I have completed the next and I think last (at least for this year) version of the DÆ phono preamp v2.1. A pdf of the schematic for the v2.1 phono preamp is in the link. Click the word “schematic” to retrieve the pdf.
DÆ Phono Preamp v2.1 Top view.
The first thing you will notice is the heat sink for all the input and DC bias transistors. This keeps the transistors at a lower and more stable temperature. I have also changed the DC bias circuit to use an LED instead of a resistor divider to set the bias current. The LED should give a more stable bias condition. I added a balanced output with an XLR connector so the preamp now has both single ended and balanced outputs.
Other major upgrades in this version include a much smaller printed circuit board because I have used electrolytic instead of film coupling capacitors. I made this change after significant testing using the QuantAsylum QA401 Audio Analyzer. There is no measurable difference between the performance of the preamp with the electrolytic and film capacitors. I will have to put the latest version through it’s paces with listening tests to determine if there is a sonic difference I can hear.
The circuit design has been optimized for low noise and an appropriate gain for a high output moving coil cartridge. The measured performance of the phono preamp is:
The gain is 40 dB at 1 kHz;
The frequency response follows the RIAA curve to within +/- 0.025 dB. This is remarkably tight because of the hand tuning of the capacitors and resistors;
The output noise is less than 30 µV. This is measured with a 100 Ω source resistor which closely matches the resistance of a high output moving coil cartridge. 30 µV is equal to -88 dBu or -90 dBV;
The distortion with a 1 kHz test tone is 0.01% at an output of 1.7 volts. This output corresponds to an input voltage of 17 mV;
With an input voltage of 5.6 mV or -45 dBV, the distortion is 0.004% or less from 300 Hz to 5 kHz and slowly rises outside of that band.
DÆ Phono Preamp v2.1 Bottom View. From left to right notice the balanced XLR output with the single ended RCA output below it, the binding post for the ground terminal and the input RCA connector.
There is a cut out in the PCB just to the right of the XLR connector to accommodate a multi-turn potentiometer used to fine tune the gain of the left channel. The right channel shown in the picture is pre-adjusted to a gain of 40 dB. The multi-turn potentiometer allows the gain of the left channel to be fine tuned to match the right channel using a test record. This compensates for any minor gain differences between the left and right channel of a phono cartridge.
Two more views are provided below.
DÆ Phono Preamp v2.1 Profile Showing the Heatsink.
DÆ Phono Preamp v2.1 Profile Showing the Rear of the XLR Connector.