"After four years of building a number of different baffles, trying many drivers and fine-tuning passive crossover points, what conclusions have I arrived at? Only one. Anyone reading this can easily and for little cash build a substantially more open, palpable and real-sounding speaker system than they are likely living with at present regardless of cost."
That's a big claim, but the author seemed a genuine guy with some credibility, and I put it in the back of my mind to investigate further at a later date.
The author was describing an open baffle system, (no box, just a flat panel with drivers mounted on it) with field coil drivers.
Field coil drivers use an electro-magnet instead of a permanent magnet to generate the magnetic force to move the voice coil.
They require an external DC voltage to energise the electro-magnet. It's how the very earliest speakers were utilised, before permanent magnets were viable .
These days they are expensive and exotic, and one of the best designers and manufacturers is the French company Supravox.
Recently one of my preamp owners asked me to build him a power supply for his Supravox field coil drivers.
Its a little out of my field of expertise, but an interesting challenge.
Bob sent me a pair of Supravox 215-EC's to experiment with ( and then gave them to me- thanks Bob!)
The easiest way to get field coils going is with a car battery, the 215 are happy with 12V so a 12V deep cycle battery minimum 25 AH will last through an extended listening session.
After a warm up I was impressed enough to go further with open baffle and field coils.
I should mention that my friend Martin Seddon of Azura Horns has been using field coil drivers for some time now, specifically the Line Magnetic LM555W , which is a clone of the Western Electric 555 compression driver.
The realism of his system is extremely impressive , and initially piqued my interest in field coil drivers, hopefully I could pull the same palpable sound from the Supravox.
As you know, i'm a big fan of active speaker/amplification, ie using a seperate amplifier for each driver with either passive or active crossovers.
The Supravox is essentially full range , in the same vein as the Lowther , Coral, Feastrex full range drivers , but the Supravox doesn't have the shouty upper midrange accentuation of these drivers.
However these drivers are known for their dynamics and ease of portrayal of micro-dynamics, they are very revealing, but in an easy, almost relaxed way.
They tend to be in-accurate, but the excitement they display tends to forgive their sins over the short time.
The 215 are reasonably accurate, so don't suffer from the same fatigue, they can be taken very seriously as a transducer of the highest quality.
But, like all full range drivers, they're not really full range and have roll-off at bass and treble extremes.
Eminence make a 15" bass driver designed for open baffle. They cant do deep sub-woofer bass on an open baffle, but these days we have some great tools to coax reasonably flat bass response from these drivers.
However, there's a real problem integrating other drivers into use with full range drivers.
The beauty of full range drivers is that they dont use crossovers, they are used "full-range" and this lack of crossovers preserves the phase response of the driver.
Crossovers ALWAYS do some damage to phase coherence, which leads to a diminishing of openess, soundstaging and imaging - the big 3 when it comes to realism.
Of course no full range driver can do 20Hz- 20Khz , so if the frequency extremes are required , bass woofers and treble tweeters can be added, although crossovers, whether active or passive , will be required.
What I like about the Supravox is it has a nice roll of at the bass and treble points, quite smooth, and this enables us to blend in bass drivers and treble tweeters with crossovers just on these drivers.
We can get away with no crossover on the 215 at all, and this preserves the phase character of it, the magic remains even with the bass and treble crossovers, as they are not effecting the 215 at all.
I used two Eminence Alpha 15" bass drivers per speaker with a DEQX digital crossover with a steep 24dB crossover at 200-250Hz. I also used the DEQX equaliser function to counteract the open baffle bass roll-off, to a certain degree.
In part 2 I will go further into the implementation, power supply and sound quality of this splendid speaker.
