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I also happen town a pair of David Berning ZH270 power amps driving Maggie 3.6 speakers. The Bernings use a large output capacitor instead of output transformers. I don't think I hear any downside to this design. They are spectacular amps. I realize that the planar forum has become basically a Maggie forum and although I own Maggies I wish there was a greater diversity of planar speakers discussed on that site.
Futterman amplifiers are capacitor-coupled. The Berning ZOtl amplifiers are not; they work by raising the audio signal so it "rides" on a frequency around 250 KHz, then the carrier frequency is filtered out at the output by use of a small inductor, rather than an output transformer. Because the carrier frequency is so high, only a small inductor is required which makes it possible to achieve very wide bandwidth and low distortion. I'm sure this is a gross over-simplification, but for sure David Berning is a brilliant guy. Pretty much nothing he designs is ordinary. Absent that small inductor, the Berning ZOtl amps are direct-coupled.
EDIT. I'm sure there's an electronic network involved too, in filtering out the carrier frequency, not just an inductor.
Of course you are correct on all counts. I was thinking of the single ended OTL by Transcendent sound that uses a big output capacitor. The Berning just has a small input capacitor to keep DC out. Direct coupled the rest of the way
"Of course you are correct on all counts. I was thinking of the single ended OTL by Transcendent sound that uses a big output capacitor. The Berning just has a small input capacitor to keep DC out. Direct coupled the rest of the way"
Looking at the ZOTL patent, it seems that the MOSFETS in the output stage, which are actually switching the high currents that pass through the loudspeaker, are DC isolated from the previous stage (which is vacuum tube), by the high-frequency transformer.
I think one way one can view the ZOTL is that what is essentially a switching-mode power supply is producing a high voltage output, which is then loaded by the "output" tubes. As the anode current drawn by the output tubes increases, so too the current draw in the low-voltage MOSFET switching circuit side of the "switching-mode power supply" must increase. The change in current on the low-voltage side is correspondingly much bigger than the change in current on the high voltage side. The high current on the low voltage side passes through the loudspeaker, and thus a large current change through the loudspeaker is produced as a result of a much smaller current change through the "output" tubes. In effect, the "switching-mode power supply" acts as an impedance matcher between a high impedance "output" tube and the low impedance speaker.
It is true that the impedance matching operates all the way down to zero frequency, so I suppose in a sense one could say it behaves as if the output side with the loudspeaker is DC coupled to the input side with the "output" tubes, even though there is a high-frequency transformer isolating one side from the other.
It's an ingenious design, and although I don't think it is fair to call it OTL, it certainly has a novel way to impedance match a tube "output" stage to a low--impedance speaker, without requiring the tubes to pass the high currents needed by the speaker. So it achieves the impedance matching without a conventional OPT.
What is not clear to me, though, is why such a MOSFET/tube hybrid amplifier is likely to be any better than a more conventional type of MOSFET/tube hybrid amplifier, in which vacuum tubes are used in order to drive a conventional MOSFET output stage.
People say it sounds very good, and I'm sure they're right. But probably conventional MOSFET/tube hybrids can sound very good too.
For your far superior explanation of the ZOtl circuit. When I wrote my own response, I was in no position to research the subject, was relying on fragmentary memory of an early review of the first such amplifier to come from DB; I think it was in Positive Feedback. This is not to say that I understand the circuit as well as you apparently do.
There are single ended (SET)amps. A single tube for amplification. 2A3, 45, 300B, 1605, etc.
There are parallel single ended amps. They use two tubes, paralleled, for amplification. Twice the power.
If this is true; a single ended OTL can use only one tube for amplification.
Question. Is the 2A3, 45, etc. a gain stage?
Could a 6SN7 be replaced in one of Ralphs amps by a 2A3? (I know there's wayyyyyy more to it then that, but in theory, could it be done?)
-Question. Is the 2A3, 45, etc. a gain stage?
- In conventional SET amplifiers 2A3 ,45 ,and similar output power triodes connected as anode follower gains both voltage and current gain , resulting in specific output power gain ,
however those SET amps usually needs at least one more voltage gain stage to work properly , since voltage gain only from those output power triode tubes is not enough to interface with low level signals coming from standard sources , as CD player , pre-amp ,PC card , etc ,....
-Could a 6SN7 be replaced in one of Ralphs amps by a 2A3? (I know there's wayyyyyy more to it then that, but in theory, could it be done?)
in some way all Atma-Sphere OTL power amps can be considered as two same SET-OTL power amp units joined together forming unique OTL Circlotron Push-Pull balanced power amplifier unit .
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Your explanation was perfect.
When I read about SET OTL's, I scratch my head. By their very nature, A-S OTL's are a type of SET.
I remember once asking another manufacturer 'your SET amplifier has an XLR, but is it a true balanced circuit?'. 'No', was the response, 'it would be too expensive to build. It would need another amplifier built in, and another output transformer'.
The single ended OTL by transcendent sound is no longer made Each channel had a driver tube and 4 output tubes in Parallel. 1-1/2 watts per channel. You can do a search for single ended OTL and see a lot of info about this amp
David Berning makes an SE OTL, called the "Seigfried". I think it utilizes his ZOtl technology, but I am not sure.
I have the Atma-Sphere MA-1 set up. 140 watts.
I have the Tenor WP75 waiting for another chance. 75 watts.
If the Transcendent was 1.5 watts, with 4 output tubes, then SET's are most often low powered.
The obvious exceptions being the 211, 845, 1605 etc tubes.
I believe I'm starting to understand.
by paralleling more and more output tubes, but inevitable mismatching among the output tubes and additional requirements of power handling in the output transformer, are said to result in a deterioration and loss of SE "magic", such as it is. 100W would be about the practical limit, though. Those Kron amplifiers from Europe were an interesting attempt at high power SET. SE amplifiers are best mated with very high efficiency, and high input impedance, speakers, such that low power output is more than adequate. Real aficionados seem to feel that the lower the power output, the better the amplifier will sound.
Have you auditioned those Tenor amplifiers? I am curious to hear them some time.
You are right ! , since my experience with DIY SET amps are that 10W max is top optimum limit , SET with more output power than 10W cause SET magic to gone ,
and of course that those low power SET amps have to be very carefully matched with very high-efficiency LS for best sonic results ,
IMHO the best SET amps are those simple two stage based ,RC coupled ,
any way , except hyper liquid treble sound which come from typical SET amp I was never impressed or big fun of that sort of amps , there is so many things that miss for me , special at reproduction of lower bass range same as at highest sonic range ,
Best Regards !
General Warning , I have no intention to start OTL vs SET war with nobody !
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Tenors claim to fame was 'Harmonic Structural Integrity'.
To that end they succeeded.
However, the Atma-Sphere's do most of the other stuff better.
Once my new speakers are complete, I'll try the Tenor's again.
My second hand information leads me to believe the Tenors used voltage regulation on the output stage, unlike most tube amplifiers, let alone Atma-sphere amplifiers. And they use a 12AX7 for voltage gain at the input, instead of a cascode a la Atma-sphere. And they are not dual-differential at the input, I don't think. But it's dangerous to assume from design or specs that one can therefore judge the amplifier. It may be excellent and does have its adherents.
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