12V Power supply gets a revamp, part 2 - maximum overdrive...

So yes... a bit carried away.  It all starts with buying some large chokes... then a big case... then a big chunky 2x12V 225VA encapsulated toroid.

And before you know it you have a pretty silly brute force unregulated supply that could deliver something like 15A.  Yes, ideally you wouldn't have this iron in such close proximity, but this is all I could do in the case... these chokes are pretty large!

The supply has an off-load voltage of around 19VDC though it doesn't take much to drop it down to 17VDC.   This is much higher than I need plus the off-load losses are significant - I was seeing 4W dissipation with nothing connected.  Ouch... some of that will be in the 17V zeners, but this all seems like gross overkill.

It will make the beginnings of a good audio PSU but this perhaps isn't what I want for a network supply.

Now I had the NAS, I wanted to try running it with the existing 12V PSU I built early last year, but found that it just didn't have the grunt to cope with the spin-up of the HDs in the NAS.  Sticking a big cap on the output only made it hiccup, so figured a better solution would simply be a beefier DC/DC conversion stage.

To keep efficiency, it had to be a synchronous buck topology, and a bit of research suggested that the LM3150 would be a good option.  TI sell evaluation boards for these, so a bit of a play in their Web Bench suggested what parts I'd need to reconfigure the boards to do my bidding - they are made on beefy 2oz copper with wide tracks so should have no problem in delivering >10A of current.

Thankfully pretty much everything was available from Mouser.  To handle the increased current, the N-channel MOSFETs needed to be changed.  As it so happens, TI make some pretty good ones.. the catchily-named CSD16321Q5C is a 25V, 100A (okay, 31A in most real world cases) part which has more than enough current capability, and is also designed to be heatsinkable from the top if you need more current handling!

A literally big component change was replacing the inductor with a much beefier one - a SER2915L-682KL... it barely fitted on the board!

The aim was to try and keep copper losses as low as possible... this inductor is a >=20A part so should have no trouble at all delivering 5A, even in a box with very little ventilation.


To swap over the FETs, I used my new hot air gun, an Atten 858D.  I saw the review on the EEVBlog a while back and it did seem to be the best tool for the job for a fairly small amount of money.  When it comes to 2oz copper board with wide traces, even an 80W soldering iron will struggle to get enough heat in to remove parts - this is where a hot air gun really comes in handy.   I don't think there's any way I would have gotten the solid caps off with my iron.

So I tested up the boards with crocodile clips - success - a clean 12V output!  Although the regulation seemed a little poor... I later found out that the crocodile clips had a whopping 0.2 ohms resistance - horrendous for any kind of significant current testing!  Need to make some fresh ones with thick copper cable...

 The new DC/DC seemed to be working okay, but the existing 50VA transformer in the PSU box just wasn't going to have the oomph required.  Even with a single HD, the cable modem, router and NAS were going to peak at 35W or so consumption when the drive spins up... the 50VA will be well out of its comfort zone.

Fortunately I had an 80VA handy... the catch being that it's a 2x9V rather than a 2x12V.  Standard transformer voltages are a little bit annoying... the standard windings go along the lines of 6V, 9V, 12V, 15V... thing is that no transformer offers perfect regulation so there can be a significant disparity in off-load vs. full-load voltage.  A 12V transformer after rectification and smoothing is likely to be something like 17VDC with only light loading, unless you put a big choke in before the cap.

2x9V would have been perfect for a 12V supply, but I need more than that to charge a battery... ideally between 13.5V and 13.8V for a 12V, depending on temperature.

To handle UPS functionality, I decided to go with a PicoUPS board.

This is quite a neat little board which is based around a couple of ORing controllers that let it use beefy NMOS as diodes, essentially... the controllers have a comparator which decides whether to switch battery or mains sourced DC to the output... when the mains voltage drops below the battery, it should seamlessly switch to the battery.

In order to charge the battery, the board uses a P-channel MOSFET with a little controller IC that appears to provide constant current.  As far as I know, it doesn't apply any continuous voltage limiting, although the board itself should not be run above 17VDC input.

While I had a nice efficient UPS solution now, the transformer was a worry.  9VAC would barely rectify to above 14VDC, even off load... ideally this needs a 10VAC output transformer but such things are custom jobs and as such very expensive.  The fact that the transformer is epoxy-sealed eliminates the possibility of adding extra turns... so let's try to make things as efficient as possible...

After some searching, I found what appear to be the very best diodes you can buy short of doing it all with MOSFETs and a controller.. the "Super Barrier" range from Diodes, Inc.  The forward voltage drop is considerably less than any other diode I encountered, so combined with thick 30A copper cabling, should help to minimise losses.  The particular model is the SBR10U40CT - a 2x5A rectifier in TO220 format.

Technically it is a dual diode, but as far as I can tell, they are fabricated on the same die and thermally connected so providing the resistances are well matched, should be okay to wire in parallel.  I wouldn't recommend running them up to 10A in this configuration but it should be plenty good enough for 5A continuous, I'd imagine.

A simple means of mounting them was to use the mounting hole for the toroid and drill/cut out a piece of stripboard to mount them on.  These things run very cool - not surprising at all given their very high efficiency.  You do have to wonder why they even bothered putting them in a TO220 package as it's kind of self-defeating... the heatsinking is so good that they don't get warm enough to give anywhere near their best voltage drop performance.

The thing about diodes is that the voltage drop tends to decrease as the temperature goes up - so you'll get the worst performance Vf-wise at room temperature or less... at 85C, you'll get a considerably lower drop.  On the minus side, the leakage is much higher and the devices lifespan will be impeded.  Swings and roundabouts.

In any case, when I get around to buying one, cutting off the heatsink tabs with a high speed disc will probably help save a fraction of a volt - before doing this, will need to check the part temperatures in a closed box after sustained use as they may well be getting much warmer now!

Putting it all together... in goes the new DC/DC converter board, face up.  Plenty of mylar tape to prevent shorts with the bottom of the case, lots of insulation on the mains wiring.  There's a switch on the front to give the option of disconnecting the PicoUPS from the DC/DC converter, just for when you *really* want to cut the power.

So does it work?  Indeed, yes!  Voltage to the battery with the NAS powered down is about 13.9V, within acceptable limits.  When the NAS is operational, voltage to the battery decreases to about 13.5V, which should just about enough to float charge it but probably marginal.

Switching off the mains seems to give a seamless switch to the battery, and it seems to be able to power the router, cable modem and NAS with no problems... I think it would have a tougher time starting them from battery, but definitely okay for switchover!

Efficiency seems to be about the same as before.  The new gigabit router uses about a watt more than the old Linksys, so now up to about 8W with the NAS switched off.  Not bad, though, given that now includes trickle charging the battery.

I should note that I haven't put in undervoltage protection - this was present on the old DC/DC converter board but didn't have a chance to figure out a good solution for the LM3150... this essentially means that in an extreme situation, the battery would discharge to the point of damage.  However, this would be a very extended run on the battery (without NAS, I figure at least 4 hours) and the equipment attached is likely to stop drawing much current before things get really bad for the battery.  Hopefully the desulphator won't get called into action too often!

12V Power supply gets a revamp, part 1...

Doesn't time fly... I have been doing stuff at home... just not as much as I'd like!

I decided it was time to get a NAS and get a proper storage solution sorted out for 2013... so ended up going for one of the Synology boxes in January.  I wanted highish performance (>50MB/sec) and lowish power consumption (<10W for the CPU idle) in the same box... I was hoping to hold out for the Atom SoC based stuff but it seemed to be taking forever to materialise.  Of course, now they have been announced.

While it is relatively power efficient (I measure it at about 9W before the HD spins up, at which point it sits at around 14W) for the speed of it, I was concerned out what would happen in the advent of a power cut.  They are not renowned for coping well in such a scenario.

So a UPS seemed like a good idea.  An off the shelf inverter-style one would be not very efficient, so wanted to go for something that could output 12V cleanly and easily.  Something that uses a cheap 12V lead acid battery and is designed for efficiency.

The first approach was to try modifying the switching supply that the Synology came with to output 13.8V.  It's a standard block power supply, efficiency rating V so not bad at all in terms of waste power.

After finally breaking my way into the case (they really don't want you to open these things!) and removing some extra metal shielding, I was greeted with this...

 

I took a look at the electrolytic caps inside and didn't like what I saw.  These might be a perfectly reputable brand (indeed, I was able to find a datasheet) as far as Korean caps go but felt more comfortable replacing them with more familiar Panasonic units of a 25V rating... particularly as I intended to up the voltage output.

 

A look at the underside reveals that the supply is based on an LD7578 PWM controller.  The output section is based around a neat UM603 which contains a dual op-amp and a 2.5V reference wired to one of them.  Logically, there had to be a voltage divider near the chip... indeed there was.  As an added bonus there was an unpopulated part which had pins in parallel with the resistor in question, so I could do the mod quite neatly - you'll see a diagonally placed resistor near the main chip in the picture below.

 I now had a perfect 13.8V output.  The problem was that the supply was now out of kilter.  While it will happily provide 13.8V at very light load, any significant loading of the supply was triggering protection circuitry and the supply would go into "hiccup" mode, which was audible through the iron.

I'd had enough of this hassle before with switched mode supplies, so decided to go back to the original 12V supply design with a linear first stage, though I went a bit over the top...

Toothbrush time...

It's been a while since I've had time to post, in between working on Sunrise and full-time work, but I thought I'd post a few shots from a teardown of my Oral-B toothbrush, as it was badly in need of a new battery.

 The toothbrush in question was the top of the line one - I believe later versions are a bit fancier now, but it was pretty expensive, and after a few years use, the battery life had dropped from 4-5 days or so down to about 1-2 days between charges.

The battery is a NiMH tabbed cell, soldered in.  The choice appeared to be between approx. 55 quid for a refurbished brush, or £8.30 inc. P&P for a new cell.  I decided to take the latter option... despite Braun's warnings that doing it yourself will knacker the water seals.  Frankly, for less than a tenner, you're not going to lose much even if the brush does suffer from water ingress at a later point.

Getting the thing open wasn't too hard - there are tutorials on Youtube, but the internal plastic retaining clips were a little more fiddly than anticipated... thankfully didn't snap the induction coil wire when it gave way!

Under the LCD screen, there was lurking an MSP430, which was slightly surprising, as I'd have thought they'd be using a more cut-down controller for something like this.  Certainly in the lower cost brushes, I seem to recall seeing an unknown uC.

In any case, even an MSP430 cannot run off 1.2V so there must be a boost converter  on there... probably that little SOT package just below, as there's a chunky-ish inductor just above it.  I presume that the motor runs directly off the battery via. a MOSFET under control of the MSP430, as the boost converter is unlikely to be that efficient at this low a voltage.

The charging circuit looks pretty simple.  I haven't traced it out, but noticed that it's a pretty slow charge, so probably nothing exotic.

Here's the old battery in place, with the retaining spring removed between the motor and the battery.  Note that there's actually quite a lot of spare length in the chassis - the original battery is 42mm long, but you could probably fit another 10mm or so in there.  Such a shame that it wasn't ever so slightly longer to take a regular AA.

They've also decided that a 0.1uF cap across the motor terminals was necessary... wonder if this is for RFI or whether it was adversely affecting the microcontroller in some way.

New battery in place.  The cell is rated 2000mAh, and is allegedly a Sanyo, so of decent quality.  The terminal strips were a little wider than the original but are easily cut down with scissors as the metal is so thin.

And then, put it all back together again... still charging, but seems to be fine so far... :)

JLH gets regulated (temporarily)...

Over the weekend, I tried to get modify some of the high efficiency DC/DC converter boards to output 26V for the JLH, by replacing most of the passive components with different values suggested by TI's design software, but not a great deal of joy was had - while they will happily do 26V into no load, the under voltage lockout/ramping doesn't appear to be right as they only output around 3.3V when connected up to the amp... ah well.

So why not try plugging into the linear bench supply?  If memory serves (I built it a few years ago), this is LT1084 based, so capable of decent grunt with the die cast aluminium case acting as a heatsink.  The outputs have been configured for a voltage of around 33V.

First results are promising with an 8 ohm load - modulation is now down to almost nothing, and the noise floor is much flatter... but there's a lot of high order harmonics present that weren't seen before in the unregulated supply.  Let's try reintroducing that hand-wound 2mH 0.03 ohm choke between the supply and the JLH... hey presto, much cleaner!

(Note: signals are still being normalised to 0dBFS)

It's important to remember that most linear regulator ICs are not good at suppressing higher frequency noise... that said, it is surprising just how big an effect the choke is having.  The performance is now very impressive indeed for a simple circuit, and fully satisfies a basic objective criteria for a "clean watt".

Let's go further and try it with a 15 ohm load again, this time comparing to the measurement reference DAC being used as a source in these tests.  The DAC is based around an old WM8740 evaluation board whose heart has been replaced by an AK4396 in the name of better objective measurements... here's what it looks like inside...

All the mains circuitry in the picture is purely for charging - when in use, the design is disconnected from the mains and the DAC runs fully off battery power, with digital input coming via. optical for electrical isolation - this eliminates the possibility of annoying ground loops at the input end.

The measurement reference DAC is driven with a 24-bit 96kHz test signal, captured by a modified LynxTWO-B board with AK5394 A/D conversion... this is as high as the board will take in its current form via. S/PDIF, and appears to give good results... while it is certainly possible to achieve lower distortion than this (the simplest means being high order filtering around the test tones), the results are plenty good enough for a basic check point.

Putting to one side the second and third order harmonic, the JLH closely mirrors the distortion and noise of the input signal... in fact, if anything, you could say that the JLH appears to have "cleaned" the signal up a bit - this suggests that the output of the DAC probably would benefit from a little bit more filtering.

This is pretty good performance and suggests I should build up some linear regulators as soon as possible for the JLH so I can go back and listen to them again!  While a discrete regulator would be interesting to play with, I've ordered some LT1083 to do an initial first run with, as the design seems to work so well with even a traditional series regulator.

Class A contender no.1 - JLH 1969

When it comes to audio, in my opinion, quite often simple is good, sometimes best.  It's very easy to overcomplicate designs and introduce additional problems, only having to add additional parts to ameliorate basic flaws.

A perfect amplifier, as someone once said, is a piece of straight wire with gain.  While there are plenty of line level ICs such as opamps capable of vanishingly small levels of measurable distortion, achieving the same with much higher levels of current and voltage is much more taxing.

As has been said, running in Class A gives a head start in terms of low distortion, as it avoids the problem of crossover distortion in Class B amplifiers, where the amplifier switches fully from sourcing to sinking current, or vice versa.  To lessen the problem, Class B amplifiers are often offset biased to become a Class AB, so that the amplifier operates effectively in Class A for a fraction of its rated power.  This is very effective at reducing crossover distortion, but as Class AB amplifiers tend to be designed for much higher powers (and therefore, typically run on higher rail voltages), the bias amount tends to be fairly small to keep consumption down.

For the sake of a single clean audio watt, I believe that (with mains power at least!) Class A is probably the best way of achieving the goal... as an added benefit, Class A circuitry tends to be simpler which means from my perspective, less componentry to sully the sound.  An excellent reference on the subject of Class A amplifiers is the Class A amplifier site, run by Geoff Moss.  The site focuses on variations of an amplifier developed by John Linsley Hood, or JLH as he is often referred to.

JLH came up with quite a brilliant little Class A design which was the subject of a Wireless World article back in 1969, and is still a reference today.  It uses just four transistors, one input level PNP, a mid powered NPN for phase splitting and two beefy NPN power transistors for the output.  As was common for the time, this is a single rail non-complementary design which means you can really go to town on just one rail of power supply and not worry about how complementary your output pair really is.

One of the reasons why the design needs so few active parts and can run single rail is due to the simplistic biasing and that both the input and output are AC coupled (so coupling capacitors in the signal path).  Some audio enthusiasts run a mile at the thought of capacitors in the signal path, but I am not one of them.  Providing the cap is of high quality, I'll happily take one over a far less linear active device which will leave a much bigger sonic imprint (to my ears, anyway).  They also provide a degree of safety over that inevitable time when then the bias "wanders" and there is an unpleasant amount of DC going where it is not desired...

I won't go into the circuit operation in detail as far wiser heads than me have debated it in great detail over the years, but it can be read as a simple three stage amplifier.  While I've grown to have a fondness for FETs over the past few years, the sound of a JLH amplifier has always stuck with me, so thought it was an excellent bipolar-based design to start with.

As someone who's designed a few bits of audio kit in their time (and still do, when time permits!), it feels a little lazy to use someone elses' PCB, but you have to value your time when it comes to these things... there is often little point in reinventing the wheel, particularly when going after a fairly faithful recreation.

I decided to start with the first iteration of the JLH design, as this is the simplest (and you could argue, purest) form of a bipolar Class A that you could wish for.  I found a seller on eBay that does what appeared to be authentic looking 1969-design PCBs with the added bonus of supporting more modern component pinouts if you wanted to try different parts.

On the whole, I was keen to stick close to the original transistors, with the exception of the output transistors... by all accounts, the OnSemi MJE15003 are considerably superior to the originals in this design and so could be used without hesitation.  TO-3 packages are a pain to mount compared to the more modern TO-247/TO-3P as they usually need an angle bracket when using with a PCB... this is then thermally coupled to the primary heatsink.

For extra security against misalignment, I put PTFE sleeving around the TO-3 pins... this ensures even if the TO-3 packages somehow wiggle their way to making contact with the metal, no shorting should occur.  Fancy alumina ceramic shims (about 1.5mm thick or so) were used to thermally couple but electrically isolate the TO-3s from the heatsink, with liberal use of good quality thermal paste in the sandwich. This naturally gives a bit more lead inductance but reduces stray capacitance.

I chose to keep the original 2N3906 PNP (using a Magnatec part which should be very close as a second source to the original Motorola version) for the input transistor, though for the NPN splitter, I ended up going with a 2N1711 branded part with lovely-looking gold plated leads - the 2N1711 was endorsed by JLH as a superior replacement so feels authentic enough.  All transistors were tightly Hfe matched, across both pairs and channels.  I would have preferred higher Hfe parts but out of 16 power transistors, the "best" were around 50ish, with a lot of them much lower.

There didn't seem much point to throw exotica at this first attempt, so you won't see any teflon capacitors or tantalum resistors here - the input cap is a salvaged WIMA Polypropylene from an amplifier refresh, and the output capacitors are effectively "no name" Forever-branded units of basic merit.  The decoupling capacitors are good quality Rubycons, bought in for the job

 I socketed R5 in the picture so that I could tune the output to be half the rail voltage as JLH recommends, but the fixed 100K was so close to half rail already that I took the trimmer pot out and put the 100K back in.

For the sake of getting things up and running quickly, I wanted to skip the regulated supply and try one of the many simple unregulated supplies floating around here of many voltages - unfortunately never quite the *right* voltages, it seems!  Commandeering a set of 2x25V 160VA toroids from Antrim, back when Maplin used to sell more interesting componentry, I put the secondaries in parallel, rigged up a simple full wave rectifier and threw a big Elna Cerafine on the output.  Hm... a bit high... off-load DC voltage was something like 45V!

This design is meant to run off 27V for 8 ohm loads... more could certainly be tolerated by the parts in question, but that it going to get properly toasty at that, never mind being worried about the health of the some of the parts from a voltage point of view.  A bit of thought, and I remembered that there were a couple of 100VA toroid cores spare which I'd intended to wind chokes with... a spool of 30A wire and a patient Anna resulted in two simple chokes of approximately 2mH each, and very low DCR.  These were put in series with the rectifier, and kept well away from the mains toroid in use.

As the angle brackets were rather oversized for the job, it was thought that it would function as a basic heatsink for now - after all, it should be only 30W or so per channel.

Both amplifiers were gingerly powered up, and gladly showed signs of sane biasing the first time around, rail being between approximately 32 and 35V.  The bias starts off fairly low and stabilises at a considerably higher point, being similar for both channels... the bias appears to be very sensitive indeed to temperature - just grabbing the heatsink with your hand is enough to affect the bias to a significant degree, which does sound like an element of the design that will benefit from slightly more complexity!

Ok... enough of this faffing about, let's get them into the main system and see how they sound.  First night impressions were very positive indeed... while I wouldn't call the resulting sound "airy", it certainly came across as more beguiling and of fine definition.  I've experimented with many amplifiers over the years but have usually come back to my humble Arcam Alpha 8Ps... a quite traditional (and relatively complex) Class AB amplifier with a complementary MOSFET output stage - the JLH was certainly bringing something new, though quite hard to define.

Let's see what some measurements show.  For sake of brevity, I'm just going to show some normalised 24/96 65536-point FFTs of a 1kHz input... load is a 25W wirewound power resistor of either 8.2 or 15 ohms.  This lets you see the harmonic spectrum, and give a great deal more information that any single THD figure will.  While these traditional measurements only give, IMO, a small insight into the sound quality of a device, the order and shape of the harmonics can be quite revealing.  There have been alternative tests proposed for quite some years, but this a reasonable starting point.

Let's start with a 15 ohm load first.  This was done at considerably less than a watt as the MF+HF units will rarely get anywhere near a full watt... if they do for any length of time, I'll probably have my fingers in my ears, and perhaps the neighbours might want a word...

The first thing that is immediately obvious is that the JLH output is being modulated, probably from mains harmonics... at a relatively low level, but nevertheless impacting on the sound.  This is likely to be a combination of the very primitive unregulated supply and the primitive biasing arrangement.

The second thing that struck me is how much (relatively speaking) high order harmonic distortion the Arcam has.  While this amplifier will no doubt measure very well in terms of a THD figure, the harmonics of this single test tone are spread across the whole frequency range.  Second order harmonic distortion in my experience is fairly benign and is generally overwhelmed by the speaker contribution, so isn't worth worrying about unless of a very high quantity... third order is a bit more concerning, and I'll generally like to see it below -80dB on the reproduction front, and fifth and above odd harmonics preferably below the noise floor.

A high noise floor is evident on the Arcam, possibly down in part due to the high gain that this amplifier offers, considerably higher than that of the JLH - I suspect when the JLH is given a better power supply and better biasing, the noise floor will drop further to go with the lower modulation.

Let's have a look at an 8.2 ohm load now, at close to a watt - this is considerably harder than I'm putting on the amplifier at the moment due to a resistive attenuator network for gain matching, but is interesting for comparison purposes.

Note that with the more difficult (albeit still only mostly resistive) load, the modulation on the JLH output has dropped considerably.  The JLH second harmonic is quite high (which would lend a rather poor THD score, for what that's worth - not very much) and slightly higher on third harmonic, but aside from a few odd glitches, higher harmonics are pretty much absent.  Odd order harmonics on the Arcam are visible all the way up the 15th, and this isn't even with extra averaging.

So the Arcam isn't very good objectively at low power levels... consistent, yes, but not particularly low in even basic distortion tests.  The JLH is already sounding good, although these results do suggest that the modulation will be impacting on the sound - hard to know if this is being perceived positively or negatively at the moment.

In any case, there is more work to do on the JLH... whether I will attempt to modify the existing boards or start afresh, I'm not sure, but it's certainly worth some more listening hours!  :)

The perfect watt...

And so we move neatly to the subject of amplification.  The amplifier in a system is generally dictated by the speakers, as different speakers have different requirements.

Broadly speaking, the majority of modern commercial speakers are in the region of 86-90dB/W/m, which is what I'd class as "mid efficiency" units.  These can be driven to modest levels by almost any amplifier, but ideally 20 watts or more, depending on how wild the impedance curve is.

Heading into the realm of drive units designed for high SPLs (a typical example would be for PA use) are units in region of 95-100dB/W/m... I class these as "high efficiency" drive units, which typically have very powerful magnet systems and lighter diaphragms.  There is a penalty (other than the typically high price) in that the lighter cones often lack the critical damping of lower efficiency units, which can lead to considerable colouration in the sound... however, the gains can be worth it.

Once you are beyond 100dB/W/m, you are in "super high efficiency" territory.  While even a 100dB drive unit may only be technically a few percent efficient in terms of converting electrical power into sound, these units are vastly more efficient than a conventional unit due to the log scaling.  The use of horns in particular can allow (given enough space!) very high SPLs from only a handful of watts.  If you are prepared to spend serious money, then 110dB/w is feasible!  At this point, an amplifier is practically unnecessary - a liability, even.

Back in the real world, we have limited resources, and without the space for large horns (or wanting to deal with their own set of issues), "high" efficiency is a reasonable goal.

My own speakers are essentially divided into two.  A bass driver operating essentially in free air, which is very inefficient, and a midrange and HF unit of relatively high efficiency.  While the bass driver requires a powerful amp, ideally of at least 100-200W in power handling, the midrange and HF unit are never likely to see more than a watt in typical use.

You can further lower the workload on the MF+HF amplifier by taking advantage of the fact that there tends to be more musical energy at the low frequencies than the high.  By moving the high pass filter from the speaker crossover to before the amplifier input, you can reduce the load by 3dB or more, depending on the music.  Do note that this is fine for a midrange like the TD15M Apollo which has insane (>500W) power handling as even if the amplifier goes DC, the speaker won't care, but it's a really bad idea to DC couple any kind of high frequency driver without suitable protection in place.  Be warned that it can be an expensive lesson...

The high power demands of the bass driver realistically limit choices to a Class AB or a Class D solid state amplifier, which aren't very interesting from a purist point of view, and this drive unit is only covering a few hundred Hz with fairly quick rolloff, so let's not concern ourselves with that for now.  What is interesting is the watt for the MF+HF, which covers roughly 250Hz onwards... the bulk of the sonic spectrum.

So we want a good watt... how to get it?  The immediate answer is Class A operation... whether it be a single output device or a push pull pair, "always on" operation yields the lowest distortion, but unfortunately also the lowest efficiency.  For a single clean watt, we can sacrifice efficiency and still keep power consumption within manageable limits.

(Do note that I've seen a Class D amplifier that idles at over 20 watts, so it can be dangerous to make assumptions purely based on topology!)

There are some very well known Class A designs that put out a few nice watts, and I've been endeavouring to build them up to try with the speakers... time to build, listen and measure... in that order.  :)

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...