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Thread: Field coil DC power supply help

  1. #1
    Join Date: Apr 2016

    Location: Gravesend and France

    Posts: 1,419
    I'm paul.

    Default Field coil DC power supply help

    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.
    Bakoon 11R Denon DP80 Stax UA-70 Shure Ultra 500 in a Martin Bastin body with jico stylus, project ds2 digital Lii audio crystal 10 tqwt speakers

    Office system, DIY CSS fullrange speakers with aurum cantus G2 ribbons yulong dac Sony STR6055 receiver Jvc QL-A51 direct drive turntablesub. JVC Z4S cart is in the house

    Garage system another Sony receiver, cassette deck


    System components are subject to change without warning and at the discretion of the owner.

  2. #2
    Join Date: Jan 2014

    Location: Northants

    Posts: 244
    I'm Martin.

    Default Field coil DC power supply help

    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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  3. #3
    Join Date: May 2009

    Location: Southampton

    Posts: 375
    I'm Rob.

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    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 .

  4. #4
    Join Date: Apr 2016

    Location: Gravesend and France

    Posts: 1,419
    I'm paul.

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    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.
    Bakoon 11R Denon DP80 Stax UA-70 Shure Ultra 500 in a Martin Bastin body with jico stylus, project ds2 digital Lii audio crystal 10 tqwt speakers

    Office system, DIY CSS fullrange speakers with aurum cantus G2 ribbons yulong dac Sony STR6055 receiver Jvc QL-A51 direct drive turntablesub. JVC Z4S cart is in the house

    Garage system another Sony receiver, cassette deck


    System components are subject to change without warning and at the discretion of the owner.

  5. #5
    Join Date: Apr 2016

    Location: Gravesend and France

    Posts: 1,419
    I'm paul.

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    Quote Originally Posted by disarmamant View Post
    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.


    Sent from my iPad using Tapatalk
    400milliamp I believe
    Bakoon 11R Denon DP80 Stax UA-70 Shure Ultra 500 in a Martin Bastin body with jico stylus, project ds2 digital Lii audio crystal 10 tqwt speakers

    Office system, DIY CSS fullrange speakers with aurum cantus G2 ribbons yulong dac Sony STR6055 receiver Jvc QL-A51 direct drive turntablesub. JVC Z4S cart is in the house

    Garage system another Sony receiver, cassette deck


    System components are subject to change without warning and at the discretion of the owner.

  6. #6
    Join Date: Jan 2009

    Location: Essex

    Posts: 22,864
    I'm openingabottleofwine.

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    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
    Barry

  7. #7
    Join Date: Apr 2016

    Location: Gravesend and France

    Posts: 1,419
    I'm paul.

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    Quote Originally Posted by Barry View Post
    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.
    Bakoon 11R Denon DP80 Stax UA-70 Shure Ultra 500 in a Martin Bastin body with jico stylus, project ds2 digital Lii audio crystal 10 tqwt speakers

    Office system, DIY CSS fullrange speakers with aurum cantus G2 ribbons yulong dac Sony STR6055 receiver Jvc QL-A51 direct drive turntablesub. JVC Z4S cart is in the house

    Garage system another Sony receiver, cassette deck


    System components are subject to change without warning and at the discretion of the owner.

  8. #8
    Join Date: Jan 2014

    Location: Northants

    Posts: 244
    I'm Martin.

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    Quote Originally Posted by paulf-2007 View Post
    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.
    Last edited by disarmamant; 23-02-2020 at 16:56.

  9. #9
    Join Date: Apr 2012

    Location: London

    Posts: 651
    I'm James.

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    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?

  10. #10
    Join Date: Jan 2014

    Location: Northants

    Posts: 244
    I'm Martin.

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    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/21s...aida-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!
    Last edited by disarmamant; 24-02-2020 at 12:37. Reason: typo

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