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08-04-2020, 08:45 PM
Hi Guys
IM will result in the sonic impression described above: harsh, cold, loss of detail, smearing of the sound - it leads to fatigue because none of the harmonic products are musically related to the original frequencies.
Where IM is generally though to be simply the sum and difference of two frequencies, it will result in an explosion of sums and differences that extend far above and far below the original frequencies. This is demonstrated quite well when you try to power audio circuitry with SMPSs. The basic hiss of the audio components will take on an 'edge' and the tone will be 'off'. You cannot see this on most analog scopes but a DSO shows it clearly - the visible noise will astound you and turn you away from at least the cheapo SMPS bricks.
There is always debate about how sensitive the Human brain is to THD and to IM. THD is generally unnatural but can some times be produced in nature, so our brains can work at ignoring the extra information. IM is completely unnatural and causes fatigue as we try to sort out the unrelated information. This means that we might find a tube amp with whole-percent THD to be "acceptable" even if we also feel that it is not "accurate", where even one-part-per-million of IM is objectionable (1ppm = 0.0001%).
Fortunately, we can minimise certain causes of IM by using Galactic Grounding and by making the circuit immune to out-of-band ingress, which means adding a low-pass filter at the input and shielding the circuitry. Every little bit helps here.
Plate caps and other high-frequency shunts throughout the circuit help to reduce noise while shaping tone or not, and reduce the THD and IM of subsequent gain stages.
IM will result in the sonic impression described above: harsh, cold, loss of detail, smearing of the sound - it leads to fatigue because none of the harmonic products are musically related to the original frequencies.
Where IM is generally though to be simply the sum and difference of two frequencies, it will result in an explosion of sums and differences that extend far above and far below the original frequencies. This is demonstrated quite well when you try to power audio circuitry with SMPSs. The basic hiss of the audio components will take on an 'edge' and the tone will be 'off'. You cannot see this on most analog scopes but a DSO shows it clearly - the visible noise will astound you and turn you away from at least the cheapo SMPS bricks.
There is always debate about how sensitive the Human brain is to THD and to IM. THD is generally unnatural but can some times be produced in nature, so our brains can work at ignoring the extra information. IM is completely unnatural and causes fatigue as we try to sort out the unrelated information. This means that we might find a tube amp with whole-percent THD to be "acceptable" even if we also feel that it is not "accurate", where even one-part-per-million of IM is objectionable (1ppm = 0.0001%).
Fortunately, we can minimise certain causes of IM by using Galactic Grounding and by making the circuit immune to out-of-band ingress, which means adding a low-pass filter at the input and shielding the circuitry. Every little bit helps here.
Plate caps and other high-frequency shunts throughout the circuit help to reduce noise while shaping tone or not, and reduce the THD and IM of subsequent gain stages.
