OTL amplifiers

Thank you Detlof, I have no plans or desire to replace ANY of my "sources".

re: SET OTL amps

Same goes for Atma-Sphere.

Perhaps Twl is referring to single tube SET's.

One of the reasons I purchased Atma-Sphere M-60 amps was low tube cost. A speaker that is a good impedance match is a good idea, but can be overcome with the use of ZERO autoformers ZERO Autoformer Website. I couldn't be happier with the performance of my amp/speaker duo without the autoformers however. The Atma-Sphere's do everything I could ask for. I think they're incredible and I love them. In fact, they gave me the same warm fuzzy feeling I had when I met my wife.

I agree with Trelja that finding an OTL friendly speaker is the optimal way to go, but I did have exceptional results with the ZEROs and a previous set of speakers that, on paper anyway, were a good match for OTL amps (nominal 8 ohms that never dip below 6.5 ohms). I obtained these results by using an alternative 2X configuration that I understand involves using less of the windings in the autoformer. Using this configuration, I was able to retain the refinement of the Atma-Sphere M-60s, in the process though bass performance suffered. I then put a second pair of ZEROs, using standard 2X configuration, on the woofer binding posts and the overall performance of the system went through the roof. The whole spectrum was improved, bass performance was phenomenal, and the mids and highs benefited in major way as well. The resolution of the system improved enormously. I had strong expectations that by using the second pair of ZEROs I would get the bass performance back that was lost when switching to the alternative 2X setting. I ended up with performance that vastly exceeded the sum of the parts - making me a wildly enthusiastic believer in the ZEROs. I now use a pair of speakers that are rated at a 10 ohm nominal impedance and I achieve better results without the ZEROs.

The alternative 2X setting I refer to use the white and black leads attached to the speaker binding posts and the blue and yellow leads attached to the amplifer posts. This configuration brought back the "magic" OTL sound, but resulted in a small loss on the bass side of things. If I only had one pair of ZEROs or did not have bi-wire capability I would have easily, and happily settled for this setup and accepted the loss of bass output for the improvements gained elsewhere.

Not entirely content to leave it at that however, I contacted Paul Speltz and he suggested I try a second pair in standard 2X mode on the woofers and like I said before, the results were phenomenal. The best of both worlds and then some.

I don't know why Paul doesn't promote this setup more at his website. I don't know if my results would be achieved on other speaker models, but on N.E.A.R. 50Me II loudspeakers the gains were absolutely awesome.

One method I never tried, but would be curious to hear the results of, would be to bi-wire using just one pair of ZEROs.

On the N.E.A.R.s I would use the configuration described above on the high frequencies and another alternative 2X such as brown/green or gray/green on the low frequencies. If anyone tries this, you might as well try the reverse as well, brown/green or gray/green on the highs and white/black on the lows, to see if there are any gains.

Refer to Group 2 Configurations at the ZeroImpedance.com website to see a diagram of the alternative configuration options I refer to.

I know that this bi-wire with one pair of ZEROs is an option, but for safety, please check with Paul Speltz before trying.

Dfrigovt, since much of what has been posted here is way outside the scope of what you originally requested (especially from me) and is probably very confusing, let me answer your question more directly as pertains to Atma-Sphere M-60 Mk. II.2 amps.

Advantages: high-resolution, effortless sound; extended in both directions, rich (but not syrupy) and dimensional

Disadvantages: speaker choices are more limited, unless you use a large OTL or a corrective device like the ZERO autoformer, and possibly heat (hasn't been a problem for me though)

Since there are plenty of excellent speakers out there that work well with OTL's, I don't see OTL's as being problematic in any appreciable way.

BTW, do you have a particular speaker that you would like to pair with an OTL?

Dfrigovt, I think you have received a lot of misinformation about OTL's that has confused the issues.

Like Trelja, I can only speak about my experience in relation to Atma-Sphere M-60 amps. I have used my M-60's for over 3 years without one problem arising. Yes, the M-60's do require the right speaker, but that can be said about just about every amp out there (too small, too large, etc.) You can pair the Atma-Sphere M-60 and the smaller S-30 successfully with just about any speaker with a nominal impedance of 8 ohms or higher, this is not a small number of speakers by any means. With the ZERO autoformers you can pair them with any speaker that is efficient enough to be driven by 30 watt and 60 watt amplifiers.

With the Atma-Sphere's you will not have to retube frequently, you will not have to worry about blowing your speakers or amps, you will not have any difficulty finding speakers that sound good with these OTL's. I have no knowledge by which to assess other OTL designs, but it is obvious that Tenor, Joule, Transcendant and Graaf are other OTL designs that people are using successfully as well.

OTL amps are obviously not the only amplifier type that will bring nirvana, but the special qualities they possess just might be nirvana for you personally. As you mentioned in your initial request, you are a tube fan, there is no practical consideration that should prohibit you from auditioning these aside from, perhaps, geography.

There is nothing about using Atma-Sphere amps that will rule anything in your life other than a simple 5 minute biasing procedure that should be done every 6 months. It would be a shame if you missed your nirvana by dismissing OTL's out of hand because of wrong information provided by others. There is nothing pertaining to modern era OTL's that will throw you any curves. I don't know why people keep bringing up issues that don't apply any longer, and haven't for years. This is like saying that it is problematic to fly more than a couple hundred feet because the Wright brothers could do no better.

If you haven't already done so, I encourage you to read the ZERO's Story at ZeroImpedance.com, it elaborates on the effect impedance has on the matching of an amplifier to a speaker. If you don't have the motivation to read all of the document, just read the Technical Sidebar near the bottom. This information is useful whether you end up with OTL's, ZERO's, or none of the above.