As promised, here is an update on the latest DÆ phono preamp design. After some listening experience and testing with the QuantAsylum QA401 Audio Analyzer I made a few changes to my phono preamp design. The test results for the v2.0 phono preamp showed very low noise and a very accurate RIAA response curve as expected. The circuit uses a gyrator to simulate an inductor to establish the DC operating point. In the literature, the gyrator is normally shown connected to ground and simulates a ground connected inductor. I found that connecting the gyrator to a DC supply voltage other than ground also worked but - the gyrator circuit is implemented with op-amps that run very hot even after a few are used in parallel to share the DC bias current. Also the v2.0 phono preamp had higher distortion than I wanted and I attributed this to the feedback assisted current mirrors and gyrator circuits that I used.

A pdf of the schematic for the v2.1 phono preamp is in the link. Click the word “schematic” to retrieve the pdf. The v2.1 design uses emitter degenerated BJT current mirrors to set the DC conditions and also mirror to signal current from the input differential pair over to the RIAA filter. This design is much less expensive but - there are still a few issues I need to solve. The distortion is actually greater at low frequencies than the v2.0 design and I may go back to a feedback-assisted current mirror for at least the signal current. However I will use an opamp that can better handle the high common mode voltage than the venerable NE5532.

Also the BJT current mirror transistors used to set the bias conditions run hot because there is 20 mA running through them and a voltage drop of 14 volts or 280 mW of dissipation. The transistors can handle this but any air current passing the transistors changes the bias conditions slightly. The changes in bias are quickly compensated by the DC servo but I would rather mount the transistors on a heat sink to reduce the influence of air currents. The same applies to the transistors used in the input differential pair.

Finally the v2.1 design has some excess attenuation at 20 Hz compared to the v2.0 which is flat down to 20 Hz. This due to the changes I made to the DC servo design and I may revert back to the v2.0 design.

More to come…