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I don't own and probably can't afford OTLS / ESLs. Even though from reports it might be where I would like to be.I noticed a recent picture here of a review - written some time back - stating that ESL's very low impedance made OTL's unsuitable for this task, given their own high output impedance.
But, I have been aware for quite a long time that a lot of people do use OTL's for this job.Further it is my understanding that the capacitive load at HF is an issue for amplifier stability, but that the low impedance that accompany's it just isn't a big issue, because at HF the power demands are quite low.
It would be good to have this laid to rest here, perhaps by Ralph, et al?
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 07/04/15Follow Ups:
I think the problem was due to the fact that the original Quad ESLs were a very reactive load (loudspeaker impedance rises with frequency), but that's not the case any longer.
Please correct me if i am wrong.
The real complications have arisen during the advent of transistors, when some ESL manufacturers have tried to make their speakers easier to drive with transistors. This has resulted in some rather insane impedances, such as the 0.5 ohm impedance at 20KHz seen in a number of the Martin-Logan designs.
But more traditional ESLs have more moderate impedances and work quite well with OTLs; Quad ESL57s and ESL63s, Sound Lab, Beveridge, some Acoustats and Audiostatic for example.
A full-range ESL usually has an impedance change of about 10:1 from bass to 20Khz. A Sound Lab for example is 32 ohms in the bass and about 3 ohms at 20KHz. The ESL57 is about 45 ohms in the bass and 4 ohms at 20KHz.
Something really important to understand about ESLs is that the impedance curve, unlike a box loudspeaker, is not also an efficiency curve. The speaker will have similar efficiency at all frequencies as its impedance curve is based on a capacitor rather than a dynamic driver in a box.
It is for this reason that transistors often sound excessively bright (that is to say, brighter than the bright that such amplifiers are normally known for) as they can and will put out more power than they should into the lower impedances present at high frequencies (this is also why ESLs often get docked for poor bass, which is undeserved; the real problem is transistors don't do well driving higher impedances). This is why tube amps, generally speaking, tend to work better with ESLs. The reason OTLs work even better is that most tube amps have an output transformer. Remember that 10:1 impedance curve? For any transformer to work right, the load and source impedances have to match the windings ratio of the transformer. If they don't, the transformer can ring (distort) or simply roll off, depending on the nature of the mismatch (too high in the case of the former and too low in the case of the latter).
An OTL can drive the higher impedances of the speaker without power loss, and if large enough, will not see significant power loss at the high frequencies either. IME, most ESLs are a moderate efficiency overall, which means you need some power to drive them for best results. The most efficient/least power handling is the ESL57, which can easily be driven by any 60-watt OTL (such as our M-60). Smaller OTLs can be used as well if they have feedback (and the use of feedback generally speaking is probably a topic for another thread) to correct for HF losses. However the more feedback the amp employs, the more it will behave as a voltage source and as outlined earlier, that does not work ideally for ESLs.
The bottom line is if you are wanting ESLs, OTLs are an important consideration, as they offer the best performance that the speaker can achieve.
.
marc g. - audiophile by day, music lover by night
Go look at the picture of the - old - review of the - old - Futterman H3aa, in the post you can click on down below.Second column page 1
"It shouldn't really be used ..... "
No one in that thread or the one I started seems to have actually read that review.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 07/06/15
Consider the source.... a man with more firmly held opinions than most who rarely let facts get in the way.
I had a customer that turned into a friend. A very interesting character from the DC area he knew David Berning well since he started and used his amps.
The guy actually built a loom to make his home brew ESL speakers! OTL works well w/ESL as others have said.
E
T
Whomever made that comment has no clue what they are talking about. OTL amps (Atma-Sphere) anyway are very capable of driving Electrostats. It is actually a marriage made in Heaven. Why do you think so many Soundlab owners use Atma-Sphere amps...Most if not all electrostat speakers have very low impedance at high frequencies where minimal power is necessary and very high impedance in the lower end of the frequency range where lots of power is needed. This plays right into the OTL's happy space...I know as a former owner of MA2s and U-1s...SO when the time comes you need not worry. Get your Stats and OTLs and enjoy the music...
The impedance of an ESL is really a function of the turns ratio of its audio step-up transformer; the higher the turns ratio, the lower will be the impedance seen by the amplifier. Plus the fact that an ESL is like unto a giant capacitor; native impedance is very high at low frequencies and vice-versa at high frequencies. In the "transistor era", some ESL manufacturers have geared their product to work well with solid state amplifiers, unfortunately. But here and there there are good matches to be found. With Sound Lab ESLs the problem was mostly related to their use of two audio step-ups in parallel, one for bass and one for "treble". The bass transformer is fronted by an inductor that rolls it off at midrange frequencies. This causes no problems and leaves a nice high Z at low frequencies. The treble transformer is fronted by an RC network that serves as a high pass filter. Problems arise, because the value of R in the RC network has ranged from as low as 4-5 ohms to 8 ohms, over time. That R sets an upper limit for impedance independent of the treble transformer turns ratio, at the worst possible place, in the midrange. Some of the more vintage SL speakers have an impedance dip as low as 2 to 4 ohms at 500 to 1kHz. In recent years, SL have addressed the problem in several ways. As many know, I cured my problem with my 845PXs by ditching the RC network altogether and using an aftermarket treble transformer, but one need not go to such an extreme to enjoy the OTL/ESL match that is indeed made in heaven.
Quad 57s and to a lesser extent 63s are a good match. Later Quads may be excellent also. KLH 9s are perfect for an ESL. Martin-Logan CLS model I speakers are great too. Later iterations of the CLS suck.
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