There's even a separate switch for each channel to keep the left and right signals separate and signal paths short. The gain and input impedance for mm and mc operation could have been made adjustable, but that would have been an unnecessary expense for virtually no benefit in terms of sound quality, so fixed values for gain and input impedance were chosen (42dB and 47k for mm, and 62dB and 100 ohms for mc). The budget saved on unnecessary adjustability is better spent on power supply filtering - seven stages in total are used in the Rialto to maximise channel separation and inter-stage separation. Even the power on/off switch was deemed unnecessary since the total power consumption is so low (less than most domestic equipment in standby mode).
The result is truly audiophile sound quality at a very affordable price.
op-amps versus discrete transistors
Although op-amps can give excellent performance and certainly boast impressive specs, they aren't designed specifically for the ultra low signal levels of moving coil cartridges which are typically in the region of a few hundred microvolts. The lowest level signals which an mc phonostage has to handle can be less than a single microvolt (the tail end of the reverb is 60dB below the peaks), and signals this low are best handled by circuits designed for the job. It’s a little known fact that op-amps have class AB, or even class B, output stages which rely on large amounts of negative feedback to minimise crossover distortion. Having a class AB output stage allows an op-amp to swing large signal voltages into low impedance loads whilst maintaining modest quiescent current drain, and although op-amps excel with input signals of 20 volts or more where residual crossover distortion from their class AB output stages pose no problem, handling signals of a few microvolts is another matter. So, discrete transistors were chosen for the Rialto. The moving coil gain stage is based around low noise discrete transistors and designed to amplify the delicate signal produced by a moving coil cartridge with maximum care, operating entirely in class A to eliminate crossover distortion without resorting to negative feedback (there is no crossover distortion in a class A amplifier whether negative feedback is used or not).The moving magnet section of the Rialto also uses discrete transistor gain stages.
zero global feedback
A moving coil phonostage typically has a gain of about 60dB, but due to the RIAA equalisation curve it actually has more like 80dB at the low end of the audio spectrum. 80dB is huge amount of gain and is best spread over more than one gain stage. Many designs employ two gain stages, but the Rialto has three to allow each gain stage to operate with optimum performance. Each gain stage uses just two discrete transistors and employs local feedback to achieve extremely low distortion. The high linearity of each gain stage means that overall global feedback (widely acknowledged to be good for achieving good THD figures but bad for perceived sound quality) can be eliminated.
The Rialto uses a passive circuit to create the RIAA curve (accurate to better than 0.5dB) and employs multiple audiophile grade polypropylene capacitors which not only allows non-standard capacitance values to achieved (to obtain the accuracy in the RIAA curve) but also to act as mutual bypass capacitors, minimising ESR (equivalent series resistance) and maximising audio performance.
The transistors too are audiophile devices, carefully selected for maximum fidelity and minimum noise. Different transistors are used in the moving magnet section of the Rialto and the moving coil section so that both moving coil and moving magnet cartridges are amplified by gain stages optimised for their specific requirements.
low noise performance
Low noise cannot be achieved merely by using expensive components. The circuit design has to be right and the circuit board tracks have to be very carefully routed. Both these aspects of design have been meticulously addressed in the Rialto so that background noise is as low as possible. The power supply goes through seven stages of filtering so that the circuit is operating from a perfectly clean voltage supply, allowing the music to appear out of silence, unsullied by residual hum, buzz and hiss.
Don't be fooled by the Rialto's affordability. This is a top-flight phonostage.
The Beginner’s Guide To Vinyl Playback
For anyone new to turntables and confused or unsure of what is required in terms of electronics to start playing vinyl we have written a short guide to explain the basic requirements. This simple guide let’s you know what you need and why you need it. click here.