Futterman OTL

Letter to the Editor

(Julius Futterman)
 

"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 all purpose 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