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cv wrote:

I think the main advantage of the EM is reducing flux modulation. There was a post recently by one of the attendees who went with Feastrex claiming that Dr Kilppel (of famous measurement tools suite) measured one of their drivers and it featured the lowest level of flux modulation he'd seen yet. With that in mind: you can think of the voicecoil and field coil as forming a primary and secondary of a transformer.

Well, I think, we have to be very careful with it and the things are not what they are. People like to talk about the redaction of flux modulation then unfortunately they are juts the smalltalk. I personally feel that the sonic softness of the electromagnets in fact come form the very heavy flux modulation of the electromagnets. The electromagnets, as I see it, have no instantaneous bufferness to hold the magnetic modulations coming form the voice coil and therefore they “dive” when the short blast of signal run across the VC. In my excrement I was thinking to have a combination of the small permanent magnet that would deliver 10-15% of magnetic force and the rest coming form the field coil. I would like to have some “code” in the “transformer”. Also, I do not know is the deduction of the flux modulation is completely positive direction to do go: did you try to drive a car with too stiff suspension and over-inflated tires? Also, I do not know about the Feastrex. The sounded phenomenally horrible, regardless the flux modulation level they had. I wonder what credibility the Feastrex people might have if they deliberately ended up with THAT sound, regardless if it come form the electromagnets or form other problems. (I comment very negatively about the Feastrex despite the fact that I do like Christopher Witmer and respect his devotion to that company, I juts do not play the games of moderation of my perception of the actual result by my relationship with the people)

cv wrote:

To reduce flux modulation to a minimum, I wonder if one would like the secondary (field coil) and power supply to have as low an impedance as possible, ie resembling a shortiing ring.

Yes, I thought about it and it is how I would do it. I would also go for much higher voltage: ~50-100. The Cogent gays went for 12V and use the Radios Shack PS. I do not know what motivated them to stay with 12V, perhaps Steve dug it in some OK RCA manual that they used 12V and imitates it. I remember when I experiment with the supplies for my old field coils it was quite affective to the drivers sound…

cv wrote:

2) Use a FC supply having a negative impedance to offset as much of the DCR of the coil as possible (til you get stability problems)

3) Presumably we need to maintain good behaviour at HF as well, in which case, do we want to layer the FC windings in order to reduce interwinding capacitance - ie much as if we were designing an anode choke etc. Need to balance this with the reduction in flux available and reduced heatsinking.

Hm, this is very interesting… I have to think about it….

cv wrote:

You can also take the following with a pinch of salt, but people have reported drivers sounding better with a SlA battery supply rather than a bench one., so much so that they are willing to string 10 or 20 in series to get the required voltages.

It might be correct. I remember that the size of the last cap in the DC supply for my magnetizing coils was affecting sound and the larger I when with that last cap (it was CRC filer) then “harder” sound was…

The caT

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