Features
Available in Britain direct for £850, the LO PRO is a complete redesign of the MC PRO, bolstering performance while adding immunity to the setup issues that often plague low-output cartridges. Consequently, it offers exceptional RIAA accuracy, very low distortion, and an even higher signal-to-noise ratio than the original.
LO PRO, frontal view
The LO PRO carries over the proven architecture of its predecessor, re-implemented with high-quality surface-mount components and several key upgrades. The new front end uses a compound loading network that reduces insertion loss while isolating the typical LOMC radio-frequency resonances that can otherwise be detected by the sensitive input transistors. The LM4562, now driving the discrete input amplifier, extends the head’s linear bandwidth and further improves immunity to RF pickup. Its increased driving ability allows a lower feedback resistance, reducing the network’s noise contribution.
Low-noise BC327 audio transistors are retained in the front end, with the parallel pairs of the MC PRO gaining an extra device per channel for further noise reduction. A quieter constant-current source replaces the previous RC rail filter. It supplies the transistors with a greater bias current, resulting in lower voltage noise and an SNR exceeding 82 dB.
Rear panel, showing connections
The head drives a similar shunt-feedback equaliser, this time using higher stability 1% tolerance ceramic capacitors and a lowered impedance for reduced noise. Doubling these precision components in parallel arrays achieves a typical accuracy far beyond published specifications. Unlike a common moving magnet equaliser/amplifier, the shunt-feedback configuration tracks the RIAA curve below unity gain at high frequency without the need for a ‘correction pole’. As the equaliser operates with no common-mode voltage and with less gain burden due to careful structuring, it is exceptionally linear.
Further downstream, the new 3rd-order subsonic filter uses triple parallel arrays of distortion-free C0G capacitors, yielding a flatter bass response and a sharper roll-off. The lower turnover point of 20.5Hz reliably delivers 20dB of attenuation in the troublesome 10Hz region thanks to tighter unit-to-unit consistency. Applying the final 6dB of gain after the filter prevents subsonic disturbances from eating into the final headroom.
PCB layout, showing new surface-mount components
Increased capacitance in the subsonic circuit diminishes its noise contribution along with the turnover points of the low-frequency cross-feed function. When enabled, this gently blends the two channels together below the selected frequency, progressively cancelling vertical rumble and revealing subtle low-end detail, especially with headphones.
NJM5532 output amplifiers deliver over 20V RMS throughout the audio band into a challenging 2kΩ line load, with less than 5ppm THD. RF-stop networks on the output side improve robustness. Muting relays with a 6-second start-up delay ensure that transients aren’t heard as the head and its DC servo stabilise during power-up and power-down.
The revised ±17V split-rail power supply includes over-voltage protection against incorrect adapters. Keeping the linear transformer outside of the small enclosure prevents its magnetic fields from affecting the audio circuitry.
Specification
| Parameter | Measurement |
|---|---|
| RIAA Accuracy | ±0.1dB, 35Hz to 25kHz |
| Channel balance | ±0.1dB, at 1kHz |
| Signal to noise ratio (220Hz to 22kHz) | 82dB, ref 500µV, 10Ω cartridge |
| Total harmonic distortion | <0.0005%, 20Hz to 22kHz, at 21V RMS |
| Maximum output (20Hz to 25kHz) | 21V RMS (XLR), or 10.5V RMS (RCA) |
| Maximum input at 1kHz | 8.2mV RMS at 62.2dB gain, 4.1mV RMS at 68.2dB gain |
| Maximum input at 10kHz | 40mV RMS at 62.2dB gain, 20mV RMS at 68.2dB gain |
| Overload margin, ref 500µV, at 1kHz | 24.2dB at 62.2dB gain |
| Overload margin, ref 200µV at 1kHz | 26.2dB at 68.2dB gain |
| Gain at 1kHz (RCA) | 62.2dB, 1290x, or 68.2dB, 2580x |
| Gain at 1kHz (XLR) | 68.2dB, 2580x, or 74.2dB, 5160x |
| Minimum load impedance | 1kΩ (RCA), 2kΩ (XLR) |
| Output impedance | 90Ω (RCA), 180Ω (XLR) |
| Input impedance | 150Ω // 100pF |
| Subsonic filter | 20.5Hz, 3rd order, for -20dB at 10Hz |
| Low-frequency crossfeed | 50 to 450Hz, -4.8 to -19.4dB at 40Hz |
| Dimensions (W*H*D) | 172*60*155mm |
| Power consumption | 6W max powered on, 0.2W standby |
The input loading seen by the cartridge is fixed at 150Ω // 100pF. This combination is designed to over-damp inductive resonances in the 1-10 MHz region of all LOMCs currently on the market, without introducing any significant response variations in the audio band.
The equivalent input noise voltage of the LO PRO is not significantly greater than the thermal noise generated by a 15Ω resistance. As a result, the common 1:10 cartridge-to-input resistance matching rule (intended to reduce the effect of insertion loss on signal-to-noise ratio) no longer applies once cartridge coil resistance exceeds 15Ω, since the cartridge’s own thermal noise dominates. The LO PRO can therefore be used without penalty with LOMCs having coil resistances up to 50Ω.
