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In Reply to: RE: OTL vs SET posted by saki70 on January 11, 2015 at 13:48:31
that you have to consider first. If you are running very high efficiency horns with very high impedance (e.g., well above 8 ohms nominal), then the two types of amplifier can compete on a level playing field. In which case, I agree with the others that an OTL will be, shall we say, less colored. If you are using some other less efficient type of speaker, but still with a high-ish impedance, I would think the OTL would prevail more obviously.
By the way, the SET lovers that I know seem to prefer 2A3 and 45 tubes vs the 300B, but those two types give even less power and require even more specialized types of speakers, as you probably know.
Follow Ups:
If the impedance of your speakers is too low for an OTL, it may be possible to change the impedance as I did.
My speakers (GR Research LS-9's in kit form)are made of a string of 12 6.5" base/mid cones and a string of 9 BG Neo 8's. Each string was configured in a series/parallel arrangement (6x6 for the woofs; 3x3x3 for the tweets) and put together in parallel at the crossover. The result was an overall impedance of about 3.5 Ohms--pretty flat across the spectrum, but also pretty low. With my Atma M-1's there was noticeable droop at both ends.
Ralph suggested that I rewire the speakers to bring up the impedance by putting the two strings more in series. I changed the arrangement to be 6+6 (series)for the woofs and 4x4-1 (four in parallel with four, and disconnecting one tweet)for the tweets. By doing so (and making a couple of simple crossover changes)I brought the impedance up to 12 Ohms across the spectrum. Now the suckers really sing.
If your speakers are in array form, you might be able to do the same thing.
It's hard to believe that the designer of that speaker would let it go out the door with such a low nominal impedance, when it was such a simple matter to revise the circuit so as to obtain a much more amplifier-friendly impedance. But changing the crossover in such a design, once you've altered the connections of the drivers, is often not at all a "simple" matter. Were you able to make impedance measurements of the separate revised driver stages so as to maintain the same crossover points and slopes that the designer originally intended? I guess this could be done if the crossover is first-order, more easily than if it is a complex 2nd or 3rd order design. Anyway, regardless, the only thing that counts is that you are happy with it now.
Yes to all of the above. I do not know why the manufacturer designed the speakers as he did. He uses them primarily with conventional tube amps (Dodd's)with transformers. But also, they would work just fine with solid-state amps as well.
You're right about x-over changes. We did measure the strings individually and together. Despite the x-over being 3rd order, all it took was the change of one capacitor and we achieved a very good acoustic response. Better to be lucky than smart, says Confucious.
We are going to experiment with a whole new x-over with first-order slopes. According to BG, this will not hurt the tweets any, even at 300 Hz, versus the 850Hz we have now. Anyway, we can put it back if we don't like it.
Cheers,
Chuck
BTW, Lew, I believe I passed on the news that Lou Johnson had passed away. Imagine my shock when I bumped right into him coming off the elevator at the Venetian at CES last week. Rumors to the contrary, etc.
Catastrofe ;
Very interesting , I have Di Capo i's . Do you like the way they sound with the big OTL's ? Do you find that the Auto-formers effect the sound or have a flavor ? Have you ever tried the Di Capo's with an SET ?
I know some have used SET"s and say it works . But as Ralph iterated , how much of the true SET sound is heard if one must crank them up passed the 20% point .
I didn't care for the sound of the De Capos with the Autoformers. The sound was "bloated" and overly full. I love the way they sound fed directly out of the MA-1s.
I've not tried an SET, as I don't have easy access to a quality amp.
Thanks to all of you .
I had forgotten about the high impedance requirement for OTL's .
I have 92db. speakers that dip down to 6 ohms .
Happy Tunes
saki70
MA1 should drive that speaker easily and well.
A lot also depends on the particular OTL and the design of the speaker, as well as its impedance. More at the url below.
The Atma-Sphere amps don't run feedback, so the speaker might require more care in the choice, but Merlins are similar to what you describe and they work great with our M-60 and other OTLs too. So impedance is not the last word.
However, FWIW:
There really isn't a good argument for a 4 ohm speaker if your goal is sound quality, regardless of the amplifier you use. Look at the specs of any amplifier made and you will see that the 4 ohm distortion value is consistently higher regardless of the amp.
The kinds of distortions in this case happen to be of the type that are easily audible (higher ordered harmonics). The result is that the amp will sound harsher and less detailed. So a simple method of getting any speaker to seem smoother and more detailed is to simply build to to be a higher impedance.
If sound pressure is your goal then you have a 3 db argument for 4 ohms.
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