Class D amplifiers have defined a new discipline in audio amplifier design and parameters, similarly as digital CD audio after its arrival in 1980-ties. After some time after its arrival, it was apparent that CD measurement methods would need something else and more than previous, purely analog audio. Similar situation is, IMO, now, with class D amplifiers. I appreciate greatly Amir's work on standard audio measurements in audio band, like SINAD, THD vs. power etc., and also HF spectrum. However, I think that class D amplifiers need another approach as well. This is based both on theoretical assumptions and measurements on real amplifiers.
A year and half ago I borrowed HYPEX NC400 amplifier from a friend and I had it several days here on my test bench. Standard audio measurements are (after the 50kHz LPF is added at the output), excellent and confirm datasheet parameters and other independent tests. But class D amplifiers are basically different from linear amplifiers and this should be taken into account. They operate at high frequency switching frequency with full swing square impulses, these impulses are modulated and after LPF demodulation and removal of the carrier frequency we get the resulting analog signal. This is in a way similar to sampling process in such regard that we have to care about rejection of aliases when input signal frequency is above half of carrier frequency.
To make long story short, I found that HYPEX NC400 is sensitive to even low level input signals with frequencies close to Fc (carrier) and 2Fc.
When the input frequency approaches to Fc (about 460kHz) and 2Fc (about 920kHz), one can see aliases, difference frequencies Fc – Fin (or 2Fc – Fin) appearing at the output of the amplifier. I measured about 56mVp-p output swing of the alias signals (my input signal was only 21mV), in the audio band. I have used a 2xRC analog low pass filter (27kHz) at the scope input to remove 460 kHz switching frequency of the NC400, to see the aliases. So, the input signal containing HF components may, under certain circumstances, create audible alias frequencies at the NC400 output. This is not very good and I guess that a better input anti-alias low pass analog filter would cure the situation. I had some correspondence with Bruno P. and he has confirmed my findings and said that NC400 was a closed chapter, because of design work on Purifi project. It might be interesting if Purifi is free of this issue.
This may seem unimportant, but especially 920kHz frequency is close to those of AM MW transmitters and RFI pickup in mV order is not impossible into common poorly shielded and unshielded link cables and loops and it would create audible artefacts.
HYPEX input driven with 915kHz 10mV sine wave. Output alias frequency is 5kHz.
HYPEX alias wave on the scope screen
HYPEX carrier frequency at the amplifier output binding posts
A year and half ago I borrowed HYPEX NC400 amplifier from a friend and I had it several days here on my test bench. Standard audio measurements are (after the 50kHz LPF is added at the output), excellent and confirm datasheet parameters and other independent tests. But class D amplifiers are basically different from linear amplifiers and this should be taken into account. They operate at high frequency switching frequency with full swing square impulses, these impulses are modulated and after LPF demodulation and removal of the carrier frequency we get the resulting analog signal. This is in a way similar to sampling process in such regard that we have to care about rejection of aliases when input signal frequency is above half of carrier frequency.
To make long story short, I found that HYPEX NC400 is sensitive to even low level input signals with frequencies close to Fc (carrier) and 2Fc.
When the input frequency approaches to Fc (about 460kHz) and 2Fc (about 920kHz), one can see aliases, difference frequencies Fc – Fin (or 2Fc – Fin) appearing at the output of the amplifier. I measured about 56mVp-p output swing of the alias signals (my input signal was only 21mV), in the audio band. I have used a 2xRC analog low pass filter (27kHz) at the scope input to remove 460 kHz switching frequency of the NC400, to see the aliases. So, the input signal containing HF components may, under certain circumstances, create audible alias frequencies at the NC400 output. This is not very good and I guess that a better input anti-alias low pass analog filter would cure the situation. I had some correspondence with Bruno P. and he has confirmed my findings and said that NC400 was a closed chapter, because of design work on Purifi project. It might be interesting if Purifi is free of this issue.
This may seem unimportant, but especially 920kHz frequency is close to those of AM MW transmitters and RFI pickup in mV order is not impossible into common poorly shielded and unshielded link cables and loops and it would create audible artefacts.
HYPEX input driven with 915kHz 10mV sine wave. Output alias frequency is 5kHz.
HYPEX alias wave on the scope screen
HYPEX carrier frequency at the amplifier output binding posts