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OTL, Output Transformerless Amplifier User Group.

ESLs and OTLs have been the match made in heaven since the 1950s.

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.


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  • ESLs and OTLs have been the match made in heaven since the 1950s. - Ralph 07/15/1509:52:20 07/15/15 (0)

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