For electrostats they are exceptional.
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I have Joule Electra OTL monoblocks, and I agree with Glide 3 that the performance is somewhat speaker dependent but on suitable speakers, the performance is stunning. I am unfamiliar with the Kharma speakers, but basically, OTLs work best on speakers with higher and relatively stable input impedance loads (definitely above 4 ohms and preferably in the 8 ohm and above range). As for specifics about the sound? My experience is that the OTLs have an extra special "purity" and "clean-ness" that on well-recorded material really makes one appreciate how beautiful music can be. Can other amps of different but well-executed design do as well? They're so many good ones out there that the answer probably has to be "yes." I've just not heard them, so for now, it's the extraordinary purity of OTLs that makes the difference to me.
Be careful with that combo, the Kharmas are basically a 6 ohm load, which gets to be borderline with an OTL. I have the CE 2.1s on order Merle, why not go with Lamm? The Lamm M1.1s (at an appropriate discount of course) are only slightly more expensive than the Joule OTLs or the bigger Atma-Sphere OTLs. I have the Lamm L1 preamp and it is so amazing, I implicitly trust anything Vladimir designs at this point. The Lamm/Kharma combo is a particularly synergistic one.
I have Transcendent Sound Stereo OTL driving my Merlin VSM-Millennium and love the combination. The Transcendent monos (80 watts/side) can drive down to 4 ohms successfully. I prefer them over Atma-sphere as they don't run as hot and are more dynamic. As far as price goes, they are reasonable as they are sold factory direct (www.transcendentsound.com) with a two-week trial period.
Dkuipers, you and I are certainly on the same page...I have the Kharma's with the L-1 amd ML-1 combo and it is amazing. I also use a Komuro 35watt set with the Kharma's and it is the equal to the Lamm's (better at some things not as good at others) Merle, I don't think any of the reasonably powered OTL's would have a hard time driving the relatively benign Kharma load. As always try and audition before you buy (I agree with the above posts that the Transcendent and Joule's are very nice I would also add the Graff OTL to this audition list).
I'm using Atma-Sphere M60 II monoblocks with Quad ESL 63's, and need only half the output tubes in a medium-size room. Sound is much improved over a Pass Aleph 3, not too shabby itself. For info on your speakers, why not contact the Atma-Sphere designer: firstname.lastname@example.org? I've found him very accessible and helpful. You can allow for some bias, but I'm sure he can be trusted on whether your speakers and his amps make a good match.
Merle - Just in case you do have an opportunity to demo (which probably can be arranged through Jed Barber of Joule, if there is no dealer in your area), let me add that from the other posts, the Joule OTLs may indeed be a possibility for use with your Kharma speakers. I use them with Merlin VSM-SEs, the speakers Joule Electra is most often associated with, and the nominal impedance of the Merlins is 6.8 ohms. If your Kharma speakers are basically 6 ohms and fairly stable (i.e., don't drop way down at some point in the frequency spectrum), the Joules will work fine. All the amps mentioned have great reputations. However, if you are curious about OTLs, you should try to give them a listen. Their magic is not at all limited to the mid-range. On the other hand what I hear and love about my OTLs may or may not strike your fancy as much. At the level of all the amps being commented upon, "taste" probably counts more than anything.
Like Vtvu, I have the 25wt stereo Transcendent Sound OTL and for $1,900.00, it is extremely good. The sound is very transparent and lets you hear everything going on in the recording, which can be good or bad. Because of it's incredible "clearness" it seems to somewhat mask the delicious juicyness that SETs give you. However! I'm now driving the woofer of my Proac Response 2.5s with the Transcendent Sound amp and the tweeters with a pair of Cary 2a3 monoblocks. This set up shows the wonderful midrange clarity and body that OTLs are capable of since the 2.5s cross over somewhere above 2500HZ.
Check out the David Berning products. Just a note, the Berning ZH-270 increases power as ohm load increases, unlike other OTL's, in which it is just the opposite. Really, it has the most advanced designed layout of any OTL. Check out the Berning website for more info. This little, non obtrusive amp had the most power and drive I have ever heard period. The bass had to heard to be believed. Advanced technology and design layout are extremely fascinating, but the sound is where this amp shines. Incredible. Note : To the man that posted above about purchase of the Berning - Hurry there are only a few new units left. David Berning does not build many period. If I had the extra money I would sell my beloved little Blue Circle BC22 (solid state) and scoup one up. BTW the ZH-270 has an estimated 20 year tube life. Amazing.
We are currently using Harbeth Compact HL7ES's and Jean-Marie Reynaud Twinn MKll's with the ZH-270. The David Berning ZH-270 works extremely well with dynamic speakers such as these and I am confident that it would also mate well with various other dynamic speakers with modest power requirments. There has been no indication that the David Berning OTL is limited in any way for exclusive use with an electrostat speaker. Our dynamic speakers match perfectly with this OTL in every aspect. This amp must simply be heard to appreciate. It has impressed me more than any single piece of gear I have heard, period.
Interesting thread. Although there are several references to OTL's being particularly suitable to esl's, how do they handle wide impedance swings... say, the 1.5 ohm to 32 ohm swings of my CLS's? Switching from ss to tubes was a huge improvement with my speakers -- are OTL's (like the 150w Atmasphere's) sympatico with my speakers' characteristics? I'm intrigued with the descriptions of OTL sound posted here.
Philefreak, no I have not had the opportunity to listen to any other OTL's. A friend of mine gave me a very strong recomendation for use of the David Berning ZH-270. He had used other OTL's before and said this one is one to have period. His strong opinion of this particular OTL stems almost exlusively from the amps excellent sonics and the ability to handle various impedance loads with high current (and runs cool!!). Often times other OTL's are limited for use with speakers with a flat impedance curve and relative sensitivity. This little monster is a high current musical animal with the most controlled, taught bass I have ever heard. The ability to control and convey dynamic passages within the music are unparalleled by no other amp I have used. Seriously, this ugly little 10 pound amp will knock you silly with dynamics and musical ability. For technology information check out the David Berning website. No other OTL uses this kind of advanced technology.
Pjilefreak, I don't have detailed specs for my CLS's (original version), but the guys at ML say that the impedance drops to 1.5 ohms at high frequency and increase to 32 ohms in the mid-bass. I know that the next incarnation (CLSII's) dipped to about .5 ohms but didn't swing nearly as wide in the other direction. The IIz electronics ,ake the speakers a bit tamer all the way around. Again, I have the original version which always did well with tube amps.
I have quite a bit of experience with the David Berning ZH-270 and I whole heartedly agree with Thorty40. From reading all of the posts on this thread, one of the biggest concerns with OTL amplifiers is there ability (or in some cases inability) to drive certain speakers due to impedance requirements. Just in a technological aspect the Berning is different from ALL other OTL's. The Berning is based on an impedance matching system which automatically matches the output impedance with that of the speaker. So it then all comes down to power requirements. The Berning creates 70 of the most powerful watts I have ever heard. Of course there are just some speakers that will just suck the life out of this amp, but not most. As far as sonics are concerned this amp is truly special in many ways. I have never heard an amp with more inherent speed and control over the music. Period. The midrange is silky smooth and more alive than I had ever thought possible from an amp, with highs to match. The bass is something which, put simply, must be heard to believe. If you think that tube bass can't ever match solid state, be prepared to be disappointed. I have owned many solid state amps and the bass can not even compare to the Bernings. All of the other OTL's mentioned above are all very respected names, and I am in no way implying otherwise. What I can tell you is that the Berning is a new kind of OTL which does not suffer from some of the problems that the aforementioned amps do. Put simply the David Berning ZH-270 is the most sophisticated amp in the world. A stroll through the David Berning website will confirm this as there are not many designers who can even decipher Davids theories and topology schematics. The Berning tips the scales at a mighty 10 lbs., runs cool enough that you can place your hand directly on the amp anywhere, indefinitely. Idles at one hundred watts and has a peak consumption of about 300watts. And yes, it only uses 4 output tubes! The most striking quality though, is the amps ability to effectively match the impedance of the speaker at any given time. It does all this, while maintaining a superior musical quality which at times is truly hypnotizing. By the way the Berning automatically bias's tubes on a cycle by cycle basis at all times. If you are in the market for an OTL amplifier(or any amplifier for that matter)make sure to put the Berning on your very, very short list. The Berning is an investment which will give its owner many years of enjoyment(there is a manufacturer claim of 20 years on the tubes)which will NEVER disappoint.
After listening to a pair of Transcendent mono's a month or so back, I knew I had to go OTL. I just received mine a few days ago and to say that this amp "transcends" any type of electronic signature is about the most apt description I can give. Just music; pure, ultra low noise floor, natural, dimensional and real. I can't imagine it can get too much better than this providing you have the right speakers. OTL's must be the best kept secret in high end unless maybe folks are too concerned about their less than dependable reputation. If you have the right speakers, love music and want to forget about the upgrade train, an OTL amp ANY OTL amp is a MUST audition. Another thing, this amp actually runs cooler than the ARC it replaced.