Monday, March 22, 2010

Mongrel Dog Audio valve phono preamp

At the time of writing this kit form valve based phono preamp for MM had not been released. I was invited by Branko of Oatley Electronics to help put the finishing touches on his latest kit. After the replacement of four small caps the phono preamp was sounding very good. This kit would couple well with the K272A tube preamp.

The kit employs a switch mode 5V wall wart to power it. The beauty of this kit is the wall wart comes with auto mains AC voltage sensing and multiple international mains adapters. So no matter which country you are in the world you can plug this kit straight into the wall.

Inside the 5V is converted to 30V with yet another SMPS. Not to worry Branko has gone to a lot of trouble to ensure that no SM hash reaches the sensitive ccts. To produce a truly linear output the tiny JAN6418 valves had to run on 30V. Not only that valves are operating in pentode mode. How may phono preamp kits do you know are using pentodes? This kit is a first on two very distinct fronts. Firstly a truly international mains power supply and a two stage pentode phono preamp.

But wait there is more: Each tubes stage is buffered by low noise FETs. This phono preamp would have to be one of the strangest ever. Pentodes, FETs, SMPS, international SM wall warts. And it sounds so good.

The kit is complete and comes with very standard componentry. If you are a first time builder don't change a thing. The more experienced constructor may swap-out some of the components for more high-end parts. But isn't this the great thing about inexpensive kits that you can put your own signature on the build. Try the kit if you have an analogue system (TT) and see if you are not very impressed by the quality of the kit and it's performance.

The cct. brds. are solder masked and the brd. plated through. Most of the cct. brd. is earth plate. This keeps noise extremely low. Apart from a small amount of valve hiss the board is dead quiet. At one meter from the speakers (and mine are 91db sensitive) no noise can be heard.

The enclosure is a double layer of Hammered Copper epoxy enamel paint. A layer of bituminous heavy Al foil lines the base and lid for enclosure dampening and additional shielding. Gold plated isolated RCA connections are on the back with the power socket. Tightly twisted telephone wire is used for the internal hook-up. Why telephone wire? One it is a very pure copper and two it is single stranded. Tightly twisted: noise is locked out with no shielding capacitive issues. The feet are small rubber finished types, cheap but nicely isolate. A rubber seal is clamped between lid and enclosure to provide even more resonance deadening.

The sound is excellent.

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