Tuesday, September 6, 2011
Oatley/Mongrel Dog Audio - JFET phono preamp cct.
Oatley Electronics of NSW Australia has released an all JFET two stage phono RIAA preamp. Previously their phono preamp was a JAN 6418/JFET hybrid type. I liked this kit immensely (see earlier post) and made two and sold two. It has a passive RIAA compensation network following a high gain front end (over 100X). The first JFET is a 2SK170 and the second a 2SK246.
Inexpensive MKP, MKT and green caps (polyester) capacitors are used as well as a lot of carbon resistors. The amp cct. is reasonable standard. The PS though employs a capacitor multiplier which filters the DC particularly well. Also the PS (which is part of the kit) uses an extremely small power transformer with dual voltages (110/240).
I found the gain stage excessive. By cutting away the bypass capacitor on the first JFET the gain fell slightly. A bypass capacitor for the second JFET is provided but not required unless you are using a MC cartridge. By having local feedback on both the active devices the sound stage is deep and detailed. See the earlier post for a description of the sound.
It is necessary to house the preamp in a metal enclosure to ensure AC and radio frequency shielding. The kit is easy to assemble and comes complete with PS. I would recommend the kit to novice and expert.