Field coil speakers

oldgears · « **on:** 22 Mar 2009, 11:02 pm »

I picked up a pair of full range 8 ohm no name 8" field coil speakers. Can i just hook them up to my tube amp just like normal speakers without damaging them. Thanks.

richidoo · « **Reply #1 on:** 23 Mar 2009, 12:10 am »

Your tube amp is safe, but a damping factor of 4 or higher would be nice, you should have that if your amps have output transformers. Use the 8ohm or 16 ohm tap.

You will need a DC power supply to power the field coils too because there are no permanent magnets in a field coil driver, they are electromagnet coils, and they need current to make a magnetic field for the signal coil to work against. If there is no voltage spec on the no label of the no name driver

then maybe start with 6VDC, make sure the field coil doesn't get too hot. Increase voltage for more control and clarity, but don't overheat the field coil with too high voltage. 12V should be enough. Try to find out what it calls for.
Have fun! The field coils I have heard were pretty cool.
Rich

oldgears · « **Reply #2 on:** 23 Mar 2009, 12:47 am »

Thank you Rich for your help on this. I will power them with my tube amp. Fun fun fun....

nullspace · « **Reply #3 on:** 23 Mar 2009, 03:11 pm »

Can you give us a DCR reading for the field coils? High DCR field coils frequently need a high voltage, low current power supply -- think several hundred Vdc and <100mA -- while low DCR field coils often use a low voltage, high current power supply -- maybe 12Vdc & 1.25A.

Also, a picture would be helpful in trying to figure out what you have.

Regards,
John

oldgears · « **Reply #4 on:** 23 Mar 2009, 05:40 pm »

I think mine are Rola as they have same code as the one at the auction site:

http://cgi.ebay.com/Vintage-Speaker-ROLA-12-Field-Coil-Loudspeaker_W0QQitemZ350175690878

Sorry John but I dont know how to measure DCR. I did hook them up last night and they sound pretty good with nice tone.

nullspace · « **Reply #5 on:** 23 Mar 2009, 05:58 pm »

It's worth mentioning that both questions directed to the seller you linked to asked about the field coil DCR... All you need is a multimeter to measure the resistance of the field coil. If yours are also 600ohms, you're probably looking at ~85Vdc and 15mA power supply needed, or somewhere in the ballpark of that.

Regards,
John

doorman · « **Reply #6 on:** 23 Mar 2009, 07:00 pm »

At the risk of derailing the topic, what are 'field-coil" drivers, advantages/disadvantages?
(please move post if not appropriate!)
Don

richidoo · « **Reply #7 on:** 23 Mar 2009, 08:41 pm »

Quote from: doorman on 23 Mar 2009, 07:00 pm

what are 'field-coil" drivers, advantages/disadvantages?

This might help you Don:
http://www.audiokarma.org/forums/archive/index.php/t-90189.html
Feastrex fans have observed greater resolution with the field coil models. They run at 12Vdc nominal.