Trying something different

It's been a while (well, over half a year!) since the last post, as haven't had a lot of free time... but something I have been working on is trying some new speaker designs.

For a while now, I've wondered if gains could be had by using a midrange driver that is more naturally suited to, well... being a midrange.  The Lambda TD15M Apollo is a remarkable driver in many ways, but despite its curvilinear cone, is probably best crossed below 3kHz due to the off-axis response pattern - which is fundamentally limited by the size of the radiating cone.

I was after more flexibility and also the chance of using a pair of midranges in MTM around the Raal unit.  In my own experience, I've generally enjoyed listening to MTM equivalents (for example, JMLab Utopia Mini vs. Micro) somewhat more... a part of this is probably due to the radiating pattern being a little closer to the ideal point source when the crossover is suitably done, in addition to the usual benefits of increased power handling.

At first, I was looking at conventional woofers.  The SBAcoustics Satori range have had extremely good press... a very advanced motor system combined with a free-breathing basket and a paper cone with well-controlled breakup modes... in fact, on initial inspection it looks like it could be usable up to 10kHz!  Very impressive indeed.

A couple of small flies in the ointment.  The efficiency is poor, as it is not designed as a pure midrange...  it's in the region of 88dB/W, and there's what looks like a very big dip at 1.2kHz.  And as it's probably designed to go in a vented cabinet, that powerful neodymium magnet system results in a Qts of 0.33.  I've no interest in putting my midrange in a cabinet, so this is going to be underdamped and with somewhat rolled off upper bass... a quick sim suggests a -1dB of around 200Hz free air, plus there's the baffle step on top of that... and I don't like having any significant baffle to speak of.  Tricky.

Ok... so maybe these aren't the best choice for me.  What else?  I thought about some Jordans... they are nice sounding units in my experience, but once again, low Q and very low efficiency - a poor match on paper to the Raal ribbons.

So... something that is designed to be used free air is a good starting point.  What about a planar?  The B&G Neo 10s might prove to be a good fit... they had a nice write up on Zaph Audio... and over time he hasn't liked many planars - mainly due to their poor measured performance... these managed to shirk the trend.

I do not have a great past experience with planars.  I did have a pair of Stryke Audio RTW2s (also known as the Silver Flute YAG20), a long time ago.  They were cheap, and had a sound to match.  It was smooth, sure, but incredibly muddled... when Zaph measured the Silver Flute version, it became apparent exactly why... the graphs show strong harmonic and intermodulation distortion.  I had no interest in purchasing any planars after that until I'd heard some good things.

Bohlender and Graebener have had a good reputation for planars as long as I can remember.  Before the new Neo family, they were probably best known for long planars such as the RD75.  And I mean long... the 75 indicates 75 inches long!  While the RD range are reasonably expensive, they are far from unreasonable to have a driver that covers such a wide frequency range.  Their disadvantages... ferrite magnets and a steel frame makes for a fairly inefficient and heavy unit.

The Neo10s, as the name suggests, use neodymium magnets instead of ferrite.  While this hasn't had much effect on the weight, due to the heavy steel frame to give the unit rigidty, it has had a considerable effect on the efficiency,  In fact, the Neo10s are more efficient that the Raals.  It's also had another, less desirable effect... cost.  The Neo10s are around 270 euros each before tax here in europe at the time of writing.

But still... they looked promising... so I bought a quartet to try.  As with the RDs, the number gives an indication towards the diaphragm size...

 
Based on a few implementations I'd seen, I thought I'd try a vertical arrangement... probably staggered and mirrored to reduce edge diffraction and gain a more even tonal response.  To do this in a low cost manner, I decided to use the thinnest plywood feasible and mount it up using PVC waste piping.  This meant a self-supported frame for a total cost of probably less than 30GBP.  The piping was hot-glued to the back of the plywood, and then angled pieces were used to create a basic frame...

It isn't the last word in rigidity, but it wasn't handling the bass frequencies... the crossover is around 500Hz so the air shifting is mostly being handled by the bass unit.  I've misplaced the measurements, but it was quite smooth across the critical area in the listening position, but the baffle was providing too much colouration, despite the bloom working very well in some recordings.  Dipole systems do not respond well to thick baffles - you gain in rigidity, but the sheer bulk creates other problems... the ideal (for me, anyway) is to not create any early reflections.

So I reconsidered my options.  Perhaps a metal frame would work better.  So I went back to B&Q and got some perforated frame... this is the kind of stuff that shelving is sometimes made from... angled steel with elongated holes.  Steel angle is very rigid, even in long lengths, so once the drivers are mounted, it should not need any real bracing if I keep the form narrow.

I would be able to mount the Neo10s directly to the frame, but the Raals are a little narrower.  I bought some aluminium sheet and made some helper pieces to allow it to be mounted in place.  Pairs of L brackets allowed the unit to stand up with reasonable stability.

I thought they looked promising, and in some ways the sound was dramatically better... the "woody" sound had gone, but they were extremely position sensitive.  I'd forgotten one of the important aspects of any MTM... combing.  Depending on listening position, sound from the top and bottom midrange drivers will cancel out... the wavelength dependent on the distance between the two units.

As everything here is mounted vertically, the null point is in quite an audible spot... right in the midrange.  Not good.  In addition to this, the extremely narrow baffle in combination with the free air mounting meant the Neo10s were starting to roll off around 1kHz... agh.  That's too high, the Dipole15s are not happy going above 500Hz without some serious EQ, and they should really be left to lower frequencies to get a nice, even response with no breakup.

Ok... so the vertical arrangement was not working well.  What if things are turned 90 degrees?  Well... the sweet spot vertically will be much smaller, but the drive units can be brought into a much tighter cluster... this moves the combing much higher up where it is generally less audible, and having the Neo10s horizontal will extend the bottom end much further as the baffle width increases.

By some remarkable coincidence, the holes for the Neo10s and the holes for the Raals match up perfectly.  Chances of that?  Pretty small, but I'll gladly take it.  This allowed me to bring the acoustic centres as close together as feasible with no need for a custom faceplate.

So this is what I ended up with.  A much wider spacing also meant the bass driver could sit in between the legs, although things were going to get much more wobbly near the bottom - to ameliorate this I added a single supporting strut from angled aluminium to stop the frame from twisting excessively.

The result?  Good integration between the Neo10s, much better than before.  Plus, the Neo10s now go down to 500Hz meaning they also fit much better with the bass drivers.  The crossover still needs some work, but already there are things I'm hearing that I very much like.

Although about this time, the potential arose to try out a slightly different solution to the midrange upwards...

Speaking plainly...

Audio is difficult.  This is the conclusion I've managed to come to after a decade or so of practising in the art.  Many systems are good at one or two things, but to cover all the bases is incredibly hard.  Being pragmatic is to decide what particular aspects are most important to you, and to focus on achieving those primarily.

Take my speakers, for example.  I'm certainly not a speaker designer by trade, but I know what's most important to me... low distortion and an even tonality with no undue emphasis, particularly in the upper midrange.  I also hate cabinet colouration, which led me to come up with a speaker with no cabinet, or baffle for that matter.  Throwing away these things greatly hurts efficiency and creates potential nulls in bass response, but that is the price that needs to be paid for a small, relatively light footprint which doesn't suffer from the usual smearing of sound as the cabinet resonates.

Losing efficiency in one area of the design like this requires high efficiency, high power handling units to compensate for the shortfall.  I've long since used Lambda Acoustics TD15M Apollo for mid/bass duty (a big 15" unit with stupendous power handling and a curvilinear cone for wide frequency response) but the thought occurred to me that it could be used for a pure midrange.  With the Raal ribbons on top, that is potentially a 95dB/W midrange upwards block, which could be driven by its own small amplifier.  No baffle step compensation should be necessary here, as that can be incorporated into the bass driver crossover, which will have to be driven relatively hard to achieve any bass.

I had been mulling over the idea of a baffle-less speaker for a while, but this created a problem... how to support the drive units?  A bit of a discussion was had with a talented carpenter called Russell who's turned some of my (rather poorly drawn) ideas into reality before, and eventually settled on the idea of a V shape as a support.  This had a potentially interesting property of dividing the rear wave energy, which may help ameliorate nulling problems to a degree.  He builds out of thick birch plywood, which is an excellent material acoustically - a light year away from the typical MDF used in most speakers.

The magnets of the drive units neatly slot into the supporting holes.  The idea was to bolt the drive units together, but in an attempt to reduce transferral of energy through the front baskets, oil-based clay was used to cement the units into the mounting holes - this proved sturdy enough to support the drivers, with some sorbothane spacers keeping things at the right height.

I haven't said much about the 15" bass drivers so far, which are high Q units similar in design to the TD15Ms... I had a bit of a trial with them, discovering that one unit had a cracked basket long after I'd purchased them - the speaker company in question was not particularly helpful in the matter, but thankfully I was able to epoxy the basket to what appears to be adequate strength, and distortion performance appears unaffected.

As said, the nature of the design means that the bass drivers need to be heavily equalised in order to produce any reasonable bass.  In the case of these drivers, a low pass filter was pretty much all that was needed to bring things into line, after attenuating the midrange+HF suitably.

The attenuation of midrange+HF to get a decent bass response worked out to approximately 12dB, which really isn't too bad, although does mean the effective efficiency of the bass driver has been reduced to roughly 78dB/W!  With roughly 200W power handling, that is enough to play reasonably loud, but it won't blow the house down... that was not on the requirements list.  :)

The crossover is relatively simple... second order on the bass driver, first (high pass only) on the midrange, and the tweeter is effectively fourth order, if memory serves.  Response looks something like this, about 15-20 degrees off axis...

It's a fairly even response with mainly dips rather than peaks, and a gentle roll off at the top end unless you're sitting bang on axis... the limited vertical dispersion of the ribbon is a bit of a price to pay for the excellent sound quality it provides, though this could be ameliorated with an "ambience tweeter"... something I've been meaning to try for over a year now but haven't got around to!

Once the crossover was gotten into a reasonable shape, I stopped messing with them and just started to listen to music... which is how it should be, really!  They have their limitations (primarily the small sweet spot), but perform well with a wide range of musical genres.

The big gap in efficiency between bass and midrange+HF suggests biamping would be an excellent idea - perhaps a high efficiency Class D driving the bass, and a high quality small Class A driving the midrange+HF.  Now there's a thought...