Monday, December 26, 2011

Le Pacific pre-preamp for MC

To compliment a two stage tube phono preamp I added this Le Pacific jFet based Moving Coil pre-preamp. It uses only a single jFet and a few other components. This very simple pre will deliver good voltage levels to any phono preamp and being battery powered it does it quietly. The total construction was on a single piece of veroboard (proto strip board) no PCB.

The output capacitors are Russian PIO the same used in the retro-thermionic tube phono preamp. The loading is 100ohms and 1nF. Silver soldered and housed in a heavy cast Al enclosure sprayed in a delightful Sungold epoxy enamel paint. With a 0.35mV Audio-technica AT-F3/III cartridge this preamp produces good dynamics and a strong output.

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.

Monday, January 17, 2011

Piezo pickup preamp

This Mongrel Dog Audio piezo pickup preamp was built to preamp a piezo pickup in the bridge of a double bass. It has a 10M ohm input impedance and a single 2SK170 jFET. A PIO Russian military cap and two polypropylene caps add quality components to an excellent build.

The case is polished die-cast AL with a polyurethane coating and all is powered by a single 9V battery. I increased the storage cap to a 200uf low ESR variety. Tough , simple and good looking.

JFET phono preamp-in a chocolate box

The finished jFET boozhoundlabs-Mongrel Dog Audio incarnation is "Gold". I dropped the whole cct. brd. into a very nice gold embossed chocolate box. The box is lined with bitumanised Al foil. It also has a clear plastic cover which covers the inside. The preamp can be run from of batteries or external power source.

This is by far the best phono preamp I have ever built. It is far superior to any of the others. Not that they were bad just that is so good. So very good and not just my opinion. Current draw is 13.5mA. Wirewarp wire is used as hook-up wire. Heavily twisted WWW (earth/hot wire) is used to bring the inputs and power in.

JFET phono preamp-in a chocolate box

I was never completely happy with my phono preamps weather tube or OPAMP base. I made the Boozhoundlabs two stage jFET "Le Pacific" style preamp. Once more I went for the vero-brd. Also, though I wanted to build it with the Russian PIO military caps I did not have some of the values I wanted. I opt for a build using high voltage (630V) brown polypropylene caps.

The cct. brd. above uses pure silver 1mm earth links, 2SK170 BL jFETs and silver based solder. Low ESR 220uf caps (snubbed) sit across a decoupled V+ rail and a single 10,000uf caps (snubbed) feed the V+ through a 1ohm resistor (a La Pass). Currently running under 27V of lithium batteries.

Ferrite beads sit across both the ground and hot wire to the cct. brd. from the input Also the PS input has ferrite beads also. No shielded wire is used (I don't like it) but the input wiring is heavily twisted, hot wire and ground. See previous post for chocolate box enclosure.

Friday, January 14, 2011

K295 Production Version

In an earlier blog I constructed a prototype K295 for a bass player. It proved to be a great success and produced richer harmonics making the bass sound fuller and a lot warmer. This build is the production version of the K295 and the kit can be purchased from Oatley Electronics in Australia In this version only a single tube is used. The PS, including a tiny mains power transformer, is mounted as part of the whole cct. brd. but can be snapped away.

As in the earlier preamp I used a plastic case lined with bitumanised Al foil. Single pots were used to allow separate channel control and large numbered knobs used so setting can be noted for future used. The central blue "on" led marks "0" for both pots. Guitar 6.5mm jacks are on the back to allow music instruments to be plugged in. All hook-up wire is twist single strand copper.

On a short listen through my system the preamp appeared to work very well providing gain and and good control over volume levels. The left channel has a slight buzz due to it being closer to the tiny transformer than the left. A larger case would solve this problem by allowing the input and output jacks to be mounted further from the power tranni and mains (110 or 240V). Or break the PS away from the amp cct. brd. and mount it separately. All in a good build and good result.

Saturday, January 1, 2011

K295 - JAN6418 buffer/preamp

In this small plastic enclosure is the new Oatley K295 kit. But in this build an early dual tube prototype brd. has been used. The intended use for the build above is for preamping of a double bass for a player in a Rock-a-billy band. Red LEDs are mounted on the brd. to light the JAN6418 tubes.

From early reports Ray, the bassist, is very impressed with the sonic results so far from his electric bass.