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In Reply to: RE: Keeping the world safe for horns. posted by mac55 on July 22, 2009 at 16:46:29
Mac
If you have'nt already heard them, you should definitely listen to the Cogent field coil horn rig (hopefully Cogent will again be at the '09 Rocky Mountain Audio Fest in Denver this Fall). It's difficult to put this rig in perspective. I heard it in 2007 and it's very much "a thing unto itself". The only thing to compare it to from the "recommended component list" audiophile stuff at the show was the Wilson Maxx's, and the Cogents imaged as well, or better even, and were more dynamic with much less power. I could'nt say what the field coils contributed to this unless I heard the rig with some permanent magnet drivers for comparison. Steve Schell and Rich Drysdale of Cogent have a preference for conical horns (which tend to be deeper than curved wall horns), perhaps they can address the reasons for their choice of horn type. I used to make fun of conical horns until I heard some very good sounding examples from John Sheerin and Tom Danley.
Paul
Follow Ups:
Thanks for the recommendation. I did a search of Cogent. If these are the ones you are referring to:http://www.teresaudio.com/haven/cogent1_lg.jpg
and if they sound as good as they look, then they would indeed be impressive. I would like to have the chance to hear them, although I am not sure when or where this might ever take place.
My interest in field coils is currently limited to this massive old 1945 Wurlitzer tone cabinet I am trying to get running as a guitar cabinet. In this function, it looks impressive with its solid wood construction, slightly Art Deco styling, and load of drivers with two 12" Jensen field coil, and two Magnavox 15" field coils.
Edits: 07/22/09
MacThe cabs you describe seem like they were for an electric organ originally. The better contemporary electric guitar speakers (which are also of interest to horn designers) are usually low Q/over damped types and, because of their relatively large magnets, they tend to have a bass roll-off an octave or more higher than the resonant frequency. With this in mind, it may be useful to adjust the Q of the driver, as some of the other posters have suggested is possible with a field coil, for your application
Paul
Edits: 07/23/09
Yes, it was for an old Wurlitzer organ. The cabinet does have amplifiers, but I wish to use my own, thus I want to power the coils separately and provide signal from my amp.A friend suggested modifying the amplifiers to provide just the field coil power, since they are already set at correct values. He is very competent, but far away and busy, so I am not sure when I will have advantage of his help.
I might have had a simpler time adapting a separate DC power source myself, but I just received the schematics, and if I am reading things right, the voltage is 342 volts for the small speakers and 394 for the large. This sounds like a lot, and I have not seen any DC power supplies on ebay that go past 300V, thus using the amps might be the most economical way to go, although I will lose the flexibility of tinkering with current. But I am now wondering if I can just plug in a variable AC power control for a simple control. Would this work?
Edits: 07/23/09
Mac
I would go with the original amps, as long as the iron is good, as this would be the simplest and cheapest way. Typical amp topologies from that time and application would be 6SN7 or 6SL7's driving 6L6 outputs, and this is similar to a guitar amp anyway. See the link for some ideas for converting old industrial amps for guitar amp duty. If the original field coils were run with D.C., then running them with A.C. is definitely NOT recommended, but someone else would have to advise you on this. You will probably wind up with more bass with the rig in question than you need, the bass E string on the guitar is 82 Hz. Good luck, and let us know how it goes.
Paul
Thanks Paul for the input and the link. It looks like an interesting read, and I will pour through it over the next little while. I should clarify that I was thinking that plugging the amplifiers into an AC power rheostat rather than the wall with its fixed 120V would allow me to control the amount of voltage seen at the coils through a different means, but perhaps this is still not a good idea. I will leave this thought aside for a while regardless.
You were spot on with the 6SN7's and 6L6's (there is a 5U4G tube as well), but I am hoping to use only essential amp circuitry to power the field coils. I have an excellent 100 Watt head with much more flexibility in gain and tone control, and I would like to use it for the voice coils. It is a Hughes and Kettner Triamp MKII.
I will ultimately have help from someone with plenty of experience for the actual rewiring, and I will report back once some measure of progress is made. I will also start a new thread instead just for simplicity in reading. Thanks again to all for the information provided.
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