That discussed driver tech which is interesting but I ment why don't CD seem to reduce above 2-3k?
I.e. not a KEF specific question ...
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I.e. not a KEF specific question ...
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A bit more data regarding source impedance and distortion—
I posted this on ASR a while ago, but I'll re-post some of it here. My big DIY 2-ways originally had the drivers directly wired to the amplifier outputs (voltage source) with all filtering done digitally. More recently, I decided to experiment with adding passive networks to increase the source impedance at high frequencies.
Schematic, lookback (source) impedances, and filter magnitude responses:
Measured distortion, 86dB SPL at 1 meter; first is voltage drive, second is with the passive networks:
And again at 96dB SPL:
The reduction of odd-order distortion appears to be largest at lower drive levels in this case. At 86dB, H3 and H5 drop some 10dB in the top octave of the woofer's passband (500Hz-1kHz). The tweeter's H3 drops ~5dB at 2kHz and above.
I posted this on ASR a while ago, but I'll re-post some of it here. My big DIY 2-ways originally had the drivers directly wired to the amplifier outputs (voltage source) with all filtering done digitally. More recently, I decided to experiment with adding passive networks to increase the source impedance at high frequencies.
Schematic, lookback (source) impedances, and filter magnitude responses:
Measured distortion, 86dB SPL at 1 meter; first is voltage drive, second is with the passive networks:
And again at 96dB SPL:
The reduction of odd-order distortion appears to be largest at lower drive levels in this case. At 86dB, H3 and H5 drop some 10dB in the top octave of the woofer's passband (500Hz-1kHz). The tweeter's H3 drops ~5dB at 2kHz and above.
TNT: "why don't CD seem to reduce above 2-3k?"
You have to consider why current drive works to reduce distortion. The change in inductance with voice coil position results in a change in impedance. The motor force is proportional to current. So with voltage drive the change in impedance results in a change in current in the motor coil (voice coil) as the coil position changes. With current drive the change in inductance, resulting change in impedance has no effect on the current. This is the mechanism for reducing distortion. At higher frequencies the displacement of the motor is so small that the inductance, impedance and current for voltage drive do not change much so this component of the distortion is already very small so a reduction isn't possible.
You have to consider why current drive works to reduce distortion. The change in inductance with voice coil position results in a change in impedance. The motor force is proportional to current. So with voltage drive the change in impedance results in a change in current in the motor coil (voice coil) as the coil position changes. With current drive the change in inductance, resulting change in impedance has no effect on the current. This is the mechanism for reducing distortion. At higher frequencies the displacement of the motor is so small that the inductance, impedance and current for voltage drive do not change much so this component of the distortion is already very small so a reduction isn't possible.
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That's probably a timely reminder to use multi-tone / IMD type tests. Mid-to-high frequencies may not contribute much to cone displacement, but they're nearly always riding on top of some lower frequencies to start with.
That's only part of it, and probably not the main factor for mid band harmonic distortion. At frequencies where excursion is large, the inductive component of the electrical impedance is small. At frequencies where the inductive component is dominant, the excursion is small. Inductance as a function of voice coil current, Le(i), is more significant in this case.
As @abstract mentioned, the effect of Le(x) will show up much more in multitone measurements.