Low noise phono preamplifier

The stereo ( functionally dual mono ) phono module is an absolutely no-compromise design which uses a three stage, distributed gain, series and shunt feedback arrangement. The first stage is a series feeback gain block for lowest noise, followed by a two stage shunt feeback system to implement the RIAA phono correction. The low frequency and high frequency RIAA time constants are implemented in separate gain blocks so that they may be set precisely and without mutual interference; whilst the shunt feedback arrangement ensures adherence to the RIAA curve at high audio frequencies. The series feedback ( only ) arrangement, common in non top end equipment, deviates from the RIAA curve at high audio frequencies unless an additional time constant is incorporated; in most instances the extra time constant isn't included.

Each individual power line to each gain block has its own voltage regulator and these in turn are supplied from master positive and negative regulators for each channel. The stereo phono module therefore incorporates no less than 16 voltage regulators to ensure an absolutely silent "virtual battery" power supply to the gain blocks.

The first stage amplifier is a straightforward, series feeback, very low-noise gain block which does not participate in the RIAA correction. This means that the overall gain of the phono stage can be set to accomodate any MM or MC cartridge simply by altering the value of one resistor. Details of how to accomplish this are included in the construction manual. The pcb also incorporates component locations for the correct resistive ( frequently 47Ω for MC and 47kΩ for MM cartridges ) and capacitative ( varies between 0 and 470pF for various cartridges ) termination of the cartridge in use.

The phono stage comes in two variants, standard and audiophile. The standard version uses 1%, 50ppm metal film resistors, 1% polystyrene and polypropylene film capacitors, standard electrolytics for decoupling, and the most cost effective low noise op-amps. The circuitry of the audiophile version is slightly different to the standard version and uses 0.1%, 15ppm Welwyn RC55Y resistors, 1% polystyrene and polypropylene film capacitors, Elna RSH electrolytics for decoupling, and the best low noise op-amps that money can buy which are specially biased to operate in class A throughout ( yes, that means the output stages too ).

The overall noise of the phono stage is down near the theoretical minimum for the audiophile version and is set by the feedback resistors associated with the input gain stage, rather than the gain stage itself, whilst the noise of the standard version is only slightly higher than that of the audiophile version. The overall distortion is below the measuring floor of the vast majority of test instruments for both versions of the phono stage, and deviation from the RIAA curve is <0.2dB for the standard version and <<0.05dB for the audiophile version. The phono stage, solder masked and component location silkscreened, pcb measures 92 x 103mm and normally requires a power supply of ±24V at 50mA, although lower voltages are possible at the expense of power supply rejection ratio and headroom.

Properly constructed and with a suitable power supply this phono stage is unbeatable by any ready made unit, regardless of price! The assembled and tested audiophile version of the stereo phono stage includes matching of polystyrene RIAA time constant capacitors to 0.1%.