OTL-1 Every man needs a Mt. Everest to test the mettle of his soul, and as soon as the NYAL team had perfected the Futterman circuit, we needed to get higher….because once we heard this circuit with a regulated power supply, we knew had to take up the challenge. Let me not be modest about this challenge. No other company in the history of the audio arts, that I am aware ever dares to design both a high voltage (350 volts) and high current (10 amps) power supply before or since. The OTL had TWO 5,000 watt regulated power supplies…which are testament to Ted Hamond's engineering genius.  At the time this was the most complex, most expensive ($12,000 in 1984) heaviest and most complex tube amplifier and OTL amplifier ever built, and explains why I understand the meaning of complexity, and the value of simplicity. This amplifier caused quite a stir in Japan and in 1984 Japan's Stereophile Magazine gave it an award. Now you know why I have such a short fuse when I here tube amplifier manufacturers whine that designing regulated power supplies are too hard. Both the OTL-1 and OTL-3 were rated at 90 watts into 8 ohms, but the OTL-1 sounded like it had twice the power. I used two pairs of OTL-1 on my West Electrostatic monitors. An interesting note: I was the first to start using retro-meters in America, and others soon followed…and the meters cost me $55 each. Harvey Rosenberg

 

 

OTL-2 A special triode version of the OTL-4 that we made just for the Japanese market. I had one specially made for me that I connected a regulated power supply to and it was one of my favorite 10 watt per channel OTLs. How sweet it was. Harvey Rosenberg

OTL-3 This was the most popular model and consisted of two mono amplifiers. Quad 63 owners loved them. Each amplifier had their own built in fans to keep them cool, and if used properly would give years of trouble free use. I am aware that many of the original OTL-3 owners are still using their amplifiers with original tubes…and that is fifteen years of ecstasy, because they used these amplifiers properly. On the other hand musicmaniacs would here these amps on Quad 63s and then used them on their Acoustat Monitors, where were very inefficient, 2 ohm speakers…which I advised against…and then I would get a call from and OTL-3 owner complaining that after only two months his amplifiers sounded distorted. As I suspected the output tubes were exhausted, and for good reason. The rumors started by the mealy minded that the Futterman amplifiers were unreliable arose because these thermionic feeble never understood that OTL amplifiers can only be used with a very limited range of loudspeakers…and be reliable. It is understandable that so many would try to use OTL amplifiers…because they sound so good…but the reason OTLs are not more popular is because they have been abused, and been given a bad rap. My warning about the proper use of OTL amplifiers is wire other OTL manufactures want me taken for a long walk on a short peer. My invitations to debate them on this subject, are unanswered.  Harvey Rosenberg Letter to the Editor by Julius Futterman, as a reply to a test of the OTL3, in "The Audio Critic 1979"   I am pleased with your evaluation of the sonic virtues of the H-3aa power amplifier but do take exception to two of your assertions : The power tubes I use (6LF6) are being manufactured in the USA by GE and Sylvania. They are also being made in Japan and Yugoslavia. I have been informed that they will be around for many years. In your Vol. 1, No. 3 issue, page 4, you wrote that the Berning hybrid tube amplifier using 6LF6 tubes will be manufactured by Audionics. Also please note: The Acoustat X ($2.200) uses 6HB5 tubes, which is also a TV tube. This tube was used in my H-3 stereo amplifier in the 1960's. The Beveridge 2SW-1 ($7.000) uses 40KD6 tubes, also a TV tube. My point is at these two types are of even older vintage than the 6LF6 tube, yet you did not caution a buyer of the Beveridge system about their not being able to obtain them in a few years. I am sure if tubes do a better job in an amp or preamp they will always be available from one source or another. Before taking up the second subject that makes me unhappy I would like to digress for a minute, if I may. Before transistor amplifiers the rated impedance of most high fidelity speakers was 16 ohms. In Britain practically all speakers were 15 ohms. There were sound reasons for this as, all other things being equal, a higher impedance speaker is more efficient and the crossover design is not as complex. We are referring, of course, to moving-coil speakers. Electrostatic speakers are inherently of high impedance and this is lowered by means of a transformer. The Acoustat and Beveridge speakers use a different approach. They do not use a transformer; instead they employ very high voltage amplifiers to drive the speakers directly. The KLH-9, Quad, and Koss are examples of speakers using transformers. In general, the lower the turns ratio of the transformer the better the speaker because of tighter coupling and other factors that 1 will not go into here. The KLH-9 impedance is 16 ohms, the Quad 15 ohms and the Koss 4 ohms The KLH 9 and Quad were designed for tube amplifiers, the Koss for solid state. The reason for the lower impedance of speakers today is, of course, the fact that transistor amplifiers, being voltage limited, provide more power for such speakers. As an interesting aside you implied in your review of the Tangent RS2 (Vol. 1, No. 5, page 25) that it was an inefficient speaker as you were able to make the Levinson ML-2 clip on it with a master tape of piano music. On the other hand, I can make the Tangent RS2 play very loud with the H-3aa. The reason for this is simple : The impedance of the RS2 at 70 Hz, for example, is 11.5 ohms; at 500 to 2000 Hz it is 9 ohms, and it rises steadily to over 20 ohms at 6 kHz, which is well above the fundamental tones of the piano. With the ML-2's 14 volt maximum voltage rating you can see that there is very little power to drive the Tangent. End of digression. Many audiophiles are using the H-3a and H-3aa with electrostatic speakers such as the KLH-9's that keep their impedance high up into the uppe range and also with the Quad, which does fall to low values but nonetheless sounds extremely good. For owners of double Quads I recommend wiring them in series and, it I may be allowed to boast a little, they do sound fantastic. Thank you for allowing me to comment. Sincerely yours, Julius Futterman Futterman Electronics Laboratories New York, NY   Editorial Reply to Julius Futterman's Letter It was unquestionably a miscarriage of justice that your amplifier was singled out for our general caveat about the future of vacuum-tube audio equipment. What's true of one particular design is true of them all: their longevity depends on the TV replacement market, the Russian aerospace industry and other factors outside the world of audio. We seriously doubt whether audio manufacturers by themselves could keep even a single vacuum tube factory in business through the 1980's. On the other hand, you may be quite right insofar as these other demands may preserve vacuum tubes from extinction for decades to come. Your guess is as good as ours or anyone else's. The rest of your comments all point to an implicit conclusion we have shared for quite some time, namely that the power amplifier and the loudspeaker should be conceived and designed as a single system, the "back end" of the audio chain, not as two separate allpurpose modules that never quite mate optimally. The trouble is that very few audio designers have an equal mastery of both disciplines. For example, neither the Acoustat nor the Beveridge amplifier is as highly refined as yours, although their philosophy of integrated design is certainly valid. -Ed. The Audio Critic , 1979

 

 

OTL-4 Stereo 40 watt per channel amplifier that was perfect for the QUAD 57 because it would never harm/arc the panels. Harvey Rosenberg

 

OTL CIRCUIT BOARD This why the NYAL OTLs are so easy to service. What you are looking it is the whole Futterman OTL shabang…NO OUTPUT TRANSFORMERS NEEDED. You will also note that this is a hybrid printed circuit/hard wired design with all of the high current running through Teflon coated silver plated copper wire. Harvey Rosenberg

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