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The amount of thought that Mr. Rozenblit has put into making this amplifier speaker safe is refreshing. His explanation of his choice of part values and topology is required reading for anyone contemplating building any tube OTL amplifier. The circuit comprises a series connected output stage with one bank of triodes sitting on top of the other bank. The input stage is a Grounded Cathode and the phase splitting and additional gain is provided by a Long Tail phase splitter. The balanced signal is brought to the output tube via two Cathode Followers. The connection here is not directly made, but purposely capacitor coupled. As mentioned earlier, he has considered the potential fault conditions very carefully. The amplifier's design is straightforward except for the terminating the Cathode Follower's load resistor into the output. This is the critical design trick to the amplifier, as it is equalizes the gains between top and bottom tubes and the basis for Mr. Rozenblit's patent. In all honesty, I cannot say I understand why this should make that much of a difference, but until I test bench the circuit, I will refrain from commenting. Voltage feedback is used. The feedback ratio is surprisingly low for an OTL amplifier and comes in at 22 dB. The resulting output impedance is a low 0.4 ohms.
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