Rothwell Signature One phono stage

Rothwell Audio Signature One
Rothwell Signature One phono stage

Andrew Rothwell has been quietly beavering away in the business for decades, but he has maintained a low profile in keeping with many of the smaller British brands. I call it ‘stealth marketing’ and frankly it’s a wonder how such companies keep afloat; they must be doing something right if they can stay in business without making very much apparent effort on the selling front. Rothwell makes a range of products for guitar players and another for us, the audio cognoscenti, and the latter catalogue is largely filled with components for amplifying the output of record players. This consists of step-up transformers, a head amp (a powered step-up without transformer), and phono stages. He also makes in-line attenuators that have been a boon to enthusiasts the world over; they soak up power in situations where the incoming signal strength is so high that it limits the range of usable volume control, such as DACs with high voltage outputs overloading preamp inputs. Rothwell also makes a valve preamplifier called Auriga, and the Iridium valve phono stage, but the Signature One is his top model and does not feature a single vacuum state thermionic device.

Yet the Signature One is not entirely conventional because it uses step-up transformers to bring the typically low powered output of a moving coil cartridge up to the point where it can be equalised and amplified to a typical line level voltage. This is an approach that’s popular with valve phono stages because the voltage levels are so low that noise becomes an issue: notionally, that’s less of a problem with transistors. Rothwell does not agree and states that: “Transistors cannot get as close as transformer coupling can, and op-amps are even less able to do so”. He is of the opinion that despite decades of development, transformers still outperform transistors in terms of noise performance. This opinion would undoubtedly provoke heated discussion at the annual conference of phono stage engineers, but as a long term user of a transformer volume control, I can see that he might well be right: something that uses mains power can never be as quiet as something that doesn’t! However the Signature One is not a passive phono stage – it’s an active MM stage with transformers to increase the incoming voltage. The overall noise performance of the amplifier is dependent on the quality of the active stage, but eliminating the need to amplify the sub-millivolt output of a moving coil with transistors must make things easier.

Rothwell uses Lundahl transformers that have a cobalt core and a balance of windings designed to provide provide the lowest noise, widest bandwidth, and lowest high-frequency ringing. The active section of the Signature One uses discrete transistors in a Class A circuit that eschews negative feedback. RIAA equalisation is passive and achieved with multiple polypropylene capacitors selected for sound quality and the output stage consists of a buffer with a low output impedance so that pretty well any interconnect cable can be driven.

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