6418 Tube RIAA Phono Preamplifier

  
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Last year I built and reviewed the low cost Oatley Electronics K272 Tube Stereo Headphone Amplifier Kit. In conversation with Oatley Electronics (located in New South Wales Australia), they told me that the kit became very popular and sold pretty much all over the world. The little headphone amplifier kit was based around new old stock Raytheon J
6418 Tube RIAA Phono Preamplifier - schematic

AN 6418 sub-miniature pentode valves (tubes). Using the same JAN 6418 valves Oatley Electronics has released an inexpensive ($47AU, May 2010) tube based stereo RIAA preamplifier kit for moving magnet (MM) cartridges, and they were kind enough to send me a couple of kits. The printed circuit board (PCB) is double sided, solder masked and silk screened. Almost the entire top layer of the PCB is grounded to provide shielding. To keep the kit price low, the passive components are very basic. A mix of metal film and carbon resistors, polyester, ceramic and electrolytic caps. Polyester caps are generally not used in audio circuits because some say they do not sound good. Of course one can choose to upgrade the passive parts. The contents of the phono RIAA preamp kit are shown below. A full parts list of the kit contents is included in the Oatley Electronics K282 Kit Instructions (PDF - 646kB). The external power supply is a small 5V switch-mode power supply (SMPS) with a universal outlet adapter that can be used with American, European, Australasian and UK power outlets (100 - 240VAC). The circuit uses a MC34063A switched mode power supply integrated circuit (IC) to convert the 5V external supply to 30V. The switching frequency is set at around 100kHz. The phono preamp circuit is a two stage affair with each valve stage buffered by a 2SK170 Field Effect Transistor (FET). The new old stock (NOS) JAN 6418 tubes are used in pentode...



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