DÆ Phono Preamp v2.0 Fine Tuning

This week I received a new B&K Precision 880 LCR meter. This meter is capable of measuring capacitors, inductors and resistors with an accuracy of 0.1%+2 digits.

A phono preamp RIAA filter requires two resistors and two capacitors. The accuracy of the RIAA response is sensitive to the accuracy of the components. 1% tolerance resistors are readily available and some 1% tolerance capacitors are available. To get a more accurate RIAA response than what is possible with 1% tolerance parts in common series values (E96 resistors for example), multiple resistors and capacitors can be used in series and parallel combinations. As an example if two equal value components are combined to make the desired resistor or capacitor, the effective tolerance is improved by 1/√2; so two 1% parts make a 0.7071% tolerance combination.

My resistor tester project provides the measured tolerance statistics for 1% metal film resistors to aid in understanding the likely results of combining resistors.

As an alternative to combining several parts, the DÆ phono preamp V2.0 uses two capacitor multiplier circuits. The capacitor value of the capacitor multiplier circuit can be trimmed using a potentiometer. Using the B&K Precision LCR meter the capacitors can be trimmed to 0.1%. It would take one hundred 1% components connected in parallel (or series) to achieve a 0.1% tolerance component.

Next I will test the frequency response of the phono preamp to confirm the capacitor fine tuning provides the desired very accurate RIAA response.

Fine tuning phono preamp with B&K Precision 880 LCR meter.

Fine tuning phono preamp with B&K Precision 880 LCR meter.