to try to measure it myself.
- MarkAudio MAOP-5. This driver looks a bit exotic—it has no "spider" suspension part (the corrugated fabric supporting the cone), so in contrast to MU10RB-SL, this driver has less suspension force than FU10RB. I was a bit suspicious about the consequences of this design decision, but I have found no measurements for the distortion to reveal the effect of this approach so it was interesting to try to measure this driver myself.
- Tang Band W3-1878 also has an unusual look thanks to the massive motor and a specially designed "phase plug", which reminds me of grilles that are used on some measurement microphones. This driver was measured by the same Erin on a Klippel jig back in 2011, but the results were not cross-posted to his site. In the post Erin mentions that his distorion measurements are "relative", which most probably means they are not calibrated to a specific SPL standard, and frankly I did not understand how to read this particular distortion graph, but from Erin's own comments the distortion is on a good level.
And that's basically it! I have used two more drivers for comparison:
- One is obviously the FU10RB itself, to make sure that I'm using its measurements taken under the same conditions as from the contenders.
- And the midrange driver that I had recovered from a broken Cambridge Audio Minx Go portable speaker. It is build as a "full cone", and the cone is made of paper. This one I used as an "anchor" in my measurements—it would be a miracle if it could actually beat any of the speaker drivers above, and the "miracle" would mean that my measurements have gone wrong.
Measurement Setup
I used my QuantAsylum stack consisting of the original QA401 analyzer, QA460 transducer driver, QA492 microphone preamp (this model is relatively new), and Earthworks M30 microphone. I powered both the QA460 and QA492 from a portable Jackery battery because my mains power is rather noisy, and the laptop was also running on battery power. Still, initially I had some issues with the mains-induced noise which I was asking about on the QuantAsylum forum. As that thread indicates, I traced the issue down to a poorly shielded USB cable which I used to power the QA492. Also, after the conversation with Matt of QuantAsylum, I obtained shorting BNC plugs and used them to cover up any unused inputs both on QA401 and Q492. This has reduced electrical noise to the minimum.
Since Earthworks M30 goes beyond the standard 20 kHz range, I was running the measurements up to 35 kHz and was using 192 kHz sampling rate on QA401. I was only using the log sweep method which is sufficient to get basic understanding of the non-linearities in the measured system. I did not have enough spare time to run the stepped sine method with sufficient resolution, and I was interested in making relative comparisons, so using the log sweep was fine.
Impedance, Sensitivity, and Efficiency
First, I measured for each driver its impedance, sensitivity, and acoustic radiation efficiency.
Measuring impedance is straightforward with QA460. Below is the summary graph for all drivers:
The impedance plots are abbreviated as follows:
- CA is the Cambridge Audio driver;
- FU is the SEAS FU10RB (the original driver of LXmini);
- MA is the Mark Audio MAOP-5;
- MU is the SEAS MU10RB-SL (the bass-reduced version of FU10RB);
- TB is the Tang Band W3-1878.
We can see that both MU10RB-SL and MAOP-5 have a nominal 4 Ohm impedance. The FU10RB is also 4 Ohm nominally, but the actual impedance is more like 6 Ohm. And both W3-1878 and the CA speakers are honest 8 Ohm drivers, with CA being a true "midrange" driver has steeply increasing impedance above the midrange.
Sensitivity measurement was done by by applying to the driver under test a 1 kHz test tone with 1 V RMS amplitude, and measuring the resulting SPL from 1 meter distance. Due to no-baffle mounting the result is lower than the "official" sensitivity spec. The difference between the drivers is not very critical because I use a 100 Watt amplifier and listening to the speakers from 70–100 cm distance, so even an 8 Ohm driver can work well assuming that it is efficient acoustically. Below are my results:
| Driver | SPL dBA (1kHz/1V/1m) |
|---|---|
| Cambridge Audio (CA) | 60 |
| FU10RB (FU) | 62 |
| MAOP-5 (MA) | 74 |
| MU10RB-SL (MU) | 68 |
| W3-1878 (TB) | 68 |
It's interesting that MAOP-5 despite having the same impedance at 1 kHz as the MU10RB-SL ends up being louder. However, the FU10RB is also quieter than W3-1878 (TB) despite that the former has lower impedance. Is it because the non-linearity in the 1–2 kHz region causes losses in acoustic transfer? The Cambridge Audio driver is unsurprisingly the quietest because at 1 kHz it has effective impedance 16 Ohm.
And this brings us to acoustic radiation efficiency. In this test I checked what is the SPL level for the same 1 kHz test tone at the same distance of 1 m, but this time the level of the electrical input for each driver was adjusted to achieve 105 dB SPL at the 5 mm distance from the driver's cone. Note that this is different from the sensitivity, and characterizes the ability of the cone to work efficiently as an ideal piston.
| Driver | SPL dBA (1kHz/1m) |
|---|---|
| Cambridge Audio (CA) | 66 |
| FU10RB (FU) | 72 |
| MAOP-5 (MA) | 73 |
| MU10RB-SL (MU) | 72 |
| W3-1878 (TB) | 65 |
Here, both SEAS drivers and MAOP-5 show almost the same result, while both W3-1878 and the CA driver are 6 dB worse. The point of measuring the radiation efficiency is that even if one driver has lower distortion than another, and both have the same sensitivity, still due to lower efficiency one of the drivers has to be driven with higher voltage relative to another driver in order to achieve the same SPL at the listening position.
Impulse and Frequency Response
Now the results from a logsweep. Since I was using the driver in an open configuration, the same way as it is mounted in LXmini, and my room is small, I was measuring the logsweep in the close proximity of the driver cone—about 5 mm.
Below are impulse and step responses of CA and W3-1878 drivers:
We can see that CA driver is seriously damped and the impulse decays quickly. While W3-1878 is also damped well, it exhibits quicker back and forth motion, allowing for a wider high frequency extension.
It's interesting to compare the SEAS "siblings":
They look similar, yet the "midrange" MU10RB-SL exhibits more back and forth motion about 200 μs after the initial impulse, but then the oscillations become less severe, although the overall step response ends up being almost the same.
And now the interesting part, this is the IR of MAOP-5 driver computed from a 40–35000 Hz sweep:
The lack of damping is apparent here. Is it—is it good? Certainly not. Initially I thought that perhaps this is a kind of "exotic" drivers that promise "euphonic" non-linearities for a pleasant sound? I started experimenting with the test setup: first I swapped the QA460 amp for the same amp I'm actually using in my LXdesktop setup: QSC SPA4-100, but the IR stayed the same. Then I started playing with the parameters of the sweep and figured out that if I limit the high frequency range to the standard 20 kHz then the ringing is gone:
The IR still has more fluctuations than IRs of other drivers, but at least now there are no high frequency modulations. I suppose, the stiff material of the driver's cone causes it to go into very high frequency oscillations. Although these should be above the human hearing, they still can cause more non-linear behavior when excited. For this driver it is strictly required to use a low-pass filter.
And below are pairwise comparisons of frequency responses of all drivers corresponding to these IRs. In fact, the response of MAOP-5 driver does not change in the range of 40–20000 Hz regardless of the sweep's upper bound frequency:
These are "nearfield" responses so they are not super useful for evaluating a dipole. Still, we can see that CA driver is indeed a "midrange" driver with a steep downwards slope after 10 kHz, so it's clearly unsuitable for use in LXmini-based designs.
There is an expected difference between FU10RB and MU10RB-SL in the bass response, otherwise they are indeed very similar. And finally, this is the comparison between MAOP-5 and FU10RB:
We can see that the overall shapes of the frequency responses are close. One interesting point is the high frequency behavior of these full range drivers. Since all the cones operate in "break-up" mode, the material of the cone affects the response a lot. We can see that both MAOP-5 and W3-1878 have a null at about 9 kHz in this arrangement, while both SEAS drivers have two: near 5 and 7.5 kHz.
Distortion
Finally, the graphs we have been looking for. As I mentioned, the distortion measurement is derived from the same logsweep, I did not use the stepped sine method. But I have done the sweep at two SPL levels (as measured near the driver cone): 105 dB and 96 dB. Below are the measurements for each driver done at the higher level showing 2nd to 4th harmonics (the levels of other harmonics are benign):
And the summary graph comparing them:
Looking at the graphs, almost all the drivers seem to be from the same league—I do not see an obvious winner, but I do see an obvious loser: the CA driver again. For others, as we know, FU10RB has higher distortion levels in the midrange, and MU10RB-SL is not much better, and it has these strange peaks between 2–3 kHz and 3–4 kHz, although they are very sharp so likely not audible. MAOP-5 driver has issues in the 2–3 kHz region, while W3-1878 looks like the most linear with the exception that distortion seriously increases past 10 kHz.
This is a comparative graph at the 96 dB output level:
The peaks after 5 kHz seem to be measurement artifact as I see them for all drivers, it's just they are drowned in noise for other drivers but clearly visible for W3-1878 which was measured on a different day. I suppose, for cleaner results I would need to use the stepped sine method.
Conclusions
We can see that MU10RB-SL variation is not significantly better than the original FU10RB, only a bit. While W3-1878 driver can be thought as a winner from the distortion level perspective, recall that it has lower acoustic radiation efficiency, which means I might need to drive it harder in order to achieve the desired loudness at the listening position. So, it looks like in order to make the final decision I will need to build one sample of LXdesktop with MAOP-5 driver, one sample with W3-1878, and compare them with my original LXdesktop speaker, with all samples tuned to the same target, of course. That should be a fun experiment, looking forward to it!
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