Features
| Brand | Lehmann Audio |
|---|
Product Description
Lehmann Audio Decade Jubilee phono stage is a two box unit with a separate power supply. Notably, it is a no compromise unit, with exceptional sound quality.
The PWX II LC power supply features a toroidal power transformer. The second unit houses the preamp and RIAA equalization section. Three switches adorn the front of the preamp. The first one actuates a high pass filter that will remove low frequency interference. This has a 50Hz cut off frequency.
Next comes a gain switch that will activate a 10dB gain boost. Finally, the last switch, switches between moving magnet and moving coil cartridges. There is also a set of DIP switches that can be used to match a moving coil cartridge more accurately.
The PWX II LC Power Supply
Having a separate power supply allows it to be removed from proximity to the sensitive electronics in the preamp. The cable is 2 metres long and terminates in a four-pin Neutrik connector. There is a second output on the power supply that allows it to power another Lehmann component. This is a linear power supply using bipolar transistors, not switch mode.
As a result, there is no high frequency noise to worry about. Moreover, high-grade components are used. These include Mundorf electrolytic capacitors, and the PCB has triple thickness traces. Next, magnetic shielding is used for the transformer. Finally, there is an isolation coil between the primary and secondary of the transformer. This prevents any noise from getting into the phono stage.
THAT Corporation Preamp chip.
Moving coil cartridges closely resemble microphones. Therefore, Lehman have decided to use a THAT Corporation Microphone IC for the main gain stage, of the Decade Jubilee phono stage. The THAT chip is used for professional mixing consoles, and has a very good signal to noise ratio.
This is also a dual mono design so all of the signals have no shared components. Moreover, there is no electronic switching for the three gain settings. Instead, six relays are used, three for each channel. This ensures the cleanest possible signal path.
Four layer circuit board
To keep the unit small, a four-layer circuit board is used. This uses two ground planes to help shield the high gain parts of the circuit. Next, the output stage uses complimentary pairs of transistors wired in parallel. This allows for a faster slew rate and wider bandwidth. The transistors also use less power.
RIAA equalization
Records are recorded with equalization. For example, the bass is boosted to reduce the chance of any overmodulation of the groove. Additionally, the treble is boosted to reduce surface noise. A very specific equalization curve is used. This was developed by the RIAA. As a result, the reverse curve has to be applied by the preamp. This requires that all of the filter components are of an accurate value, and do not drift with temperature. Hence why the Jubilee uses audio grade components.
Cartridge types and impedance
There are two main types of phono cartridge in use today. Moving magnet and moving coil, both have different impedance and gain characteristics. Moving magnet cartridges have a small magnet mounted on the cantilever. The magnet moves inside a coil, to generate the signal. They typically have an impedance requirement of around 47K and have an output of around 5 mV.
On the other hand, moving coil cartridges have a set of tiny coils mounted on the cantilever. This reduces the mass but also reduces the output. The output can be less than 1mV and the impedance characteristics vary more widely. As a result the Decade Jubilee has a set of DIP switches, to set both the capacitance and resistance of the preamp. The capacitance ranges from 47pF up to 1347pF while the resistance ranges from 100 ohms to 47K ohms.
