Recently I have been enjoying a YouTube channel called Audio Science Review (ASR). On this channel and the associated website our host Amir tests a wide variety of audio equipment using an Audio Precision APx555 analyzer. The data he provides is a treasure trove for an audio equipment designer because it provides a vast array of measurements to benchmark the performance of a new design.
Let me say this at the start - I think the objective comparison of equipment is critical. If you want to engineer anything you need a way to measure and mark progress. That does not mean the equipment that measures best is the right, only choice for everyone. For comparison, anyone shopping for a new car may look at the fuel efficiency numbers and/or the 0-60 time, but these numbers are far from the only factors influencing the buying decision.
ASR uses Signal-to-Noise and Distortion ratio (SINAD) as the key figure of merit to compare equipment performance. Quoting from the Audio Precision website, “SINAD is defined as the ratio of the total signal (desired signal plus the sum of all distortion and noise components) to the sum of all distortion and noise components.” So the greater the SINAD the better. The Audio Precision website also says “SINAD is usually performed with a stimulus signal of 400 Hz or 1 kHz.” The measurements reported by ASR are the SINAD for a 1 kHz sine wave.
The Audio Electronics Review and Measurements Index on the ASR website currently includes over 700 reviews. On the “Electronics Rating” tab the results are sorted in descending order based on the measured SINAD. The best measured SINAD is 123 dB and it is for the Audio Precision APx555 analyzer measuring itself on XLR connections with a 5.0 V signal level.
The equipment is sorted in to four Tiers. I understand that Tier 1 equipment is the top 25% of equipment measured which currently equates to a SINAD between 106 dB and 123 dB. Tier 2 is the next 25% of equipment with a SINAD between 92 dB and 106 dB. Out of interest, the lowest SINAD currently reported is 30 dB for a car audio amplifier - this is the bottom of Tier 4.
I, of course was interested to compare the performance of my v3.0 phono preamplifier design to the other phono preamplifiers on the list to see how far I have come and how far I still need to go. The best phono preamplifier on the list has a SINAD of 90 dB which puts it in Tier 3! The Tier 3 rating doesn’t leave a very good impression. How could the best phono preamplifier measured so far only deserve a Tier 3 rating? But why? Are phono preamp designs really that bad and digital equipment that good? And how does my design compare?
Incidentally, the top rated phono preamplifier was tested on a moving magnet (MM) input. The highest rated moving coil (MC) phono preamplifier was in the bottom tier, Tier 4, with a SINAD of 64 dB.
My preamp is optimized for the phono cartridge I have now, which is a high output moving coil cartridge with an output resistance of 95 Ω. A gain of 40 dB at 1 kHz is appropriate for this cartridge and of course the RIAA phono equalization curve is used. To establish an upper bound on performance, I used PSpice to calculate the theoretical minimum output noise from a “noiseless preamplifier”. The preamplifier has an RIAA curve, 40 dB gain at 1 kHz and is driven from a source resistance of 100 Ω - and the result was 11 µV. This estimate was over the full bandwidth of 22 Hz to 22 kHz; with PSpice I calculated the noise in the ten octave bands from 32 to 16 kHz. 11 µV is only the thermal noise of 100 Ω source resistance passed through the “noiseless preamplifier”. This “noiseless preamplifier” is better than that, it’s also distortion less. With a 1 Vrms 1 kHz desired output signal that gives a best possible SINAD of 99dB. The conclusion - a perfect “noise and distortion less preamplifier” would be in Tier 2! Tier 1 performance is not possible!
I guess in this way you would have to conclude that it is possible to get better noise and distortion performance out of digital than analog so digital is better - but more on this in a later blog.
Recent measurements of my preamp show a noise level of 23 µV so a 1 Vrms output level would give a SINAD of 93 dB. That is assuming noise dominates over distortion at a 1 Vrms output which I think is true, but I will take an actual measurement of this in the next few days. A 93 dB SINAD would put it in Tier 2.
SINAD is sensitive to output signal level so a 2 Vrms output level, would give a SINAD of 99 dB assuming distortion doesn’t creep in and a noise level of 23 µV. I use a 2 Vrms output level because this is the standard for the peak output from a digital source like a CD player. A 99 dB SINAD would put the device solidly in Tier 2. Carrying this thought experiment further, a 5 Vrms output level would give a SINAD of 107 dB which is Tier 1 level of performance.