field coil power supply

chromenuts

Member
Paid Member
2008-12-27 7:27 am
Connecticut, USA
Hi

I spoke to him about this, as I assume you probably might have if you bought them from him on eBay.

He recommended the “simplest solution” as the best which is using a variac and selenium rectifier as a power supply.

Here is a link he sent me of the circuit that he suggests.

https://cdn01.trixum.de/upload2/193600/193494/5/5c36d3e6857b1334d668f4b05fa99a2d5.jpg

I decided against purchasing his field coils at the time. I cannot give any feedback concerning how the circuit performs.
 
This is the only info he gives you.
Each of them has to be run with a variable power supply of 0 - 120 VDC. 8 ohms voice coils, 1 kohm field coils .
He is very helpful until you have brought them after that he seems to switch off.Wonderful sounding drivers though better then the Voxativ ones i was using.I am wondering if the Tentlab e-choke would work and help smooth out the ripple.
 
If you really wanted to go off the wall then depending on the required drive power you might take a 60W laptop brick and drive a push-pull convertor in step up with a manual tweak of the duty cycle. OK regulate the output current.

Use the field coil as the output inductor with no filter capacitance. I assume it has a rather large inductance on its own so ripple current will be small... Might be a bit of fun with EMI but just a thought.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
Advising you to run 120V around the listening room with inattentive people and pets sounds reckless to me. Field Coil excitation is just I*N, current times turns. Voltage is incidental loss. You can work at low voltage by using fewer turns and requiring more current. That would be *safer*. (In old days, current was costly so we often did low-I high-V field coils.)

I can't see this any way except with transformer isolation. Common Variacs do not isolate. Variac does vary voltage, but is a fairly expensive way to go.

If a 240V:120V 25VA transformer is available in the UK, this can also step your wall voltage down near what these coils need.

Have you done SEARCH? Here and on other forums there are MANY threads on the subject. I have learned many wonderful things. FC speakers can give three denominational sound! (CoE, Catholic, and Jewish?) You have to use a 300B tube to regulate. You have to regulate voltage. No, you must regulate current. Compensate for temperature rise?

I have a feeling a lot of people are fooling themselves (or trying to fool others). There is a range of optimum current for a given winding. Since the winding is a resistance, you can adjust voltage to get your target current. If you run too much current the field coil burns up (which is apparently a big cry). Because of pole saturation there's little gained by pushing the burn-up limit. Too little current and sound is small.

It is an old-school technology and I like old-school solutions. NOT Selenium!! wall:120VAC 25VA transformer (for each speaker). 400V 1A bridge rectifier. C-R-C-R-C filter appears to be plenty clean (cleaner than anything in the 1930s). Then series resistance to adjust the coil excitation. While pass-transistors and switchers are "more modern", there's nothing wrong with a box of hot resistors.

120VAC winding to 40u-150r-40u-150r-40u filter, then series resistor to FC. We won't need a "to zero" range, and probably not even down to 33% voltage, because sound falls off as fast as the field does. Because of filter un-sag we may want 3k series; ah, let's go past 5k to be sure.

A knob-pot would be best. But this wants a reverse-taper 10+W-rated pot which is not a common thing.

Rotary switch is obvious but most available parts are 50V 50mA and we need more than twice that rating. And the inductance of the coil will burn-up small switch contacts if switched "live". Dunno what the best plan is. I might mock-up a banana plug on a lead and a bunch of banana jacks in a child-resistant box.
 

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I am (not favorably) impressed by what I see.

I love FC speakers, in fact I have manufactured one myself in the 80´s, they can give you the highest field available for any structure, since you can fine tune it to *just* saturation point for that particular structure, no more, no less.

And these speakers must probably sound *good*

What I can´t understand is the absolute lack of information about a*crucial* element, such as the power supply.

*REALLY?* "110VAC or 230VAC or 240VAC?"
Which one will it actually be?

IF 240V is fine, that means rectified 353VDC, so a 1k DCR coil will dissipate CRAZY 125W

IF 110V is fine, DC will be 155V , dissipation will be 24W.
Still high for such a speaker, classic Jensen 15LL uses some 18W for a WAY larger speaker, the little 8" will quickly get very hot.

Does manufacturer offer any kind of setup suggestion?
What´s drawn on that sheet does not count as such.

Not forgetting that 2:1 "acceptable" variation means 2:1 field intensity.
If maximum is near saturation, half of it will sound weak mushy by comparison, certainly some detailed explanation must accompany the speakers.

And using LIVE offline power from an outside supply to inside of 2 cabinets, plus needed exposed wiring and even worse, connectors, is reckless.
 
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@richmain, nice to know you compared favourabily against Voxativ. Not much comparisons in the fullrange field, at last in the highend contenders, everything is obscure.
I also have Rullit 8" field coil, super aero series in my case, but I think only difference is cone membrane by yours.
I powered the unit just to check it out (using variac 3A. is minimum requirement for stereo) then selenium AEG/Grundig (also is required big expensive ones), finally a 30uF capacitor.
Sound quality I remember is very sensitive to correct phase, both audio and DC line. To set up in first I powered on in a safely 60volts range and slow increase turning the variac knob till reach certain temperature on the coil, I sorry I can't remember for sure... I thing is mandatory not to surpass 70 C degree (using IR gun). I remember confirmed increasing further up the temperature, sound degrades, and same backwise, if coil is not properly heated.
At this temperature, voltage is 80v. DC.
Sound quality is surprisingly good, never heared any driver sound like this out of enceinte!!!
Sadly I can't further advice on power supply, as I stored the drivers waiting in first to implement an speaker protection for my BTL amps.
 
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paulfk

Member
2008-10-15 3:00 pm
I am (not favorably) impressed by what I see.

I love FC speakers, in fact I have manufactured one myself in the 80´s, they can give you the highest field available for any structure, since you can fine tune it to *just* saturation point for that particular structure, no more, no less.

And these speakers must probably sound *good*

What I can´t understand is the absolute lack of information about a*crucial* element, such as the power supply.

*REALLY?* "110VAC or 230VAC or 240VAC?"
Which one will it actually be?

IF 240V is fine, that means rectified 353VDC, so a 1k DCR coil will dissipate CRAZY 125W

IF 110V is fine, DC will be 155V , dissipation will be 24W.
Still high for such a speaker, classic Jensen 15LL uses some 18W for a WAY larger speaker, the little 8" will quickly get very hot.

Does manufacturer offer any kind of setup suggestion?
What´s drawn on that sheet does not count as such.

Not forgetting that 2:1 "acceptable" variation means 2:1 field intensity.
If maximum is near saturation, half of it will sound weak mushy by comparison, certainly some detailed explanation must accompany the speakers.

And using LIVE offline power from an outside supply to inside of 2 cabinets, plus needed exposed wiring and even worse, connectors, is reckless.
I know very little about this but I assume 110v, 230v or 240v is the mains supply depending on where you live. I thought ac to DC was ac voltage peak divided by square root of 2 hence 1.4 so DC voltage is less than ac and not more as you said.
 
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paulfk

Member
2008-10-15 3:00 pm
I am (not favorably) impressed by what I see.

I love FC speakers, in fact I have manufactured one myself in the 80´s, they can give you the highest field available for any structure, since you can fine tune it to *just* saturation point for that particular structure, no more, no less.

And these speakers must probably sound *good*

What I can´t understand is the absolute lack of information about a*crucial* element, such as the power supply.

*REALLY?* "110VAC or 230VAC or 240VAC?"
Which one will it actually be?

IF 240V is fine, that means rectified 353VDC, so a 1k DCR coil will dissipate CRAZY 125W

IF 110V is fine, DC will be 155V , dissipation will be 24W.
Still high for such a speaker, classic Jensen 15LL uses some 18W for a WAY larger speaker, the little 8" will quickly get very hot.

Does manufacturer offer any kind of setup suggestion?
What´s drawn on that sheet does not count as such.

Not forgetting that 2:1 "acceptable" variation means 2:1 field intensity.
If maximum is near saturation, half of it will sound weak mushy by comparison, certainly some detailed explanation must accompany the speakers.

And using LIVE offline power from an outside supply to inside of 2 cabinets, plus needed exposed wiring and even worse, connectors, is reckless.
I know very little about this but I assume 110v, 230v or 240v is the mains supply depending on where you live.
 
If you really wanted to go off the wall then depending on the required drive power you might take a 60W laptop brick and drive a push-pull convertor in step up with a manual tweak of the duty cycle. OK regulate the output current.

Use the field coil as the output inductor with no filter capacitance. I assume it has a rather large inductance on its own so ripple current will be small... Might be a bit of fun with EMI but just a thought.

I put (3) 36 VDC / 4A Power adapters in series, controlled with a six outlet TrippLite isobar power strip. Crude, but there seems to be enough capacitance in the power adapters and the sound is good, but only 108 V DC. It was supposed to be temporary but it may stay. The power adapters run cool and both speakers draw only .26 amps combined, about what Oleg said they would. For obvious reasons I want to find a better and a variable DC power supply solution.

This seems to be the big problem that I see with field coil drivers. Unless you get the type that can take up to 12 V DC, the power supply is the real issue. It creates the strong magnetic field against which the weaker voice coil magnetic field bounces off of. And the magnetic field of the field coil must be as smooth and noise free as possible. I think that any field coil speaker manufacturer should also sell an extremely good variable DC power supply that are made specifically for the speakers. This increases the cost of course, but that is the way it is. People should not be left scrambling to find a high voltage DC power supply. Oleg, if you are listening, this could be an opportunity for you to also sell a high-quality variable power supply with your speakers. Just suggesting.

I tried a BIO-RAD High Current laboratory grade variable DC power converter which could go in one-volt increments to 250 V DC, and is really intended for electrophoresis DNA experiments. I figured that would be a pure enough DC signal and the flexibility of the voltage increments looked great. I ran it at 120 V DC and the speakers sounded better at the higher voltage. But the unit has a loud fan and was not acceptable for listening. It would’ve been nice if they had designed the unit to be well vented, and temperature sensitive so it would only turn on the fan when needed, but I guess they don’t care about noise in such experiments.