DÆ v3.0 Phono Preamp

The DÆ v3.0 Phono Preamp is a complete two channel phono preamp in an aluminum and acrylic enclosure. The phono preamp includes the following sub-assemblies that are described in previous blog posts as noted:

• Two v2.1 Phono Preamp circuits (blog post dated May 23, 2020). I know it is a confusing number system with the DÆ v3.0 Phono Preamp having v2.1 circuitry - sorry;

• One v3.0 Battery Charger/Power Supply (blog post dated September 9, 2020). The Battery Charger/Power Supply is computer controlled by a microcontroller to avoid over discharging the battery pack;

• One v0.1 “Sort of Analog Panel Meter” (blog post dated May 19, 2020). This custom designed meter has a computer controlled stepper motor to provide a non-linear voltage scale to properly portray the battery pack state-of-charge.

Power is provided by a battery pack with six, 6-volt valve regulated lead–acid (VRLA) batteries, also known as a sealed lead–acid (SLA) batteries. When fully charged, the battery pack provides low noise power to the analog electronics for a six hour playing time. Once the battery power is depleted, the Battery Charger/Power Supply switching back in to re-charge the battery-pack; it is still possible to continue listening during recharging with a (very) minor increase in noise level.

This phono preamp circuit has many interesting features including the use of a transconductance RIAA equalization stage. You can read a little more about a transconductance stage in Douglas Self’s book entitled “Electronics for Vinyl” on page 91 in the first published edition 2018. This stage allows the two RIAA stage capacitors to be ground referenced which is critical to another feature of this design - the use of capacitor multipliers.

To make an accurate RIAA equalization stage requires two very precise resistors and two very precise capacitors. The components are also typically not standard values. The required resistor values are a little easier to obtain by a parallel combination of 1% metal film resistors. More on the statistical distribution of resistors can be found in the DÆ Resistor Tester project. High precision capacitors are more difficult to obtain. To start with, fewer standard capacitor values are available and 1% tolerance capacitors are even more rare. One potential solution is to use a capacitor multiplier circuit that allows the capacitor values to be fine-tuned at assembly with a trim pot.

The schematic of the phono preamp is in the linked pdf. Click the word “schematic” to retrieve the pdf. Notice that both a balanced XLR output and single ended RCA output is provided. Also the power supply uses +/-17 volt rails to provide a very wide dynamic range.

The circuit design has been optimized for low noise and an appropriate gain for a high output moving coil cartridge. This phono preamp has been thoroughly tested using a QuantAsylum QA401 Audio Analyzer and the measured performance 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 typically around 23 µV. This is measured with a 100 Ω source resistor which closely matches the resistance of a high output moving coil cartridge. 23 µV is equal to -91 dBu or -93 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.

• With an output of 1 Vrms (10 mV input), the SINAD is 83.4 dB with a THD of -85.3 dB (0.0054 %) and a noise level of -89.1 dB. More information on SINAD in general can be found in the blog post dated May 12th, 2021. This measurement in particular is further described in the blog post dated May 21st, 2021.

More information on the development of v2.1 DÆ phono preamp circuit can be found in the Blog including the May 23rd, 2020 Blog post.