Linear microZOTL User Manual page 8

microZOTL Preamplifier User Guide
output transformer, this impedance transformation operates on both the ac and dc
components of the signal.
In the transformer-coupled amplifier, the turns ratio of the output transformer
determines the impedance matching between the output tube(s) and the speaker.
With output transformers, there are practical limits to how large this ratio can be
made because of the parasitic elements of the windings, and it is difficult to make an
output transformer with more than a 25:1 ratio.
With the ZOTL technology, the impedance matching is determined by the effective
turns ratio of the RF converter transformers (called impedance converters).
Without the parasitics to affect the audio, these impedance converters can have
much higher effective ratios, opening the door to using various tubes under unusual
operating conditions that cannot be implemented with output transformers.
In the MicroZOTL, the effective turns ratio is 168 to 1, making it possible to use a
tube for output that is normally used for input or intermediate gain stages. The
12SN7/6SN7 tube is respected for its linearity, but prospects for making a high-
fidelity output transformer for this tube are dim indeed. With the high effective
turns ratio in MicroZOTL2.0, the 6SN7 works well, and a very low output impedance
is achieved without using negative feedback.
Amplifier Maintenance:
Adjustments: The amplifier is self-biasing, and has no adjustments.
Fuses: The MicroZOTL Preamplifier has a fuse on the amplifier board and two
inside the AC input assembly accessible from the rear panel. All are the 5x20mm
type. There is a 2.5A fuse on the amplifier board inside the unit. There are two
3.15A fuses in the AC assembly on the rear panel (there is a little plastic door that is
opened with a small screwdriver).
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Linear Tube Audio