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Old 29th May 2011, 10:54 PM   #1
dobias is offline dobias  United States
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Default Field coil PS?

Could a 2A3 SET be modified for a field coil speaker (or make a separate PS)? Can one PS supply both stereo FC speakers?
There's very little to be found online.
I understand FC speakers require 350-395VDC but my present PS has voltage to spare! The old PT I used is a 436-0-436 vac.
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Old 29th May 2011, 11:19 PM   #2
tinitus is offline tinitus  Europe
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oh, I thought it required around 12V max
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Old 29th May 2011, 11:47 PM   #3
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I believe this voltage required varries from speaker to speaker. The Feastrex field
coils use about 15V.
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Old 29th May 2011, 11:54 PM   #4
tinitus is offline tinitus  Europe
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and lower voltage results in higher Qts, which would be convenient for OB
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Old 30th May 2011, 12:52 AM   #5
kevinkr is offline kevinkr  United States
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There are a variety of FC drivers requiring different voltages to energize the coils. Vintage RCA cinema drivers for example use 13.8VDC and 115VDC, average power consumption would be in the range of 20W.

There are a number of modern FR drivers that have field coils (Feastrex IIRC offers one) and conversions of certain old Altec models are also available.

FWIW I think using FC drivers warrants separate FC power supplies, and my preference for safety reasons would be for ones that operate on 12 - 16V.

Note that it is alleged that driving the VC of an FC driver without the FC energized is a potential way to fry the VC. Not convinced that that is at all the case, but my experience is very limited and only to the RCA drivers mentioned. (13.8V versions)
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Old 30th May 2011, 12:56 AM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by dobias View Post
Could a 2A3 SET be modified for a field coil speaker (or make a separate PS)? Can one PS supply both stereo FC speakers?
There's very little to be found online.
I understand FC speakers require 350-395VDC but my present PS has voltage to spare! The old PT I used is a 436-0-436 vac.

In most cases the old field coils were wired in series with the rest of the amplifier load and provided the L in the typical CLC supply filter of the time. The voltage drop across the FC would have been less than 100V in most cases.

Modern performance requirements IMHO make this approach undesirable from a safety standpoint, and also a performance standpoint. (Hum) I'd use dedicated FC supplies. This is something I am more than a little interested in myself.
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Old 30th May 2011, 02:45 AM   #7
bappe is offline bappe  Sweden
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Hi,

I would recommend a SS constant current supply. The reason is that it is current, not voltage that builds the field up and since the resistance of the coil changes as it gets heated up so will the current if a constant voltage supply is used. I use this on my Feastrex and have never looked back and the original Festrex supply is also of trhis type since a while back.

//Anders
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Old 30th May 2011, 02:47 AM   #8
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A simple regulated 12V power supply will do or a 12v battery; at least for
the Fertin. Wouldn't complicate things using an amp...
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Old 30th May 2011, 03:00 AM   #9
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The field coil was used as the L in the power supply filter as mentioned. That was before good magnets became available. The current through the field coil was small by today's standards. You can use a fixed voltage supply as the easiest solution. A constant current source is much better. If you use an adjustable current source you can monitor the voltage across the magnet coil. If the voltage starts to rise it means the coil is heating up and when you get about a 25% increase from cold you probably are close to the original design current. Any more rise and you risk burning out the coil.

The simplest current source would be a high voltage (100 volts should do) and an adjustable power resistor. I would start with .01 amp and slowly increase the current. Of course once you decide on an operating point you could then measure the voltage and use a fixed voltage supply equal to the operating voltage.
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Old 30th May 2011, 05:54 AM   #10
dobias is offline dobias  United States
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Default Feastrex

Gentlemen,
If this crowd can afford Feastrex drivers, I must be with the wrong group.
I doubt I could afford the admission cost to just listen to them, on my retirement income!
Please, let's bring the discussion down to an affordable level.

Is the advent of low voltage fieldcoils a recent development?
Which of the older drivers are preferable?
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