SynergyII\Cu LM3875 compact chip amp

SynergyII\Cu started with 3.6kg of 100% very thick copper plate. The enclosure was selected, a 1U, 19” rack mount. The challenge was to build a pure Gainclone with its three resistors and simple 1500uf storage caps in a compact 1U enclosure. Everything had to fit in this tiny space including 48VAC of tranni, power filters and rectifier cct.

To achieve this two 24V ultra low profile toroid trannies were selected, the rec. cct. brd. was constructed on a tag strip with super fast, soft recovery diodes (snubbed with 0.01uf polies) and a compact EMI\RFI power filter installed across the mains. A pseudo smoothing cct. is provided with a couple of 4.7uf polies on the differential voltage rails. The 1500uf electrolytic are snubbed with 0.1uf polies at the chips.  A simple ferrite choke sat around the power cord inches from where it entered the amp.

Solid 0.4mm five-nines pure silver wire connects the earth point on the PS to the input RCAs and speaker binding post returns. Heavy multi stand copper wire connects the differential PS, individually, to each chip amp cct. Total weight is 9.6kg. Being a DC amplifier bass is extended and controlled. The sound is clean, clear and solid with plenty of power.

La Boca - MC phono preamp

I was  asked to make a MC phono preamp for an audio club member. My experience with both my clone of a Boozhoundlab (BHL) phono stage and the Le Pacific MC preamp made it easy to put together a dedicated MC phono preamp. I used my previously made BHL Chocolate Box phono stage with RIAA equalization and a freshly built Le Pacific stage. Both of these modules are good performers.

The MC stage and BHL phono have there own battery supplies. The BHL phono stage has a large 10,000uf cap to allow bottomless drive and dead black back grounds. The cct. brds. are housed in a 1U 19" case and looks very purpose like. There is considerable RFI snubbing throughout and a light layer of Dacron is placed over the cct. brd. to control cct. and enclosure resonances. A battery test point is provided at the front. The synergy of the two cct. brd. and battery system makes for one extremely good sounding preamp. It will be missed in my system,

Boozhound Labs and Le Pacific phono stage

After building the Boozhound Labs (BHL) MM phono stage, with RIAA compensation and finding it a fantastic preamp, I added to it a MC stage. The BHL phono is the best preamp I have built and I have built quite a few. Because I moved to an Ortofon Rondo Blue MC cartridge I required the addition boost of signal from the cartridge. The Le Pacific MC stage (also reviewed on my blog) is simple to build and works well.

Both the MM and MC stage run from their own battery sets and can be turned on individually. There are separate MM and MC RCAs on the back of the preamp to allow a MM and MC TT to be connected. A single switch allows you to chooses between either input. The preamp is housed in a 1U 19" metal case. Both the BHL stage and Le Pacific stage use only Russian PIO military caps which I have also used extensively on other projects always with excellent results.

Bitumanised heavy Al foil lines the base and lid of the case and when the lid goes on a layer of sound deadening Dacron is layered over the cct. brds and fills the enclosure. All hook-up wire is silver plated fine copper wire (wire wrap wire) and ferrite beads are used extensively on all RCA wire connections. The preamp is dead quiet using only 2SK170 JFETs and quality metal film resistors and electrolytics. If you have not tried the BHL kit for the money, they are excellent value and top performers.

Jean Hiraga Bipolar MC preamp

Continuing the build of MC cartridge preamps I decided to tackle something different. In this build a Jean Hiraga MC preamp schematic was used. Jean uses low noise bipolar transistors instead of jFets.  He originally used discrete low noise transistors which are no longer available. But what is available is even better. In a single eight pin chip (SSM2220) two low noise, perfectly matched, transistors share a single substrate. The $AU11.00 chip, when used with a socket, makes construction and maintenance very easy. I use gold lined Mil-spec chip sockets. WBT 5% silver solder is used throughout.

Construction is on veroboard and the enclosure is a Hammond anodised Al type.  Where Mr Hiraga likes to run all devices from car batteries with huge capacitors I settle with a total of 1F (1000,000uf) of capacitance and 8 X AAA batteries all onboard. The batteries supply +/- 6V. To maintain even battery discharge and to guarantee all four 1F caps get equal voltage loads a resistive network is used. The capacitors are in series to meet voltage level requirements (4 X 1F @ 5.5V). Additional caps are added: 220uf low ESR and 0.1uf to ensure good midrange band pass and snubbing of noise.

Twenty five turn trim pots are adjusted to ensure zero volts at the input and Russian PIO caps are used on the output.  An exceptionally low current draw should ensure over 200 hours between battery replacements even with cheap batteries. On early listens this MC preamp sound natural and appears to be working well with my Audio-Technica  AT-F3/MKIII with an output of 0.35mV

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.

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.

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.