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Einstein Audio Components – The System – Part 2

07.06.2017 // Dirk Sommer

While working on Einstein's complete audio system with its laudable analogue source, in Part 1 of this review we had traveled from cartridge to tone arm, and from there to phono stage and preamp. Following up on that we will now take care of power amps and the semi-active speaker system, The Pure -– and, of course, the sound.

Connecting the fully balanced preamp The Preamp and the OTL amplifiers The Silver Bullet, in whose circuit balancing plays a considerable role too, we of course use XLR cabling. But more specific details on the concept of an OTL amplifier in general can be explained to you by tube specialist Jürgen Saile, who said it much better than I can... "Most tube amplifiers are designed using an output transformer that allows an impedance matching to today's low-impedance loudspeakers of four to eight ohms. However, due to winding capacitances, leakage inductances, etc., these transformers limit the bandwidth of the amplifier due to their construction. Sometimes the results will be more, sometimes less, according to the manufacturing efforts undertaken. In addition, this will, of course, entail considerable costs, when suitably designed.

But, what would happen if we simply omitted the output transformer? Julius Futterman came up with this idea as early as in the beginning of the 1930s, and presented the first commercial output transformer less (OTL) amplifier about 30 years later. This amplifier had two characteristic features: as the name implies, the output transformer was missing, and second, per channel several tubes were connected in parallel, usually eight to twelve. In later designs from other manufacturers, up to 20 tubes could be found, which then at the same time spares the central heating effect in winter. The purpose of this measure was, of course, to lower the output resistance, since the output transformer was missing.

The Silver Bullet:  The mono amps come without an output transformer. Under the chrome covers the transformers of the two power supplies are concealed
The Silver Bullet:  The mono amps come without an output transformer. Under the chrome covers the transformers of the two power supplies are concealed

The direct coupling of the loudspeaker to the power tubes promised an improvement of the impulse response and a more accurate imaging of the waveform. Nevertheless, it's not that easy to use a tube without a transformer to drive an eight ohm load. I'm choosing my words every carefully here. The standard design of an OTL circuit in the 1950s was the so-called single-ended push-pull circuit, which, however, had a drawback:  it was unbalanced. The output signal was simultaneously generated from the cathode of the one tube and the anode of the other tube. In addition to some variants of this circuit, there is a simple modification that compensates for this asymmetry, and was launched by Electro Voice:  the so-called Circlotron. With this modification, both tubes behave in the same manner. Using two 6C33C in parallel—as in The Silver Bullet—an output impedance of approximately 10 to 15 Ohm can be achieved; with several of them connected in parallel, it could be even further reduced. Circlotron amplifiers always require a balanced input signal, and the drivers should be able to drive higher capacities, depending on how many tubes are connected in parallel. One disadvantage of this circuit is that two power supplies are required per channel, which then have to work in a floating manner, that is, without mass reference, otherwise the whole concept doesn't work. A reasonably linear power supply is a considerable cost factor in such a device, so there still will be no OTL bargain at Radio Shack in the future.

This Circlotron circuit has already been used by Rolf Weiler, the developing force at Einstein, in the predecessor of The Silver Bullet. Of course, this was not done without performing some changes. While in the original circuitry the loudspeakers are driven by the cathodes, here the anodes take over like in a single-ended amplifier. So, a certain similarity is to be found here. In addition, there is a slight over-all-feedback which somewhat reduces the output resistance a bit more. The amplifier thus has a damping factor of just under 100 at eight ohms, which is a sensational value for a tube amplifier.

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