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- Project Background:
I acquired a Levinson JC-2 chassis some time ago, but it included only the rear panel connectors and front panel switching - no main PCB, amplifier modules or power supply. I've used that chassis over the years as a test mule for various circuits. Although I found some original ML-1 motherboards, my project duplicating the original JC-2/ML-1 circuit (albeit using modern integrated circuits) was unsatisfactory duplicating the entire preamp including a phono stage. I've had very good experience with the Wyn Palmer designed moving coil phono preamp, see my moving coil phone preamp here, and decided to take advantage of his version 3.2 phono design (in moving magnet configuration for this project) which conveniently includes a line stage. This webpage documents the build process of integrating the Wyn Palmer design into the JC-2 chassis.
This link to Wyn's design contains all you might want to know about his design.
The starting point. The original JC-2 chassis with most wiring, including shielded teflon wire for the inputs. All of the switches were intact and working fine, as was the Spectrol volume pot (lower right).
The RCAs have already been replaced with gold jacks at the time of this photo.
2. Levinson JC-2 Info
2. Levinson JC-2 Info
The Levinson JC-2 (For John Curl, the engineer who designed the unit) was built and marketed starting in 1974. The preamp received rave reviews when released, among others: The Absolute Sound Vol2, #7 (Winter 1975/1976) and Audio April 1976. It is considered a classic, and still a popular preamp many decades later, selling for substantial prices on the used market.
The original sales flyer follows.
Above: Original marketing flyer for the JC-2, front side bottom half.
Above is the front side, top half of the flyer, showing the internals of the preamp.
Above: Back side of the flyer with background and specs.
This shows the JC2 internals in stock form. There are five potted plug-in modules. The left two modules are for the phono preamp, with space between them for an optional moving coil gain stage. The next two modules to the right are the line stage, and the far right module is a capacitance multiplier for the power supply to the phono section. Note that the ML-1 preamp was virtually identical except for the internals of the potted modules.
Above is the ML-1 schematic. The JC-2 schematic was published in Audio Amateur magazine in the 3/1977 issue, and included schematics for the plug-in modules, however parts for those modules are long obsolete. My goal for this project was to keep all of the front panel controls functioning as originally designed, including input, gain, mono/stereo, and the stepped balance controls. The gain and phono EQ blocks in the schematic above will be replaced with the Wyn Palmer circuit.
3. Power Supply
3. Power Supply
This photo shows the JC-2 with the stock external Levinson power supply on the right. Mine will serve the same function, but look a bit different.
I used Wyn Palmer's suggested switching power supply, which is coupled with a custom filter, to supply the +/-15VDC for the preamp. This is the Meanwell PD2515, which comes fully assembled and tested.
Closeup of the Palmer designed SMPS filter PCB, after installation. Input from the Meanwell supply at the bottom, outputs to the preamp at the top. The black & white pair is for the power LED.
I built a simple 12v regulator on perfboard to provide a 12V trigger signal to turn on the associated amp, shown here. It includes a resettable fuse.
Top view of the assembled power supply, front panel to the right, rear to the left.
Rear view of the power supply, showing the 4 pin XLR power supply connector.
Front view of the completed power supply. The front panel is from FrontPanelExpress.com.
The rear of the JC-2 showing the original power supply connectors (both input and output to daisy chain another component).
This is where my new power supply will connect.
4. Phono & line stages
4. Phono & line stages
Above: the schematic for the phono preamp. Note the paralleled input ICs for lower noise and the DC servo to avoid coupling caps. The PCB was originally designed as a moving coil preamp, and the schematic shows parts values for that setup. However, the very thorough documentation includes parts changes for a lower gain moving magnet design, which I used for this project. The parts include many 0.1% resistors as well as 1% capacitors for very tight RIAA EQ.
The version 3.2 PCB for the Wyn Palmer phono preamp. It contains 4 sections, left to right: the phono preamp, a warp filter (not used in this project), the line stage, and the output relay section.
The warp filter was bypassed and not used here simply because the JC-2 has no appropriate front panel control for such a filter.
The PCB used in this project (different production run hence different color), mostly populated (except ICs). You can see my black markings indicating which warp filter parts are to be omitted.
This is one of the output stage ICs, an OPA1656. Its a surface mount chip mounted on an 8 pin DIP adapter to fit the sockets on the main PCB.
Soldering the surface mount chips is not particularly difficult with flux, a very fine solder tip, and at least for me, a magnifying lamp.
The objective of this project was to make a fully functional preamp within the JC-2 chassis, using all of the front panel controls. I used the Wyn Palmer phono stage as designed, but modified the line stage to integrate the stepped balance controls in the JC-2. Those balance controls are simply strings of resistors in the feedback loop of the line amp (as shown in the ML-1 schematic above), allowing changes to its gain.
The stock Wyn Palmer design has a fixed gain for the line amp, so I tapped in and out of the PCB to the front panel switching to integrate the switched gain setting feedback resistors into the Palmer line stage.
This is a photo of my notes indicating how the switches would be wired into the PCB.
This shows the PCB wired into the chassis. Phono inputs to the left, line stage to the right with the red and blue wires into the PCB from the front panel balance switches at the bottom of this photo. The main preamp outputs are on the right of the PCB, and the yellow/black/green twisted wires at the center right are the +15V,-15V and ground for the power supply.
A wider view of the completed chassis. Volume control at the lower right, power supply input at the upper right.
5. Performance Tests
5. Performance Tests
Above: Line stage distortion and noise measurements, unweighted. Note the THD is -98+dB (or 0.001%) and Signal to Noise Ratio (SNR) about 101dB, quite respectable performance.
Above: the same measurement, but A weighted. Signal to Noise Ratio (SNR) improves to 103dB.
6. Completed project
6. Completed project
All buttoned up, in operation, before the faceplate for the power suppy arrived.
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