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Apologies for the long post, answers and some speculation on the ribbons

Posted by cfb (A) on August 29, 2010 at 11:21:59

In Reply to: RE: Well there's the Wisdom & Apogee ribbons, a wall of SoundLabs, vintage Infinity IRS, Wilson WAMM posted by Satie on August 29, 2010 at 03:50:21:

Only bi-amping here's why. The Full Range has its own active line level 3 way crossover that is internally switchable to 2 way. Although the x/o point for the midrange is still fixed at 400hz with both settings. If one tri-amps, the tweeter ribbon and the planar bass section can be directly driven. The tweeter ribbon is about 1.5 to 2 ohms and the planar bass is a more forgiving load at around three ohms. In fact, I've even been able to drive the bass section directly with some tube amps on their 4 ohm tap. The bass panel is surprisingly much easier to drive than the true ribbons are. It also has greater sensitivity, requiring less power to reach 100db peaks.

The problems with the midrange are several; first is to drive them directly by bypassing the transformer would present a dead short to the amp. Depending on the cable connections used the 2 inch ribbon is only one one tenth to one fifth of One ohm; it works strictly on current. The only amps ever built that can handle that were the Classe Audio DR3-VHC, the Electrocompaniet AW 250, and a couple of the first series Krells way before their varying plateau biasing arrangement. I'm sure there may be a few new all out assaults on the “state of the art” in amplifiers that might drive such a load but while I've acquired some equipment $50K+ amps are out of the picture for me. Now, if you are fortunate enough to go that route hey more power to you, but even with that type of amplification one would still have certain intrinsic problems to deal with for the following reasons. Most of the solid state muscle amps, even the ones that tout driving difficult loads will, (for safety reasons), have protection circuitry kick in, blow a fuse or fry a resistor that is being used as a fail safe, when the amp is presented with a load that is less than one half of one ohm. Normally there just isn't any load that low, other than an accidental short of the speaker cable. Even with the previous amps that can drive a dead short the sound changes for the worse, there's no damping factor to speak of as the load is equal to the internal resistance of the amp, in fact the speaker cable to the midrange ribbon is going to equal or exceed the resistance of the ribbon itself. Even some of the esoteric amp circuits that have extremely low output impedance would only have a damping factor of 10 or less when driving a one-tenth of an ohm load, for illustration purposes picture a single-ended-triode amp driving a sub 2 ohm load, not good.

That leaves us with the midrange transformer staying in place, it presents a 4 ohm load to the amp and actually vacuum tube amps can drive it. But, there is another problem with the mid ribbon going down to 400hz, regardless of how much power is available to drive it at a certain point the mid ribbon will start oscillating side to side, this is generally around 400 watts. I experimented with up to 800 watts input and the amps were not clipping this was undistorted power. The problem is intrinsic to the freely suspended ribbon, when the mid is driven above 800hz it can handle more power with less distortion than it can at 400hz to 800hz. Why would one need that power? Well the ribbon has two limiting factors, one it is horridly inefficient and two it can only withstand so much power before its sound quality deteriorates and distortion rises. The inefficiency leads one to pour more power in to achieve a satisfactory sound level while at the same time having a limited ceiling of headroom for peaks. Instead of the typical speaker allowing an amp to operate for most of the signal in the first 5 to 10 watts the mid ribbon requires the amp to run at a level of 100 watts and up leaving little headroom. The ribbon has a narrow window of transparency where it gives the best performance. At low volumes it simply does not sound as alive and holographic as it will at a slightly higher volume level. It is too polite, recessed and not as transparent, it makes one want to turn the volume up for the sound to come alive. But then at that level the ribbon is requiring 80 to 100watts of power leaving little if any headroom. Furthermore, at any higher level the sound quality deteriorates yet again. It is as if it is precariously balanced on a peak, too little and it falls off, too much and it does the same.

I believe that the real solution to the problem is that a driver that does not exist is needed. If there was a second midrange ribbon handling the lower mid / upper bass range from 800hz down to say 125hz then it might solve some of the issues. This would require something like a 5” wide freely suspended ribbon and no one has made that due to cost issues with the magnetic field that would be required for a gap of 5 inches.

Why do we need to go down that low? Well I've skipped over the bass issues; it is quite clear that as good as the planar bass can be it is not of the same quality as the true ribbons. There is a distinct discontinuity in the sound quality between the planar bass and the mid ribbon, one can always hear the transition between them. What is needed is More ribbon down low and Less planar bass up high. More headroom would be welcomed too if that could be done while preventing the mid ribbon from wildly flailing about on peaks.

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