Saturday, September 24, 2011

Oatley/Mongrel Dog Audio - JFET phono preamp Red Case

See the earlier post for some more details about this new kit from Oatley, the K303. I decided to install the basic kit in a better case than the light weight "tin" box I used previously. I believe there has been an improvement in sound with the heavy weight enclosure. If nothing else it looks a lot better. An extremely good sounding phono preamp and a well presented and complete kit all for thirty dollars. It is a steal.

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.

Oatley/Mongrel Dog Audio - JFET phono preamp

I originally housed this preamp in a timber enclosure but 50Hz hum invaded the signal. This enclosure is a lot more flimsy than I might normally use but, when connected to mains earth, did provide good shielding. See the earlier entry for more information about the cct.

I did ensure that I used isolating gold plated RCAs. The signal/power earth and the mains earth are separate. Usually I don't do this. With the gain of the front stage around 100 X good shielding and earthing is necessary. The sound from this AU$30 kit is quite astounding when you consider the common parts used. The PS having a capacitor multiplier does a very good job of DC filtering .

The bass is deep and very extended. It is also well articulated and not one noted or boomy. Mids are rich and detailed with treble well extended and well defined. But what really shines through with this "build-in-an-afternoon" kit is the sound stage. Both the active stages of this preamp have local negative feed back. This is achieved by neither of the JFETs having a bypassed source resistor. I have found on my single stage valve preamp this does appear to produce a very deep and highly detailed sound stage.