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ATC SM75-150S:
A unique midrange driver
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The ATC SM75-150S is a unique midrange driver. It combines virtues of both professional
and domestic audio
drivers.
Dome shaped high frequency drivers are common in Hi-Fi speakers. They
have the advantages of simple construction, wide dispersion, and low
cost. Against them are low efficiency and limited excursion. The SM75-150S has a
relatively small dome diaphragm which maintains wide dispersion up to around
2.5-3kHz. It also has an excursion limit (Xmax) of 3mm allowing it's use
down to relatively low frequencies. Most speaker systems including ATC's
own propriety studio monitors that use the driver operate it down to
about 380Hz. Used over this frequency range it thus
covers the largest part of the midrange spectrum.
It's precision construction allows a very small gap between voice coil
and magnet assembly, along with an edge wound voice coil aiding power
handling and efficiency. It also has a massive magnetic structure with
an under hung voice coil that helps efficiency climb to around 94dB/w/m
in the middle of it's operating range.
The soft dome construction effectively damps the worst of its break up modes and longitudinal travelling waves within the diaphragm.
While the efficiency figure of 94dB sounds high, after the driver is
equalised to flatten it's amplitude response, the mid-band efficiency is
quickly chewed up and effectively drops to around 90dB/w/m.
 | on axis frequency response: |
The amplitude response is not as smooth or as flat as might be expected for driver that costs
so much or is held in such reverence. However the phase response is
remarkably smooth between 300Hz-4kHz. The above plot is
a 'nearfield' LF response merged at 700Hz with a 'farfield' high
frequency response
taken at 0.5m. The first major breakup occurs at about 4.5kHz and is
also evident in the impedance plot below. Below 1kHz the response rolls
off smoothly.
As mentioned, in order to use this driver in a system requires
considerable EQ. This isn't easily achieved with a passive crossover,
and the driver benefits from an active implementation.
| impedance of an 8 ohm driver: |
The impedance plot is well behaved with just a small glitch at 700Hz and a larger one at
4.5kHz. The driver resonates at 320Hz which is only marginally outside
the claimed operating range of 380Hz to 3.5kHz. Normally a midrange
driver is not used close to it's resonance frequency. But in the case of
the ATC mid, it works very well down low, sounding better crossed around
300-400Hz than higher up. Of course, when used below 400Hz, a steep
filter is recommended or power needs to be limited not exceed Xmax.
| off-axis response: |
The off-axis response is where the driver really shines. It's easy to
eqalise a driver to be flat on axis, but not so easy to equalise it for
a flat
power response as well. Only a driver with a consistent power response
can be equalised to achieve both. Below is an example of the ATC's
consistent off-axis performance. EQ has been applied to flatten the
response somewhat, and the consistency in amplitude response as the
measurement microphone is moved off-axis is easily seen; at 45 degrees
the amplitude response remains within +/-1dB between 400Hz and 2kHz.
Only above 2kHz does the driver start to beam and the power response
fall away consistent with size of the driver's diaphragm.
Note that the vertical scale is 1dB/division. By augmenting the ATC mid with a controlled directivity HF
driver, a smooth roll off in the system power response can be achieved.
By 4.5kHz things are getting nasty, so to use the driver up to around
3.5kHz as is found in ATC's own speaker systems suggests quite steep
crossover filters are needed.
| distortion |
Operating a small diaphragm at relatively low frequencies inevitably
means distortion will increase at the low frequency end of the spectrum.
Steep high pass filters will go some way to reducing distortion and in
total the distortion remains acceptable throughout the operating range.
| listening tests: |
The projection and dynamic impact of the ATC mid is reminiscent of
the best compression drivers but without the distortion and resonances
of the necessary accompaniment to the compression driver, the
conventional horn.
It really brings brass instruments to life with bite and clarity like
a compression driver/horn combination and yet copes well with complex
string passages giving a subtle and revealing performance. In fact
piano, and woodwind are just as convincing. The speed, accuracy and
detail bringing every nuance of the original to the fore.
The positive aspects of a driver with this kind of accuracy also
shows up every defect in the recording as well. Very little is hidden
from the listener. But having said that, listening to the SM75-150S is
always a joy. The magic doesn't fade away with extended listening.
Typically small flaws in a driver's character only become apparent after
an extended period, but with the ATC the performance becomes ever more
convincing.
| build quality & support: |
The ATC SM75-150S is a superb piece of engineering, extremely
heavy due to it's large ferrite magnet and heavy duty steel motor
assembly. Due to it's waveguide assembly at the front of the
driver it can only be mounted from the rear of the baffle making
mounting a bit awkward. But this isn't a big minus, and Solen make
a mounting ring if front mounting is essential.
This driver along with the rest of the ATC range of drivers are
used in propriety studio monitor systems and 3rd part speaker
systems and so spares will be readily available for many years to
come.
| conclusion: |
The ATC SM75-150S does so many thing so well that it is hard
to think of a driver that comes close to it's outstanding
realism. However it's not an easy driver to tame. It requires
careful equalisation to achieve it's best and crossover points
need to be carefully considered. Probably the greatest
difficulty in implementing a system design with this driver is
in choosing suitable speakers to complement it. Bass driver
integration is critical to getting the best sound and is made
difficult by the fact that the ATC mid sounds best when pushed
hard at the low frequency end of it's range. Few drivers make a
suitable complement. But in a suitably voiced system the
SM75-150S delivers on it's promise.
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