This phono preamp has many interesting features.

The preamp uses a transconductance RIAA equalisation stage. You can read a little more about this 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 the adjusted 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 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.

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

This is the first version of the DÆ Remote v0.1. The remote is used to control the DÆ Modular Stepped Attenuator.

Features:

- Capacitive touch key technology allows for ultra thin construction. The main body is only 6 mm thick!

- Bluetooth radio link to the attenuator so the no line of sight to the attenuator is required.

- An LED back light that is turned on by proximity sensing so the back light is illuminated when your hand approaches. A light senor is provided so the back light is always the right intensity. The back light is bright in a well lit room and is dimmed when the lights in the room are lowered.

- A rechargeable lithium battery to power the remote for up to three months between charges.

- A USB connector to allow charging from any powered USB port or with a cable and wall dongle. The remote back light will flash while charging; the flashing will stop when the battery is fully charged.

- An on/off switch so the remote can be switched off for an extended storage.

- After 15 seconds of inactivity, the remote goes into a “sleep” mode to conserve battery power. While in “sleep” mode, the remote is disconnected from the attenuator. This allows the attenuator to be controlled with the smartphone app. Switching the remote off also allows the attenuator to be controlled with the smart phone app.

Minimalist!

Two DAE Modular Stepped Attenuators v2.0 (even without the volume control knob) and not much more. Everything is controlled from the remote control.

All you need for great sound.

Specifications:

• attenuator has 24 steps. The steps are 0 dB, -3 dB, -6 dB …. -21 dB, -23 dB, -25 dB … -53 dB. See Modular Stepped Attenuator pages for additional details;

• attenuator total resistance = 2 kΩ;

• four inputs are provided: PH (phono), CD, 1 (auxiliary 1) and 2 (auxiliary 2);

• output cable length should be limited. Long output cables will cause a roll-off in the high frequency response because there is no output buffer (the preamp is passive);

• mahogany base, dyed red with clear satin varnish.

Late in 2017 I eagerly started reading the latest book by Douglas Self entitled "Electronics for Vinyl". In this book Douglas provides a clear and detailed description of the low noise/high accuracy design of phono pre-amplifiers. The design philosophy includes the following two concepts:

• use low value resistors to minimize the thermal noise;

• use two or three resistors in parallel to achieve the high precision and non-standard resistor values required for RIAA equalization. RIAA equalization is the specification for a filter in a phono pre-amplifier used during the playback of vinyl records. The specification was established by the Recording Industry Association of America (RIAA) to allow the recording of sound to better match the mechanical limitations of cutting a groove in a vinyl record.

On page 41 in a section on Resistor Value Distributions, I read "...what is the actual distribution of resistor values like? It is not easy to find out..." and "...measuring thousands of resistors with an accurate DVM is not a pastime that appeals to all of us. Any nugget of information in this area is therefore very welcome."

This sparked my interest - I could build a machine to automatically measure a whole box of resistor values.

1000 resistors in a box!

During Christmas break 2017, in the time between family gatherings, I designed the Resistor Testing Machine shown on this page. Parts started arriving at my house in January 2018 and the machine was ready for its maiden run on February 19th, 2018.

Over the following few days I used the machine to measure three resistor values, 220 Ω, 1 kΩ and 10 kΩ. A text file with the value of each resistor tested is available by clicking the link associated with the value. If the Resistor Testing Machine measures a bad value, a blank line is written in the text file.

All resistors were metal film Vishay MRS16 series ±1% tolerance. The results were very interesting:

• the measured average of the 220 Ω resistors was almost spot on at 219.9 Ω (-0.05%).  The measured average was a little low for the 1 kΩ resistors (995 Ω or -0.51%) and 10 kΩ resistors (9969 Ω or -0.31%) but well within tolerance;

• the measured standard deviation was much tighter at 0.18%, 0.13% and 0.12% for the 220 Ω, 1 kΩ and 10 kΩ values respectively;

• the measured resistor values very closely followed a normal distribution;

• the measured average of 211, 10 kΩ resistors reported by Hugo Kroeze and referenced in Douglas Self's book was 0.05% low which is the same percentage error in average value I measured for the 220 Ω resistors. The average value for the 1 kΩ resistors and 10 kΩ resistors I measured is even lower than the measured average of 100, 1 k resistor measured by Doug Self which had a percentage error of -0.23%. The standard deviation I measured was between the values reported by Kroeze and Self.

All values for the Resistor Testing Machine were measured using a recently purchased Rigol DM3058E 5½ digit benchtop digital meter.  The accuracy specification for this instrument in 4-wire resistance measurement mode is 0.03+0.005 (%reading + %range) on the 200 Ω range and 0.02+0.003 (%reading + %range) on the 2 kΩ and 20 kΩ ranges.

I would be happy to measure more resistor values if you are interested in supplying the resistors. The Resistor Testing Machine was designed to use Tape & Box (TB) packaging but I would consider testing with other packaging options if the resistors can be feed automatically through the Resistor Testing Machine. 1000 values take about one and a half hours to complete. Please use the contact page if you in interested in testing more resistor values or have any questions.