Field coil DC power supply help

[paulf-2007]

Considering buying Rullitt field coil drivers again. I was communicating with Oleg Rullit some time ago whilst in the middle of building tqwt for Lii audio crystal 10 that I absolutely love. So put the idea on the back burner. Now the crystal 10 are playing nicely with the Bakoon 11R I've rekindled my interest in the field coils, it's just on my list of things to try while I'm still kicking. I need a DC power supply for these from 100v to 150v DC that can be set at different outputs as this is how the q value of the driver is adjusted. I have a sketch using selenium rectifiers but it's a bit beyond me and a friend commented that it would hum. As its only powering the magnet and nothing to do with the music side of things I don't understand this but like I said it's beyond my knowledge. Bottom line is, can somebody here build me a power supply.

[disarmamant]

What’s the current rating of the driver coils? I have a couple of toroidal transformers here that might form the basis of a PSU.


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[RobsterD]

Paul Hynes would be your man whether you could persuade him or not I dunno. He designed my Voxativ AC-X field coil power supplies and they're superb as expected
If you can pique his interest you may be lucky .

[paulf-2007]

Sorry guys, haven't had notifications of posts and haven't looked here for a while. I don't know much about the drivers as yet, just arrived, I believe the field coils are 1Kohm, they run around 100v dc. James (jazid) is working on a power supply using multi tap transformers, not decided whether to have different voltages or optimise a voltage to suit the box.

[paulf-2007]

What’s the current rating of the driver coils? I have a couple of toroidal transformers here that might form the basis of a PSU.


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[Barry]

The DC supply must be very well smoothed, otherwise any 50 or 100Hz component will beat with the bass frequencies leading to low level intermodulation distortion.

If the field coil resistance is 1kOhm and is fed with 100V, the power supply needs to provide a clean and constant 100mA current

[paulf-2007]

The DC supply must be very well smoothed, otherwise any 50 or 100Hz component will beat with the bass frequencies leading to low level intermodulation distortion.

If the field coil resistance is 1kOhm and is fed with 100V, the power supply needs to provide a clean and constant 100mA current
I'm guessing at this stage but I've heard 300mA possibly 400mA is needed so the field coil resistance may be incorrect. I'll measure them when I get a minute.

[disarmamant]

I'm guessing at this stage but I've heard 300mA possibly 400mA is needed so the field coil resistance may be incorrect. I'll measure them when I get a minute.

It's imperative to know exactly the DC resistance of the field coil.

The current that the power supply needs to source will be directly proportional to the supply voltage at the coil. I think that the easiest way is with an adjustable voltage regulated DC supply. Without any voltage regulation setting an accurate supply voltage won't be easy simply by having a multi tapped secondary on the transformer. It's possible to do, but needs careful design of the transformer as the current wll rise with the voltage.

If you adopt the unregulated approach it may be better to use a variac to supply the primary of a transformer with a single secondary voltage. Still not ideal, but a good deal easier IMO.

[Jazid]

I think I have that end covered Martin :)
Start with a static DC resistance measure
Hook it up to a regulated supply to measure current to check all is accurate and closely monitor coil temperature. Internet hersay suggests regulated supplies sound a bit crap though...
Provide a simple low ripple linear supply with multi-tap trafo (I have these anyway) and variac for fine tweakery if neccessary - but I slightly doubt that will be necessary.
According to my cogitations when music is playing there will be a magnetic field from the voicecoil 'reflected' as back EMF into the field coil and thus to the PSU. I suspect that the ability of the PSU to drain that is responsible for the difference in SQ between different supplies (as reported - I have no personal experience of field coil drivers). This is the thrust of my experimentation, I have no idea how much damping (if that is the right word) is ideal. Looking forward! Any thoughts on the above?

[disarmamant]

If you already have a sutable multi tapped transformer it makes sense to try a simple low ripple DC supply.

I did think about using choke regulation with a CLCRC topology.

The output voltage could be set at different levels by adjusting the value of the first input capacitor in the CLC Pi network. I actually started modelling such a power supply around a 230V primary, 0-55 + 0-55 volt 1 amp secondaries toroid I have here using the Duncan Amps PSU designer, but using a 14H choke with a 97ohm DCR I could not get above 135volts with a 260mA load.

A good regulated supply could be made using a Neurochrome 21st century Maida regulator, up to 800mA load current, with an output impedance of 0.06 ohms at 1Khz 400volts. It should be pretty good at supplying a very clean ripple free supply (20uV noise and hum level) and with the low output impedance sinking any back EMF from the field coil. Needs a sizeable heatsink for the output Mosfet though.

https://neurochrome.com/products/21st-century-maida-regulator

I've no experience with field coil drivers either, other than with the field coil speaker I have in an old WWII civilian utility radio I have here!

[Jazid]

And does that field coil hummmmm?

Still not convinced field coils will sound their best heavily damped or under-damped, but what do I know [emoji16]


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[Jazid]

I modeled an RCRC and with available bits it had under 1mV ripple at 300mA. I presumed this would be adequate as a starting point, the speakers aren't that sensitive (if memory serves me - unlikely)

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[disarmamant]

And does that field coil hummmmm?


Well there is some hum, but this is a wartime radio probably built some 75 or more years ago. And how would one tell if the hum was from the field coil?


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[Jazid]

Now that is a very good question!

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[Barry]

Well there is some hum, but this is a wartime radio probably built some 75 or more years ago. And how would one tell if the hum was from the field coil?


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Disconnect the connections to the driver coil?

[paulf-2007]

Disconnect the connections to the driver coil?
Depends where the hum can be heard coming from. If it comes from the cone then disconnecting the voice coil will kill it anyway, switch off the field coil and see if that kills it,

[Jazid]

I presume if it is ripple in the field coil circuit you might leave that energised but short out the voicecoil? That would show any hum, but some precautions necessary in some equipment of course!

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[Barry]

I presume if it is ripple in the field coil circuit you might leave that energised but short out the voicecoil? That would show any hum, but some precautions necessary in some equipment of course!

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Agreed - disconnect the audio connections to the driver coil and terminate them with, say, 10 Ohms (assuming the radio is valved and uses an output transformer). Then short out the speaker driver coil, or terminate it with, say, 1 Ohm.

[bumpy]

The DC supply must be very well smoothed, otherwise any 50 or 100Hz component will beat with the bass frequencies leading to low level intermodulation distortion.


I agree with this. A valve based power supply just sounds better than solid state (to me) but has associated hum issues. This hum can be sat on at the expense of overall sound quality. This is why I have not progressed down this route.

[Jazid]

For others coming down this route I have a suggestion. The PSU is a doddle, hum is not a major issue in anything working adequately on PSUdesigner.

I have found the impedance of the power supply to be rather more important, if you use 1-3 ohm impedance the sound falls into place, this with the silver Aero 8 only since I have tried no other models.

Sounds rather fine actually [emoji16]

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[walpurgis]

I may be talking nonsense, but is there not a dedicated PSU for these speakers? That would seem to be the way to go.

[Jazid]

Seems not, the manufacturer is a little eccentric and does things his own way, didn't even come with an upper voltage limit.

I measured coil temperature and worked accordingly to keep it under 60C. With Paul's speakers that's South of 90V, I run them at about 70V. TBH the voltage is subjectively to me about a quarter as significant to the sound as the impedance and I keep the volts down as they aren't mine!

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[bumpy]

The PSU is a doddle, hum is not a major issue in anything working adequately on PSUdesigner.


Just a word of caution for those listening near field. Hum can be very pervasive and annoying.

[Barry]

Just a word of caution for those listening near field. Hum can be very pervasive and annoying.

Then a pretty lousy power supply is being used. A half decent power supply ought to have a minimal and inaudible hum level.

[Jazid]

Just a word of caution for those listening near field. Hum can be very pervasive and annoying.I work at approx 5cm with ears to the coils. No hum at all. Approx 5uV on the analyser. Thanks for the advice.

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[bumpy]

Field coil driver power supplies are rarely debated or reviewed.

You will obviously rail against this, but the consensus from things I have read and listened to is that valve power supplies sound better than transistor and that hum can be eradicated easily at the expense of sitting on sound quality. Perhaps when you have tried both you can help us out with your opinions.

[Jazid]

Field coil driver power supplies are rarely debated or reviewed.

You will obviously rail against this, but the consensus from things I have read and listened to is that valve power supplies sound better than transistor and that hum can be eradicated easily at the expense of sitting on sound quality. Perhaps when you have tried both you can help us out with your opinions.

Hi Chris,

I have tried both. I've tried a regualted valve PSU, a regulated transistor PSU, the one I am making for Paul, and a couple of similar variants. I cannot see the need for a valve rectified stage or those super old school selenium rectifiers. I also abhore hum but see no need for it, as Barry pointed out - it's just a badly designed supply if there's any hum. The current drawn by the field coils barely varies when playing music, far far less than in any SET for instance let alone a class B amp. Back EMF is lost in the final capacitor, so I can't see how a valve two capacitors and resistors further back in the supply would change the sound since its characteristic differences from silicon just aren't being exposed? And it's not that I like silicon, anyone who knows me knows that I'm a died in the wool valve guy.

The PSU I have made has had the trafo snubbered and optimally damped so there is diddly squat ringing from the diodes, and nothing that is getting through for sure. What does make a difference is the resistance of the final stage, howsoever achieved. As a result I imagine that the smoothing caps chosen in a valve reg PSU would likely make a significant difference: NOS or old electrolytics for example usually have far more ESR than the modern equivalents which I have used. That's why I went down the impedance rabbit hole to see if that made a discernable difference, and it seems that it does. Since capacitor ESR is a function of frequency there may be more to experiment with for sure, but for me the journey is done. The PSU is finished and encased, and being soak tested this weekend (:harp::guitar::violin:). Paul's speakers are playing very nicely with 81V on the coils and 1 ohm impedance in series with the each side's final reservoir caps, and presently to migrate to their rightful owner sadly...

[Jazid]

Edit:
If anyone out there believes open baffle speakers don't do deep bass, they owe themselves a listen to these things...

[bumpy]

Hi Chris,

I have tried both. I've tried a regualted valve PSU, a regulated transistor PSU, the one I am making for Paul, and a couple of similar variants. I cannot see the need for a valve rectified stage or those super old school selenium rectifiers. I also abhore hum but see no need for it, as Barry pointed out - it's just a badly designed supply if there's any hum. The current drawn by the field coils barely varies when playing music, far far less than in any SET for instance let alone a class B amp. Back EMF is lost in the final capacitor, so I can't see how a valve two capacitors and resistors further back in the supply would change the sound since its characteristic differences from silicon just aren't being exposed? And it's not that I like silicon, anyone who knows me knows that I'm a died in the wool valve guy.

The PSU I have made has had the trafo snubbered and optimally damped so there is diddly squat ringing from the diodes, and nothing that is getting through for sure. What does make a difference is the resistance of the final stage, howsoever achieved. As a result I imagine that the smoothing caps chosen in a valve reg PSU would likely make a significant difference: NOS or old electrolytics for example usually have far more ESR than the modern equivalents which I have used. That's why I went down the impedance rabbit hole to see if that made a discernable difference, and it seems that it does. Since capacitor ESR is a function of frequency there may be more to experiment with for sure, but for me the journey is done. The PSU is finished and encased, and being soak tested this weekend (:harp::guitar::violin:). Paul's speakers are playing very nicely with 81V on the coils and 1 ohm impedance in series with the each side's final reservoir caps, and presently to migrate to their rightful owner sadly...

James, you seem to have cracked it, well done. Although I love the sound quality and the ability tweak them, I have always steered clear because of hum. I listen near field which compounds the problem. Can you offer some ballpark figure for build and supply of a pair. Thanks Chris

[Jazid]

HI Chris,
tried to PM you but your inbox is full...

[paulf-2007]

HI Chris,
tried to PM you but your inbox is full...
James I emailed you yesterday about paying for the psu and to say you are welcome to keep longer if you wish. I'm in no hurry to get them back. Just to say I still have your Colin wonfir amp here to return and if/when I do come up I'll bring my Bakoon for you to hear.

[Jazid]

Can't wait to hear your amp Paul! Got a small revision to your PSU to make. Parts on order, nothing major, just an afterthought, ready maybe Wednesday.

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[bumpy]

HI Chris,
tried to PM you but your inbox is full...

Thanks. Hopefully have cleared it a bit now :)

[paulf-2007]

Can't wait to hear your amp Paul! Got a small revision to your PSU to make. Parts on order, nothing major, just an afterthought, ready maybe Wednesday.

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Ok James, let me know when you want me to come up, as I said you are welcome to keep them for a while

[paulf-2007]

I have had the field coils playing in my listening room for a few weeks now, no longer in the open boxes but in my quarter waves. So little room and close to the back and side walls. For those that were talking about hum, well there is none at all, dead silent. They sound great, better top end than the Lii audio crystal 10 and maybe more realistic bass, what I mean is, with many speakers they are designed to give more bass than is realistic for people's appetite for bass. These don't just do bass all the time. Boris blank etc lots of deep bass that is associated with this kind of music and more subtle detailed bass on most everything else. I have got around to trying different voltages or output impedance yet, will get around to that once I've got familiar with them.

[paulf-2007]

Well it's been nice but due to no interest I'm out of here see ya

[Metlor]

Hi Paul, thanks for the thread, I'm sure it's been interesting for many. They're certainly a subject I'm unfamiliar with, but having heard some of the Lii Audio range I expect you're listening to music with quite a wide grin on your face.
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Well done to James too, with the power supply.

[Jazid]

Hi Paul, thanks for the thread, I'm sure it's been interesting for many. They're certainly a subject I'm unfamiliar with, but having heard some of the Lii Audio range I expect you're listening to music with quite a wide grin on your face.
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Well done to James too, with the power supply.Thanks for the kind words, but TBH given the august standards on this forum, I am nothing more than an amateur dabbler, albeit a happy one!
The thread runs slowly, which is as I would expect. Pretty niche stuff here. I suspect Paul will still be watching [emoji16]

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[paulf-2007]

Thanks for the kind words, but TBH given the august standards on this forum, I am nothing more than an amateur dabbler, albeit a happy one!
The thread runs slowly, which is as I would expect. Pretty niche stuff here. I suspect Paul will still be watching [emoji16]

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