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65.6.174.198
Full range, good off-axis response.I am in Miami, so the smaller the amp they can get away with the better.
Just gatherting a list at this point, so price is open.
KP
Follow Ups:
Recently I've been working on designing a speaker specifically intended to work well on a high output impedance OTL amplifier, and it might be useful if I shared some of my observations.The amp I'm working with is the Atma-Sphere S-30, which puts out 30 watts into 8 ohms and 45 watts into 16 ohms, and Ralph recommends you not use a speaker whose impedance goes below 7 ohms in a region where there's lots of music energy. So right off the bat we come up with our first rule of thumb for matching speakers with high output impedance OTL amps:
1. All else being equal, high impedance speakers work better with high output impedance OTL's than low impedance speakers do.
Sixteen ohm woofers are pretty scarce, especially with the other characteristics I wanted. I had a pair of prototype 16 ohm woofers built for me, and while they were promising on paper and measured pretty well, they didn't sound as good as I hoped. A couple of 8 ohm woofers I had on hand sounded better, presumably because the manufacturers took more time to optimize them. So we come to the second rule:
2. A good 8-ohm speaker will sound better with a high output impedance OTL than a mediocre 16 ohm speaker will.
In addition to the Atma-Sphere amp I have several other amps on hand, including a single-ended triode amp and a couple of solid state amps. I tweaked the speaker until it sounded balanced on the Atma-Sphere amp, then tried it on a very low output impedance (high damping factor) solid state amp. To my horror, the measured frequency response changed by about 4 dB in the midrange! At that stage in the design, the impedance curve had a huge peak in the midrange. When I switched to the solid state amp, the result was a dip in the frequency response where that impedance peak was because I had optimized the design for the OTL amp. So I learned the hard way, and am now paying much closer attention to the impedance curve.
Now you won't find any published frequency response curves measured with OTL amplifiers - the magazines that do serious testing (Sterephile and SoundStage online) use solid state amps. But they do publish impedance curves, and that will help you to evaluate what effect an OTL amplifier will have on the sound.
At the link below you will find SoundStage's measurements of the Coincident Total Eclipse, which is a speaker that works well with high output impedance OTL and SET tube amps. The frequency response curve shows the Total Eclipse to be a bit weak in the bass region, and there's a large dip in the region between 1 and 3 kHz.
Now scroll down and look at the impedance curve. Notice that there's an impedance peak at 50 Hz, and another fairly broad peak centered on 1.5 kHz. So if you connect this speaker to a high output impedance OTL or SET amplifier, the frequency response curve will be boosted at the impedance peaks! Israel Blume has put the impedance peaks right where they will definitely improve the speaker's frequency response with that type of amplifier. Israel if you read this, nice job!
Which brings up to the third rule:
3. By looking at the measured frequency response AND impedance curves, you can make an educated guess as to how a speaker will work with a high output impedance OTL amplifier.
Hope this helps.
Duke
Good observations. One thing I see, however, is the frequency response midrange dip is centered around 2 kHz (pretty even at 1 kHz) while the impedance rise is from 1 to 2 kHz, which could translate to a peak around 1 kHz when driven by an OTL amplifier. Also of interest is the minimal phase shift - the speaker system is a fairly benign load in that respect, important for use with SETs especially.
Hi Brian,Good eyes, Brian! Yup, I think you're right - the response dip and the impedance peak don't quite line up, and the result would probably be a response bump at 1 kHz or so with an OTL amp.
Duke
Thank you for the replies, guys, and I realize my question is overly board but was gathering a list of suspects right now.OTL's may not be the way to go for me, considering the heat, and I now generally for OTL's you want high impedance more than anything else, but I am learning that whenever I simplify I tend to like the results. Running the Rogues in triode had a much greater impact in my enjoyment of music than I would have imagined. Nothing overt, the change was subtle in the short term, but in the long term it has been most profound. STE's seem to require even more narrow choice of speakers, I think, so OTL's might be a more viable choice for me.
Listening room size is 18x15, I listen to mostly acoustic, but with broad dynamic demands. Plus, when I do play something that's electronic/amplified, I don't like to run out of power. Currently using ~100 triode watts with low 90's speakers, and volume is acceptable.
Willingness to use Zeros? Sure, but I am a little wary since this going away from the path of 'simpler is better' as I mention above.Duke, please fell free to send me any info you wish.
KP
Many of the speaker manufacturers Ralph mentions offer models that would work well with the S-30. I've heard Classic Audio Reproductions speakers driven by the S-30, and the sound was magnificent. I think Ralph has a pair of C.A.R. T-3's in his home.If you have corners available but aren't on a C.A.R. Hartsfield budget, you might consider the PiSpeakers 7 Pi. I don't know what the impedance curve looks like, but assuming it's an honest 8 ohms (no dipping below 7) it should work well.
Ralph invited me to share a room with him at the upcoming Rocky Mountain Audio Fest in Denver. He's bringing the S-30 and I'm bringing some fairly affordable speakers that are specifically designed to work well with the S-30. Shoot me an e-mail if you'd like more information.
Best of luck to you,
Duke
Easy to Drive Speakers (works with M-60) that work without the ZEROs:-a short and very incomplete list-
Quad 57s and 63s, Classic Audio Reproductions, Spendors, ProAc Response series, Merlins, Lowthers, most horns, Talons, JM Labs, nearly all 'full range driver' speakers, Silverline, NHT, Coincident, Wilson Audio, ZU Druids, Gallo, AvantGarde, Reimer, Soliloquy, Vandersteens, IOW if the speaker is 8 ohms in the woofer region and 89 db or more it will probably work...Works without ZEROs but need more power (MA-1 or MA-2):
Avalon, Sound Lab, Wilson Audio, newer Quads, Magnaplanars, Appogee, MBL (easier to drive than you would think!), Audiostatic, bigger Vandersteens, Kharma, Cabasse, Meadowlark
There's a bit of crossover (no pun blah blah) in this list depending on models in the manufacturer's lineup. As you can see, in the bigger amps 4 ohms isn't such a big deal.
ZEROs can help smaller amps with lower impedances and often the MA-1 too. Anytime you can get the impedance up the amp will make less distortion, but you can run into tradeoffs so the general rule is 8 ohms=0 ZEROs :)
Have fun!
What about Tannoy? I'm considering Yorkminsters....Just thought I'd get the question in while the thread was active.
-there's a lot that I missed.
I hesitate to disagree slightly, but in my experience with Sound Lab M1s, they have a dip in impedance to about 4 ohms at 500Hz and then go down as low as 2 ohms at high frequencies. I found a tremendous benefit to using Zeros with my MA240s (about the same power as MA1) driving M1s. Without Zeros there is a noticeable midrange "suck-out". With Zeros, it's "all good". Also, Apogees and Magneplanars?????? These are 2-ohm and 4-ohm speakers, respectively.Somewhere I once read in Paul's writings that the Zeros were designed to work with amps producing up to 150W. When one uses Zeros with MA2s (rated at 220W) or other high power tube amps, do the Zeros themselves limit the power made available to the speaker? I once queried Paul about building a pair of "high power" Zeros, but he never responded.
Hi Lew-yes, there is some varience in the Sound Labs!! Depending on the vintage, ZEROs can help a lot. I recently heard a *new* set of A-1s running with MA-1s and the tonality was utterly neutral with a presentation stunning in its overall musical nature- I totally get why the speaker has such a following!
Paul Bolin based some of his comments on the MA-1 in his review several years ago while using Appogee Duetta Signatures, which are 4 ohms and arguably the best of the Appogees. Other Appogees, like the old Full Range, need either ZEROs or the Atma-Sphere Z Music device to work with almost any tube amp.
The people at Magnaplanar maintain that the best they ever heard the MG-20s was with MA-2s. MA-1s make a good showing on them too. To do well with M-60s the ZEROs are required.
Ralph,does using the Speltz Zero box make an OTL amp an effective non-OTL amp? (one has inserted a transformer/autoformer between the amp & speaker effectively making it an transformed amp). One could/does lose the true benefits of an OTL amp as the "purity" of the technique - OTL directly driving a loudspeaker - is lost?
Thanks!
The effectiveness of the ZERO is the fact that the amp driving it already is comfortable with a 16 ohm load. Most tube output sections are driving between 1000 and 5000 ohms. So the result is something that has not been done before- in effect the gestalt amplifier is not an OTL but it is not working inside the conventional transformer-coupled constraints either. This would not be possible if you did not start with an OTL to begin with.On speakers where the amp is uncomfortable (for example, M-60s driving Magnaplanars) this allows the amp to work quite well where it would not have done poorly before- in such a case there are no trade-offs. In the case of 8 ohm speakers there are trade-offs, but then you don't have to use the ZERO unless you want to- with a regular tube amp you have no option. So its something different- something that works. We generally recommend the ZERO if the amp is working with less than 8 ohms, and do not if the amp has 6 ohms or more to deal with.
With our larger amps 4 ohms is not a concern, although the ZERO is sometimes helpful with the MA-1 and speakers less than 4 ohms. If you look at the ZERO as a problem solver then you get a better idea of what its about.
nt.
Which amps?Listening room size?
Types of music and range of volume levels?
Willingness to use Zeros? (I do, with Atma M60s, to good benefit)
You didn't mention your price range, but Merlins are often matched with OTL's, Joules, Bernings, if you're worried about heat then the Berning is probably your best bet.
Also, if you invest a few hundred dollars for Zero autoformers, the list increases nicely.
After you spend 6-20,000 on amps,cant wate put AUTOFRMER,S to make the speaker,s work! Make,s me run right out to get my OTL.Thanks
I thought the same thing, why would anyone buy a OTL only to have a transformer added to the input of the speaker. After talking to several guys "in the know" I thought maybe I would give them a try, and, with a 30 day return/trial what would I have to lose to try them on my Atma-Sphere M60's. Well I still have the speakers and I still have the Atma-Sphere amps and the system has never sounded better. I did not use the 30 day return option I guess goes without saying. I was reminded that the Zero's are not transformers but autoformers, not sure the difference but they do work in allowing the ability to use a lower sensitivity speaker to be used on an OTL with great results. Again for the 30 day trial what does one have to lose, if you like your amp and you like your speakers you might want to try the Zero's They also work with SET amps on lower sensitivity speakers. I am not in anyway affiliated with Atma-Sphere or with the Zero Auto-former companies just a happy owner of both the OTL and the Zero Auto-former...........good luck...........Bob
Yes i no, glad u are happy.For my self I got more amp. my M60 Diden get it so M1,Ok now M2 an so on. $33,000 now $450 to make work?
Tubevibe, I am not quite sure what you are trying to say but, if you have $33,000 wrapped up in amplifiers and you are not happy with the sound why not try a $450.00 fix. If it still does not work for you than return the $450.00 product (auto-formers) and you are out nothing except $30.00 or $40.00 shipping. To me this is a no-brainer, nothing gained and nothing lost. If you have $30,000 amplifiers, I can't see how spending another $30.00 or $40.00 for the chance it would work is of any risk, and if it should work $450.00 is a small price to pay when comparing it to the $30,000 you have already paid, you just might be surprised.
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