AFT Fieldcoil Power Supply

How do we adjust fieldcoil supply DC voltage? 

Our goal is not highest magnetic flux density B. Obviously higher flux density provide higher efficiency and faster sound. 
When fieldcoil supply DC voltage is too low, it is slower and less efficient. However it might work better with open baffle configuration (better lower frequency).
At the sweet spot DC voltage, field coil speaker can produce magic, three denominational sound stage, depth and beautiful decay.
 
When fieldcoil supply DC voltage is way too high, it heats up the coil and can damage the driver. If the temperature is OK, should we crank it up the voltage? Not for me. For person who want FAST. VERY FAST sound, it may be the choice. When voltage is higher than sweet spot, fieldcoil speaker loose the magic! no more magical three denominational sound stage, depth and decay!!

If you are not using fieldcoil speaker as a fullrange, using in two way or three way system, adjusting fieldcoil DC voltage is a tool to align speed with other drivers. Too fast is not always better!

AFT model 7:
Optimum field coil DC power supply condition for AFT model 7 is 6.5Vdc at 250mA. They should be mono or dual mono set up. Fieldcoil magnet circuit acts like an audio transformer. If right and left channels share a same power supply, it will affect each other (Figure 2).
We recommend the power supply to be low impedance, flat phase response over audio frequency range and open loop. Figure 1 shows our recommended basic fieldcoil power supply circuit. Since they are simple open loop circuit, quality of each component is very important.

Where:
Xfrmer1: custom FINEMET power transformer
L1: custom FINEMET choke
C1:  film capacitor 
D1, D2: SiC Schottky diode
Figure 1. Basic fieldcoil power supply

Closed loop constant voltage source and closed loop constant current source are not recommended.

Fieldcoil magnetic circuit pass though music signal from music amplifier to fieldcoil power supply and vice versa. It makes that the constant voltage (or current) source and the music amplifier fighting each other, Figure 2.
Figure 2. Simplified relationship between closed loop fieldcoil power supply and music amplifier


How about batteries? They are quiet, but so far, we have not found a battery that can respond consistently over entire audio frequency. Battery also needs regulator to supply constant voltage (or current). Battery technology is changing rapidly, possibly super battery is in horizon?


Entry model:


When funding is tight in the beginning of the project, this entry model can be a good starting point. It uses a FIMENT core choke, L1. The FINEMET core choke is very fast and has very stable phase response. LC values are optimized. Larger C value does not necessary makes better results.


Xfrmer1: Oriented grain core power transformer
L1: FINEMET choke small 100mH
C1: Panasonic FC 3300uF 35Vdc
C2: CDE 944U 33uF
D1, D2: SiC Schottky diode

Figure 3. Entry model fieldcoil power supply


Entry plus model:


This entry plus model has the best of both world (cost and performance). All irons (transformer and chokes) are used in standard model is FINEMET. These FINEMET irons are very fast and have very stable phase response. LC values are optimized. Larger C value does not necessary makes better results.


Xfrmer1: FINEMET power transformer 30VA size
L1: FINEMET choke 30VA size 150mH
C1 CDE 947C 600uF (or 944U 220uF x3)
C2: CDE 944U 33uF
D1, D2: SiC Schottky diode
Figure 4. Entry plus model fieldcoil power supply

Standard model:

All irons (transformer and chokes) are used in standard model is FINEMET. These FINEMET irons are very fast and have very stable phase response. 180VA size, large L2 with low DCR is the key to the sound stage. LC values are optimized. Different C value may degrade the sound.


Xfrmer1: FINEMET power transformer 30VA size
L1: FINEMET choke 30VA size 150mH
L2: FINEMET choke 180VA size 650mH
L3: FINEMET choke 7mH 1A
C1 CDE 947C 600uF (or 944U 220uF x3)
C2: CDE 944U 100uF (or 944U 33uF x3)
C3, C4: Urushi 0.1uF 1000Vdc 
D1, D2: SiC Schottky diode
Figure 5. Standard model fieldcoil power supply​


Signature model:


All irons (transformer and chokes) are used in signature model is FINEMET core. These FINEMET irons. are very fast and have very stable phase response. 360VA size, large L2 with very low DCR is the key to the FAST and stable sound stage. LC values are optimized. Different C value may degrade the sound.

Xfrmer1: FINEMET power transformer 90VA size
L1: FINEMET choke 180VA size 0.65H
L2: FINEMET choke 360VA size 1.33H
L3: FINEMET choke 7mH 1A
C1 CDE 947C 600uF (or 944U 220uF x3)
C2: CDE 944U 100uF (or 944U 33uF x3)
C3, C4: Urushi 0.1uF 1000Vdc 
D1, D2: SiC Schottky diode

Figure 6. Signature model fieldcoil power supply



Other components:


AC supply Fuse:   
3A slow blow

Connectors:
6.5Vdc at 250mA is not difficult for many connectors, and we recommend Neutrik power connectors

Figure 7 Neutrik power connectors



Ordering information:


AFT fieldcoil power supply iron kit:

AFT fieldcoil power supply iron kit includes transformers and chokes only. (User needs to prepare all other components.) Shipping from Japan is free except Africa and South America

Entry model:            $500     (+ $40 for Africa, South America)
Entry plus model:     $1960   (+ $54 for Africa, South America)
Standard model:      $4260    (+ $128 for Africa, South America)
Signature model:     $7260    (+ $212 for Africa, South America)


Finished mono field coil power supply:

For ordering finished power supply, please contact us for availability.

Entry model:            TBA
Entry plus model:    TBA
Standard model:     TBA
Signature model:    TBA