I thought I knew what the Signal-to-Noise and Distortion ratio (SINAD) of the DÆ v3.0 Phono Preamp would be from the theoretical analysis in my May 12th blog post, but the actual measurements contained a few surprises. The measurement below is taken with a QuantAsylum QA401 analyzer. With an output of 1 Vrms at 1 kHz, the measured Total Harmonic Distortion (THD) is 0.00594% or -84.5 dB and the measured Total Harmonic Distortion plus Noise (THD+N) is -66.9 dB. Note that SINAD is the reciprocal of THD+N so this equates to a SINAD of 66.9 dB.

What? Why so low a SINAD? I was expecting a SINAD of 93 dB. Also I was expecting that noise would dominate the SINAD with the distortion being less of a factor. While the distortion is relatively low at 0.00594%, it is still clearly visible above the noise floor.

There appears to be something wrong with the measurement because the noise is much higher than previously measured. The output noise is typically 23 µV, so a 1 Vrms output level would give a signal-to-noise ratio of 93 dB. Instead this measurement of SINAD is equivalent to a signal-to-noise ratio of only 67 dB.

First measurement of DÆ v3.0 Phono Preamp with an output of 1 Vrms at 1 kHz.

Further consideration of this result revealed that SINAD measurements on a phono preamp must be done very carefully taking into account the peculiarities of a phono preamp input. The DÆ v3.0 Phono Preamp is optimized for a Moving Coil (MC) cartridge with an internal resistance around 100 Ω. The noise at the input of the phono preamp will be dominated by the thermal noise of this resistance. In the measurement shown above, the output of the QA401 analyzer is connected directly to the phono preamp and the output noise of the QA401 appears to dominate the measurement.

The phono preamp will dutifully amplify the noise at the output of the QA401 along with the desired 1 kHz test signal and apply the RIAA equalization curve. The shape of the RIAA curve is clearly visible in the shape of the noise floor in the measurement. An RIAA curve with a gain of 40 dB at 1 kHz has a gain of 60 dB at low frequencies around 20 Hz. A gain of 60 dB is equal to 1000 times which turns microvolts into millivolts or millivolts into volts - so it is a lot of gain . Any noise at low frequencies at the output of the QA401 will contaminate the results and produce erroneous measurements.

To get a more accurate reading, care must be taken to make the output noise of the QA401 equal to the thermal noise of a 100 Ω resistor.

To get a more representative measurement I purchased a QuantAsylum QA480 precision oscillator and notch filter. The QA480 has a very precise analog 1 kHz oscillator and associated notch filter to allow the measurement of very low distortion audio equipment. I didn’t use the notch filter for this set of measurements but plan to use it in the future.

To mimic the output resistance and thermal noise levels from the MC phono cartridge, I ran the output of the QA480 through an approximately 40 dB attenuator made with a 10 kΩ resistor and 100 Ω resistor to ground. This attenuator reduces the output noise from the QA480 and makes the output resistance equal to about 100 Ω to properly model the thermal noise of the MC phono cartridge. The attenuator also reduces the level of the test signal by 40 dB but it is easy to increase the test signal level so that the output of the phono preamplifier is unaltered at 1 Vrms. The resulting measurements are shown below.

Measurement of DÆ v3.0 Phono Preamp with an output of 1 Vrms at 1 kHz from a QA480 and 40 dB attenuator.

The SINAD is improved to 83.4 dB with a THD of -85.3 dB and a noise level of -89.1 dB. This is still 10 dB worse than the expected SINAD of 93 dB with noise alone but a big improvement over the first measurement. Distortion is playing a more significant role than expected (or wished for).

What about SINAD at other output signal levels? With a 1 Vrms output distortion is already slightly more significant than noise in the combined SINAD measurements. At 2 Vrms output level the SINAD is actually a little worse than the SINAD with a 1 Vrms output level because distortion is on the rise at these outputs levels. This is another surprise because I was expecting SINAD to improve as the output level is increased from 1 to 2 Vrms.

In the graph below of THD+N versus output signal level there is a broad minimum around -85 dB with output levels between -6 and 0 dBV. Remember SINAD is the reciprocal of THD+N so a THD+N of -85 dB is equal to a SINAD of 85 dB.

Below -6 dBV noise starts to dominate and the THD+N (and SINAD) slowly gets worse as the output level is decreased. Above 0 dBV, distortion starts to rear it’s ugly head and the THD+N (and SINAD) slowly gets worse with increasing output level.

THD + N as a function of Output Level for the DAE v3.0 Phono Preamp.

Next stop, study ways to reduce the distortion…