Field coil driver for OB design advice sought!

shooter

Member
2003-08-16 10:28 pm
usa
I just bought Rullit lab8 Field coil drivers, the best sounding speakers I have heard. they are still breaking in so 100 or 200 more hours to go, but already the low end is impressive, especially in a quasi-open baffle. My power supply now is marginal but I am going to try to build a full wave bridge rectifier with selenium “rectifiers“, which I believe are simply diodes.

I live near Las Vegas and it took a month and a half to get them from Nuremberg Germany. I was getting frustrated but it was worth the wait.

According to Oleg (Rullit) I can use a veriac transformer combined with selenium rectifiers and it should sound great. I would think that a big power capacitor on that and then a DC voltage meter to show what voltage is really coming out would be a good idea.

Glad you are enjoying your Rullit, bought my first pair about 10 years ago still loving them. BTW I have lots of family Members living in Las Vegas.
 
i also just bought silver lab 8, have to collect from our office tomorrow as didn't get back in time on Friday. I would be interested in your power supply, a friend is going to build mine, probably valve rectified, we haven't got started yet. Just to add, Oleg sent the drivers they took a week to get here in uk from Germany

Valve rectified sounds like it should be very good if done properly.
 

paulfk

Member
2008-10-15 3:00 pm
Hi
I have used 4 x EY500 in bridge configuration with LCRC and a final LC for every speaker. I have also a few brand new AEG seleinium bridge rectifier tested, Its the same level of sound quality. If If someone needs selenium bridge rectifiers I have brand new AEG 200V 300mA for sale
i would very much like to see a schematic for power supply with EY500. Is the tube maniacs your blog?
 
Gary,
a transformer with selenium rectifier and capacitors only will hum extensive. Its better to use RC or better LC filter chains

I was thinking that, and that’s why am thinking that Oleg (or whoever) should sell perfectly quiet variable DC power supply(s). The every day person who isn’t an amplifier builder shouldn’t be scrambling to figure out a power supply for these potentially wonderful FC speakers.
 
Gary

some people like this very much it produce hum, yes but it sounds also very open and natural. I think most important is to use low value capacitors an a absolute minimum of filtering.I think I can do also my supply much better. The power supply of field coil is more important than every power supply in the system
 

paulfk

Member
2008-10-15 3:00 pm
Gary

some people like this very much it produce hum, yes but it sounds also very open and natural. I think most important is to use low value capacitors an a absolute minimum of filtering.I think I can do also my supply much better. The power supply of field coil is more important than every power supply in the system
Produce hum through the speakers or just from the power supply?
 
Gary

some people like this very much it produce hum, yes but it sounds also very open and natural. I think most important is to use low value capacitors an a absolute minimum of filtering.I think I can do also my supply much better. The power supply of field coil is more important than every power supply in the system

If it creates hum in the output sound, I could not stand that. I even have a timer on my fridge to turn it off when listening, and then back on later, to get rid of the hum of the refrigerator.

And I can believe that the power supply of the FCs is most critical, since it is the speakers that create the sound and you can hear the most difference.

I thought the variac / selenium rectifier design used 32 mF (millifarad) [ or 32,000 uF (microfarad) ] caps. That is a lot of capacitance. Or do you mean 32 uF caps? I can understand that in general, less can be better.

I thought the DC powering the field coil should be perfectly smooth and noise-free. Are we wrong about that?

If the DC powering a field coil is not clean, and has hum, won't that cause the field coil magnetic field to "hum", and not be consistent? If the FC magnetic field is not smooth and consistent, how might that fluctuating big magnetic field affect the smaller magnetic field from the voice coil, and how the speaker vibrates, in relation to the original signal?

It would be nice to be clear on all of this before proceeding, and certainly Oleg and others like him seem like they should have a better take on all of this.
 
Produce hum through the speakers or just from the power supply?

Just guessing, but imagine a regular magnet speaker. It seems like the large magnet magnetic field would be perfectly smooth and act like a "perfect" trampoline to bounce the much smaller voice coil magnetic field off of, and then if there is any hum in the moving cone, that would be from the amp/voice coil signal only.

However, if there is also hum in the much larger FC magnetic field, wouldn't that add to the hum that you hear from the moving cone? I would think so, unless there is some way for hum to be canceled out? Is that what the old humbucker coils were for?

However it works, we want the movement of the cone to reflect only variations in the voice coil field. The field coil field should not affect the cone movement in any bad way. Does that sound about right?

So what is the answer?
 
I'm having the impression these statements are just presumptions since I recently remade the electromagnet coil of a vintage field coil driver and I had similar thought in my head... they all ended when I turned the unit on with a 12 volt power supply, the dirt cheap kind, and there was no humming, in fact my reaction was - knowing that the magnetic field has inertia, so it does not die off immediately even if you cut the power off - that it was highly unlikely... so I don't know what king of variation of current/voltage induces a hum in field coil drivers; my unit had a humbucking coil but this was serving some kind of problems of a different nature with radios from the 30s I think it was. I simply told the humbucking coil "happy retirement" after all these years, and that was the end of the thing.
By the way - forget about power supplies - just use battery for the DC you need.
 
Well... why do you think I replaced the original coil of 1470 ohms with a 15.1 ohms? The field coil need 10-30 watts of power (source: Hentai user who made a few of these from scratch, see ProjectRyu thread).. .after you make the calculations you realize you need 1-2 amps at 12 volts... that's battery territory. The magnetic strenght is currents x turns in the coil... so making a coil with less turns with thicker wire means you can get same amgnet strenght with higher amps but also you need a lower voltage at the power source.
Hentai did not use any humbucking coils.