Bottlehead Crack - Acknowledgements; Brief Circuit Theory

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ACKNOWLEDGEMENTS
ThankstoPaul“PB”Birkelandforworkingupthecircuitdesignandlayoutschemeusedinthiskit.
Thanks to Paul Joppa for the design of the Bottlehead PT-3 power transformer used in this kit.
Thanks to John “Buddha” Camille for all his mentoring in proper grounding techniques thru the
years. We miss you, boss. Thanks also to Queen Eileen Schmalle for putting up with yet another of
Dr. Bottlehead's cockamamie schemes.
What the Crack kit is about
As headphone listening becomes more and more a dominant form of high fidelity listening Bottle-
head has wanted to offer a product to that group of listeners that offers the same bang for the buck
as our speaker-oriented kits. The Crack Output Transformer-Less (OTL) headphone amplifier has
been designed as a relatively simple, highly cost effective, great sounding headphone amp kit for
higher impedance headphones (100 ohms or higher) .
Brief circuit theory
The amplifier circuit consists of two channels, each using 1/2 of a 12AU7 tube voltage amplifier di-
rect coupled to 1/2 of a 6080 tube as a cathode follower output. Only one output coupling capacitor
is in the signal path of each channel. The high voltage power supply is an efficient solid state ul-
trafast soft recovery full wave bridge feeding a C-R-C-R-C filter.
Maximum output is about 10V rms (28V pk-pk) before clipping into a 300 ohm load. Gain is about 15
dB into a 300 ohm load. Output impedance is about 120 ohms - recommended headphone load is
100 ohms or greater. Frequency response is +/- 0.5 dB from 10Hz to 50kHz into a 300 ohm load.
Phase is not inverted.