Dynamic speakers for OTL amps

Focus on speakers with easy to drive impedance, ideally not dipping below 8 ohms. While some might suggest autoformers to eliminate this concern, I would beg to differ and suggest that you avoid autoformers unless you listen to them extensively first, and decide they are for you. There are too many brands that meet this criteria to even list them.

Most single-driver speakers don't exactly plumb the depths on the low end, and aren't particularly suited for rock. I'd probably lean in the other direction.

Having owned multiple OTLs, I'd say you are in for a treat! Cheers,
Spencer

In my opinion (which is not unbiased), single-driver speakers are generally limited in clarity and/or articulation when called upon to reproduce large-scale music at high SPLs in a big room. That's just not where they excel.

Again imo, the coherence of a single-driver speaker arises largely from its lack of significant off-axis frequency response glitches which commonly occur in crossover regions. I believe that frequency response glitches (including off-axis ones) are typically much more audible than any phase non-linearities multi-driver speakers may have, which is good news because it is possible to minimize such off-axis glitches and thereby rival the coherence of a single-driver system.

For compatibility with an OTL amp, smoothness of the impedance curve is arguably just as important as having a relatively high impedance. Let me explain: If the speaker is "voiced" to sound smooth on a voltage-source-approximating solid state amp, then its frequency response may be significantly different on a constant-power-approximating OTL amp.

For example, suppose we have an 8-ohm speaker with a 16-ohm peak in the crossover region, and this speaker is voiced for a solid state amp. Into that 16-ohm peak, the solid state amp will be putting out only 1/2 the wattage that it puts out into the 8-ohm portion of the spectrum. Now drive this speaker with an OTL amp that puts out the same wattage into 16 ohms as into 8 ohms, and the speaker is now getting twice the wattage into that 16-ohm peak as its voicing anticipates! So this speaker will sound too forward, too upper-midrangey, and we mistakenly blame the amp because that's what we changed. But the real culprit is poor amplifier/speaker matching.

Having designed speakers with the goal of compatibility with both solid state and OTL amps, my suggestion would be to look for speakers with a relatively even impedance curve. The voicing of such speakers will be much more consistent from one amplifier type to another. Or, alternatively, look for speakers specifically designed for OTL or SET amplifiers; such speakers might not work well on solid state, but that is of academic interest only if you're not going to use solid state.

Finally, impedance peaks in the bass region are virtually inevitable, which can result in overly heavy bass with an OTL amp. Now if the speaker is bass-shy to begin with, the net result can be very good. But in cases where the reduced output impedance of an OTL amp results in excess bass, often the correct bass balance can be restored by lowering the port tuning frequency (either by lengthening the port or reducing its cross-sectional area). A beneficial side effect here would be deeper bass extension than we'd normally get with a solid state amp.

I haven't gone off about the sonic benefits of OTL amps, as presumably you're already sold on that. Take care with speaker matching and you'll have a really sweet-sounding system. Imo and ime.

Duke
dealer/manufacturer

I use Audiokinesis Jazz Modules with my OTL. Not crossover less, but an excellent match. The combination is great with rock music as well.

15" dual concentric Tannoy's might be worth a look.

Audiokinesis showed a 2-way speaker at RMAF last year call the Stratoprism that may well suit the bill. It was moderately priced and very easy to drive- with that kind of power and the efficiency of the Stratoprism you would have no worries in a room of that size. The speaker was very fast, detailed, cohesive, relaxed, imaged very well and had a nice extended bottom octave with plenty of impact.

For a bit more outlay Classic Audio Loudspeakers makes two models, the T-3 and T-1 that are both very easy to drive and have excellent bass extension.

Also easy to drive are the Wilsons, with the possible exception of the Sasha, which IME is not power hungry but may present impedance issues, which is solved by using a set of ZEROs (although the setup I heard was sans ZEROs, others have felt that the Sashas did benefit from them). The WP series doesn't need any such assistance.

Any of the ZUs will be effortless with a 140 watt OTL! Most 'full range' single-driver speakers will be as well any horn system.

Merlins are very popular with OTLs and are easy enough to drive with that kind of power that you should do well.

Quad ESL63s would not do badly in a room that size either.

There are many others as well, but the larger room is the consideration here.

Thanks a lot, and yes, I hope I am in for a treat! Obviously there are many variables, and doing away with x-overs may not be realistic in practice. The advice concerning a smooth impedance curve and bass excess/voicing is relevant, as are the models suggested by Atmasphere. Ideally I would try them all, since system/room matching is quite unpredictable. Realistically I will start using two Aurum Cantus 8 ohm monitors (with or without a DD18 sub), and work my way from there. I probably won't be able to resist connecting the OTLs to my big Dynaudio Consequences, although I can say beforehand that it won't be optimal. But - you never know, in this hobby, which is part of the fun.

Not all tube OTL amp have the same impedance requirement of speaker

?? Sorry- can't make sense of the last comment.

Much has happened since this thread in 2011 - but briefly, for me, this is the situation. I followed my OTL interest, buying Atma-sphere MA-1s and then upgrading to v 3.2, and invested in Audiokinesis Dream Maker speakers, and lately also LCS effect speakers. I have not looked back. Beautiful music, no listener strain.