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This project evolved from work updating the modules in my DIY Levinson LNP-2, here. Thanks to good friend and fellow Levinson/Cello fan Charles King who suggested I extend my latest design to an LD-2 replacement and helped with the design and testing.
Levinson decided to pot the LD-2 modules (both for heatsinking and to keep the design proprietary), so repairs of failed a module are essentially impossible, they just need to be replaced.
As you may know, the LNP-2 was designed by Dick Burwen who used his own UM-201 Burwen modules (based around an early integrated circuit) for the early version of the preamp, and later Levinson took the production of those modules in-house with their own discrete LD-2 module. Designer John Curl (who designed the Levinson LD-2 replacement, as well as the Levinson JC-2 among many other pieces of high end gear) indicated that the original Burwen modules actually contained an IC: a selected Harris HA-911, his comments here. This is confirmed on Dick Burwen's website: "Dick was able to greatly improve the high frequency performance of their Harris HA-911 op amps via a more complex stabilization network, and they were selected for low noise and DC offset." John Curl takes credit for designing the Levinson branded discrete LD-2 module here.
IC design for audio has come a long way in the past 50 years, so a modern IC designed specifically for audio is used in this project.
Photo of an early LNP-2 with the Burwen "Universal Mixing Amplifier" modules. Since the 2 modules on the left are the phono section, I'm guessing the blue dot indicates selected for low noise.
The more common later discrete LD-2 modules designed by John Curl.
The new modules are built around the OPA1655 IC, a recent FET input design optimized for audio use. Datasheet here. The PCB is designed with an 8 pin DIP layout, so the surface mount SOIC OPA1655 requires an adapter shown in the photos. This potentially allows changing the IC to another if the user wants to experiment, however the modules have been tested (see below) only with the OPA1655. All resistors are 1% metal film (similar to those used elsewhere in the LNP-2), and decoupling electrolytics are 105degree type for long life. The output coupling cap is a Nichicon Muse bipolar. Connecting pins are gold plated, of course.
These new modules aren't intended to improve the performance of a functioning LNP-2, but rather allow owners with failing modules to replace them with something of similar performance and keep their equipment running as well as possible. Owners of fully functioning LNP-2s, however, may want a spare pair of these to have on hand in the event of a failure in the future.
An early prototype of the new module, very much a work in progress.
Closeup of the completed new module, showing the OPA1655 SOIC IC mounted on an 8 pin DIP adapter.
Although there's no provision on the original LNP-2 to add cartridge loading capacitors (other than soldering to the inside of the phono RCA inputs) , I added a space (lower left) to add a cap if desired when using the new module for the phono section.
I've built a small number of the new replacement modules, shown here.
Unit testing of the new modules before installing in the LNP-2 at both 0 and 10dB gain.
For testing and measurements, I installed three new modules in the right channel of a good LNP-2, and kept the stock LD2 modules in the left channel. Phono section to the left, then input section, then output section.
Measurements show the new module performs virtually identically to the stock LD2 module. The spectrum analysis below shows distortion (THD) and Signal to Noise rate (SNR) for the original Levinson LD2 modules in the left channel (yellow trace) and the new module in the right channel (red trace, largely beneath the yellow). As you can see, THD is below 0.0004% for both original and new modules, and SNR is 91dB for both. Not shown below, but SNR drops to 94dB with A weighting for both the original and replacement modules. This very good performance data.
Frequency response measurements from Aux input to Output show +/-0.01dB from 20-20Khz for both the original and new modules. Examining the phono frequency response, it was within 0.25dB for 20-20Khz with both the old and new modules, quite good for a almost 50 year old design, mostly thanks to the original Levinson 1% phono EQ caps.
I have a small number of these new modules available for sale, $500/pair. Contact me below to inquire.
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