I have used Futterman H3aa mono OTL tube amps successfully with KLH Nines and Quad 57's. These both have an average impedance close to 16 ohms. The CLX is only about 4 ohms. Any OTL amp will need an autoformer to work well with the CLX. My Futterman Harvard Music stereo amp (6HB5 output tubes) with built-in Altec autoformers could do it! The original owner used it with a multiple dynamic speaker (low impedance) set up.
OTL Tube Amps and Full Range Electrostatic Speakers
I have Martin Logan CLX speakers which have a nasty impedance curve in the treble with a minimum of 0.7 ohms at about 18 to 20 kHz - or so I have read. The impedance of the CLX is, as many know, largely capacitive. Over the years I have preferred tube amps and have heard OTL amps driving various Quad speakers with great results. My own experience with the CLX's has been more positive with high quality solid state amps; I am currently using a Krell 402e which has no trouble driving this speaker well. An ARC Ref 150 did not provide the control, particularly in the treble, which this speaker needs and did not provide the level of transparency which this speaker can provide. I would appreciate hearing if anyone has experience with other tube amps with the CLX's or Sound Labs, particularly OTL's and whether autoformers provided a significant benefit. Thanks for any insights.
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Exactly roberjerman, and the RM-200’s low output impedance is a bonus with the impedance swing many ESL’s present to an amp. It is also very stable into highly-capacitive load of most (all?) ESL's. A mighty fine amp, made specifically for difficult-to-drive loudspeakers. Roger is now making a direct-drive OTL amp, specifically for ESL speakers, including his own. About $6,000/pr, I believe. Check the Music Reference website to confirm. |
Thanks to roberjerman, bdp24 and kalali for their comments. I have heard the Futtermans on Quad 57's and it was a great combination. I have used a number of highly modified ARC tube amps on Quad 63's, 989's, and 2805's with good results. I have had a number of Sound Labs - most recently the A3's - as well as many Martin Logan hybrids the CLS IIZ and most recently the CLX's. While it is probably not worth delineating all the amps - both solid state and tube, Pass, Rowland, Levinson, Krell, ARC, CJ - that I have used over the years with these speakers - I have not used OTL's with the ML or Sound Lab speakers. There have been a number of OTL manufacturers over the years, but currently I am aware of only Ralph Karsten at Atma-Sphrere and Roger Modjeski at Music Reference who are currently producing OTL amps. The CLX is perhaps the most fussy speaker that I have had both in setup, room requirement for optimal performance and the upstream equipment needed to optimize its performance. I suspect that is why one sees CLX's on the used market. Has anyone had personal experience with OTL's on either the ML CLX's or the larger full range Sound Labs? |
Some more OTLs that have been popular recently... (I haven't heard them) https://www.lineartubeaudio.com/new-page-1/ |
We have a number of customers using the RM-200 with their ML speakers. It's an excellent amp for the job. OTLs can also sound great, if Atma-Sphere I would recommend the MA-1 which is 100 watts. I'd recommend putting the autoformer in between the amp and speaker, but it may not be necessary. Ralph could advise as well. There is some debate on whether the Berning patent for his design and that is now used in the LTA amps is actually an OTL circuit. IIRC there is a high frequency transformer that follows the tubes in the circuit. Therefore, some feel that technically it can't be an OTL and I believe Berning more accurately refers to the design as Zero hysteresis since that high frequency transformer does not behave like a traditional output transformer. So the other side of the argument is if it doesn't behave as an output transformer it is still an OTL (output transformer less). |
Power increases into lower impedances!@roberjerman One thing about ESLs is their impedance curve is not also a map of its efficiency, like it is for most box speakers. Instead, its impedance curve is a result of being largely a capacitor. For this reason, what is desired is the ability to put out constant power over the range of impedances presented by the speaker rather than constant voltage (which results in increased power into lower impedances per the quote above). Of course, no amplifier really does make constant power but they can come pretty close, especially if assisted by a set of ZEROs (www.zeroimpedance.com) since any tube amp that is designed to make constant power (like ours are) will tend to have a higher output impedance. For more on this topic, seehttp://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php |
Ralph, would the solid state McIntosh amplifiers with autoformers qualify as constant power sources? Their specs indicate the same power from any of the speakers taps, e.g., 2, 4, or 8 ohm, but with different levels for the output impedance. If so, that might explain the general tendency to pair these amplifiers with Martin Logan ESLs. My MC2200 model does seem to do quite well with my ML Aerius i. |
I agree with with audioman58 that the Berning amps, particularly the larger monoblocks do mate well with Sound Labs. David Berning is an extremely intelligent and innovative circuit designer. I have heard a number of his earlier products but not the ZH series although it is interesting that at least one reviewer reported excellent results with the ZH270 and the CLX. One has to factor in room size and listening preferences. The Linear Tube Audio amps have had excellent reviews but are limited in power output. I would probably need at least 100 wpc. I would like to thank Ralph Karsten weighing in with his usual insightful comments. The MA1 with an autoformer is a possibility but 100 wpc may be marginal given the real sensitivity of the CLX - probably around 85 dB as determined in some reviews rather than the 90 dB quoted by ML although both numbers are probably accurate given the measuring conditions. My own experience favors the former figure. In regard to the MR200II, I am sure that it is an excellent amp and can handle lower impedances but I have not heard it and I suspect that monoblocks would be needed. It appears that I shall need to make a trip to hear some of these amps or purchase one of these options at a price at which I do not take a major hit if I need to resell. Any other thoughts are welcome. |