Hi Alexander,
I will try and keep this simple and please understand that I do not have a degree in Physics so there will be no complex equations.
The field coil drive unit uses an electromagnet in place of the typically used modern permanent magnet assembly. The field coil drive system uses a pair of coils, one for the electromagnet and one for the voice coil. The electromagnet coil presents magnetic field to the voice coil to work against. The voice coil magnetic field will vary in sympathy with the audio signal from the amplifier and the interaction between the coil magnetic fields will move the driver cone along the axis of the voice coil.
Applying current to the voice coil will induce a change in the current through the electromagnet coil. Therefore the electromagnet coil power supply has to cope with transient current changes that are in sympathy with the audio signal current fluctuations. This can cause a change in the output voltage of any power supply connected to the coil and this change will be directly proportional to the current change in amps times the output impedance of the power supply in ohms. For example, a change of 1 amp from a regulator output impedance of 0.01 ohms will cause an output voltage drop of 0.01 volts. This may seem insignificant to many but it is enough to cause a small efficiency drop in the drive unit causing mild compression of the musical transient amplitude.
I have not allowed for the fact that the coils have a reactive impedance characteristic, and this can cause the resulting error on the electromagnet coil to be replicated on the voice coil, time displaced due to reactive phase shift. This error signal will inter-modulate with the low level information of the acoustic output of the drive unit causing masking and smearing of the waveform of the low level signals.
The ideal power supply for this job should have zero impedance at all frequencies of operation to stop any voltage changes on the output voltage. To reduce any reaction of the power supply output voltage in makes sense to ensure that the power supply output impedance is as low as possible over as wide a frequency band as the drive unit is operating over.
Another issue would be power supply output noise. This will be coupled through the electromagnet coil to the voice coil and higher levels of noise will mask low-level information.
The quality of the power supply used to power field coil drive units is just as critical to performance as in other areas of audio signal processing. The coils are not independent of each other. They are closely coupled together. Disturbing the electrical parameters of one coil will affect the electrical parameters of the other coil.
Your Altmann DAC power supply upgrade produced some radical improvements in the musical presentation of your system. Expect more generally similar improvements with a power supply upgrade for the field coils.
If you wish to continue with a field trial, let me know the power supply voltage and current requirements of your drive units and I will slot some power supplies into the build schedule.
Regards
Paul
