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Field-coil speakers with variable FC
Hi Guys

Modern loud speaker drivers use permanent magnets to create a standing or static magnet field for the voice coil and its signal-generated magnetic field to work against. Because the speaker isĀ  akin to a "linear motor", the fixed field is the "stator" while the moving part is the "armature".

In the past, permanent magnets were difficult to manufacture except in small sizes, and as electronic amplification evolved, a clever solution was to use an electromagnet for the speaker's stator. Actually the clever part was to incorporate this winding into the power supply of the tube amp. At the same time, high-value high-voltage capacitors were expensive to manufacture but transformers and chokes were not, so you see a lot more choke filtering in older PSUs and the field coil was used as one of these chokes.

Another aspect of the amplifier design at the same time was the use of cathode-bias. This means the amp draws a fairly constant amount of power all the time and the current through the field coil would be steady, as well. It is ideal for the static speaker field to be constant so the driver performance can be predictable and consistent.

FC speakers disappeared as magnet technology and economy improved, but there has been a recent revival of FC drivers both in hifi and MI. Speakers of this design are quite expensive due to their low production volumes and the target customers tend to be economically well-off. Adding to the cost in hifi is the rather superfluous use of tubes in the FC PSU. The application is purely aesthetic.

In MI, the FC driver lends itself to field weakening which reduces the driver output and therefore can act as a loudness control if the FC power supply is made variable.

Traditional FC driver field coils had a DC resistance of 1k or less and the voice coil power would typically be in the 10-25W range. If we assume a worst-case scenario that FC power should exceed or match maximum audio power, then we might have a situation like this:
audio power = 25W
FC power = 25W
FC resistance = 1k
FC V and I: 158Vdc at 158mAdc

If FC resistance was only 100-ohms, FC V and I would be 50V at 500mA. Since the field coil is physically fixed in place and does not have to move, its weight is of no concern and it can have many turns of light wire or fewer turns of heavy wire. Coil weight is only of concern in the voice coil, as in conventional drivers.

Either way, the power supply is not that big a deal and can be made using standard linear regulator circuits. The regulator can be made constant-voltage or constant-current and both can be made variable. There is little point in extending the control range to zero, but there likely is a minimum point that is reasonable simply to have any sound. A higher limit would be "where does the sound change?". This might be a much higher FC strength than the sonic cutoff and may in fact be not at an SPL anyone would call "quiet".

As a case in point, there is a company (Fluxtone) making drivers specifically for MI that admits that their reference tone is what they achieve at 32W of input to the driver. That is incredibly loud and the player will not be hearing the speaker output accurately unless he is a hundred feet away. Even an open-back cabinet achieves efficiencies around 90dB@1W/1m, so 32W would be 105dB. Were the more usual reference of 100dB@1W/1m achieved, then loudness with 32W would be 115dB. Both levels are well beyond the limit for the Human Scale of Loudness as defined in our books (see TUT4).

The company selling these drivers claim that the SPL range extends down to 86dB, which is still beyond the Human Scale for anyone wishing to retain their hearing into old age. It may be that the manufacturer considers any volume reduction below this loudness using their field coil technology simply changes the tone and is therefore ineffective? For some players these products may provide the small amount of loudness reduction desired, but...

To take advantage of this technology you have to change your speaker to theirs. Considering the product range, this will be fine if you use low-power Jensen drivers or similar. At least you will be playing quieter.

At those high SPLs, your aural compression is being invoked quite heavily and the illusory sound in your head has little to do with the actual sound in front of the driver. Assessing the tone over 80dB is "optimistic" at the very least.

Note that an SPL change of 10dB corresponds to a power change of 10-times and a loudness perception change of 2. So, going from 100dB to 90dB will seem "half as loud" and requires a power change of 1W to 100mW. Similarly, going from 80dB to 90dB is a sonic perception of "twice as loud" and an increase of power from 10mW to 100mW. (100dB@1W/1m driver reference)
Thanks for the write up! I was wondering about how he Fluxtone approach worked. I've had customer or two mention it but seemed so limited compared PS that I steer them towards scaling instead.

It's strange to me how many players are so reckless with their hearing. So many of the older players I know are now struggling to hear after many years of playing way too loud.

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