Rothwell Signature One Transformer Coupled MC Phonostage
The Signature One is a phono pre-amp dedicated to low output moving coil cartridges and uses transformer coupling to achieve the best possible performance.
The very low signal levels produced by low output moving coil cartridges (usually less than a tenth of those produced by moving magnet cartridges) require careful handling to achieve a noise performance approaching the best theoretically possible. Transistors cannot get as close as transformer coupling can, and op-amps are even less so. Despite decades of research and development in transistor technology transformers still offer the best noise performance. This is an uncomfortable fact when the best possible performance is demanded because transformers are vastly more expensive than transistors, but it is a fact non the less.
Of course, noise performance alone isn't the only consideration when seeking the ultimate sound quality. To achieve the wide bandwidth and low distortion required for superb sound the transformers employed have to be very high quality. For this reason the transformers used in the Signature One are supplied by Lundahl of Sweden. The core material used is a cobalt based uncut amorphous strip which gives a high inductance factor (to extend the low frequency bandwidth) without the need for an excessive number of turns of wire, which in turn keeps the winding resistance low (to minimize Johnson noise) and inter-winding capacitance low (to extend the high frequency bandwidth and to minimize ringing). The unusual coil geometry also has the benefit of rejecting interference from external stray magnetic fields to minimize hum pickup.
However, the real benefit of using transformers is the sublime sound quality. This is due to the very different way in which transformers behave compared to transistors. Whereas the distortion spectrum of a transistor circuit tends to produce increasing distortion as frequency rises, transformers produce less distortion with rising frequency and produce lower levels of intermodulation distortion. This gives transformers a smooth and transparent sound without any of the unpleasant harshness which can afflict transistor circuits.
The Signature One's transformers can be configured for step-up ratios of 1:10 and 1:20, so that the vast majority of modern moving coil cartridges can be accomodated.
The Active Circuit
Following on from the Signature One's transformer-coupled inputs comes the discrete transistor section of the circuit. Using discrete transistors allows elegantly simple Class A circuitry to be used. This is used in preference to the Class B or Class A/B push-pull circuitry employed within op-amps because Class A circuits are completely free from crossover distortion, and given that the total amount of distortion a circuit exhibits is less important than the type of distortion it exhibits, and crossover distortion is the worst sounding distortion mechanism possible, discrete Class A offers sound quality which simply sounds right. Whereas op-amps use negative feedback to reduce crossover distortion, Class A circuits avoid it altogether and so sound natural and fatigue-free.
Following the first active gain stage is the passive RIAA equalization using multiple audiophile polypropylene capacitors. Then comes the second active gain stage and a Sziklai-configured output buffer for low distortion and low output impedance capable of driving any interconnect cable.
The Power Supply
The signature One's power supply is an onboard linear regulated DC supply using a toroidal mains transformer for low magnetic field radiation, oversized reservoir capacitors with two stages of filtering and a low noise discrete transistor regulator. Since the DC supply is the raw material from which the amplifier creates the output signal this attention to detail pays dividends in a high signal-to-noise ratio and by revealing low level detail. There is also an earth link/lift switch so that hum-free performance can be achieved regardless of the turntable's earth wiring scheme.
The Signature One is designed and built in the UK. Each unit is carefully assembled in-house with rigorous and continuous quality control and thoroughly tested before approval for sale. Each unit is built with the kind of pride and care that is only possible with small scale manufacture by people with a passion for quality audio and music reproduction.
- Gain: 62dB/68dB
- Input Impedance:
- Low gain: 117 ohms
- High gain: 470 ohms
- Output impedance: 75R
- Sensitivity for 500MV O/P:
- Low gain: 0.4mV @ 1kHz
- High gain: 0.2mV @ 1kHz
- Headroom: >20dB
- Signal/noise: >87dB
- Power supply: internal, linear regulated 24 volts DC
- Mains voltage: 120VAC/235VAC
- Power consumption: 2W
- Dimensions: 430 x 250 x 74 mm
- Weight: 4kg