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Old 28th June 2012, 12:03 PM   #1
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Default Stackin' Chokes

Gents, I am in the process of building a no-holds-barred dac with tube output stage. Currently I am moving to a tabletop prototype to a final built-in-chassis version.

I will be using a dual-mono CLCLC psu for the tube output stage, followed by Salas HV shunts (overkill? maybe yes). As will not surprise you, I am running into space constraints.

Would it be possible to stack 2*2 chokes on top of each other? Meaning 2 chokes on the chassis base place, and 2 mounted on feet/construction above the first 2. Mounted ortagonally of course. What would happen with EMI?

Thinking practically, one could even consider to use the above construction as covers for the chokes, for example made of copper (or better: mu metal)...

What do you think?
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Old 28th June 2012, 12:57 PM   #2
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The choke are iron core, and they usually have a small gap. So magnetically coupling is always low. You donīt need shielding. It will work properly.
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Old 29th June 2012, 06:43 AM   #3
disco is offline disco  Netherlands
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There will be coupling between chokes, depending on the magnitude of the magnetical field. How much will you tolerate for and what will be the influence? You could strain one choke to practical levels and connect an AC meter to the second for testing how much it picks up.
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Old 29th June 2012, 09:22 AM   #4
SY is offline SY  United States
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Quote:
Originally Posted by studiostevus View Post
I will be using a dual-mono CLCLC psu for the tube output stage...

What do you think?
I think you need to be VERY careful in the design. Cascaded LC sections have a way of giving you very poor transient response unless the sections are staggered by a decade or so. This is one place where simulations (especially PSUD2) can be very, very helpful.
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Old 29th June 2012, 10:18 AM   #5
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That's an interesting insight, Sy... I was not aware...

I have used psud2, but i think the effect you describe would not show up on a sim, would it?

Given that i will follow the psu with a salas hv shunt, maybe its better to use CLCRC or CLC...
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Old 29th June 2012, 08:15 PM   #6
disco is offline disco  Netherlands
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He's hinting at the resonance frequencies of the pi filters. Have a read here (dutch).
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Old 30th June 2012, 06:46 AM   #7
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Bing Translation:

www.ham-radio.nl/cursus/wisselstroom/resonantiefrequentie-kwaliteitsfactor/Resonantiefrequentie%20en%20kwali teitsfactor.htm - Translator

Quote:
Originally Posted by disco View Post
He's hinting at the resonance frequencies of the pi filters. Have a read here (dutch).
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Old 30th June 2012, 06:54 PM   #8
PGDO is offline PGDO  Netherlands
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Sy has a good point there, too much capacitance and chokes with a relative high DCR make for a power supply with a high impedance. PS impedance should be low compared to the tube circuit it is supplying. But there are of course a lot of different opinions here. My feeling is that it matters more for the power stage and I have to say that I have no experience with this yet.

The picture shows an low DCR PS design for an SE 300b design. The trick to designing a low DCR PS is
  1. Properly damped. When hit with a current transient it does not ring. Even more it cannot be over damped either
  2. Low output impedance over a broad frequency range.
    In this case impedance is dV/dI -> 2.64V/0.0157A = 168ohm
  3. Very fast recovery time, recovery should be stable within 50msec.
    If the supply does not recover from a transient by the time the next one hits, the B+ will be slightly depressed. Then the next comes along and its even lower. This can severely compromise operation if the recovery time is too long. (Quote from John Swenson)
Simulate an increase or decrease in PS load of about 15%, in the simulation I increased the load by a total of 15.7mA

To achieve these goals you have to use (very) LOW DCR chokes and not too much capacitance and use low ESR capacitors.

The circuit show has actually a bit too much capacitance to qualify as a real low DCR design I think.

I haven’t got the iron yet so unfortunately no real live experience with de design so far.
Attached Images
File Type: jpg Micro-Cap 10.0.8.2 - [Transient Analysis].jpg (546.7 KB, 146 views)

Last edited by PGDO; 30th June 2012 at 07:02 PM.
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Old 1st July 2012, 01:02 AM   #9
djn is offline djn  United States
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I did that and put a piece of mu metal between them....no issues.
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Old 1st July 2012, 05:33 AM   #10
tomchr is offline tomchr  United States
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Quote:
Originally Posted by studiostevus View Post
I have used psud2, but i think the effect you describe would not show up on a sim, would it?
Yes it would. Run a transient simulation where you whack the output with a current pulse. If the supply has issues, it'll ring like crazy. I suspect you'll have to use significant series resistance to dampen it out.

But seriously.... In a DAC output stage you're running a few 10's of mA. Why the cascaded LC? Just because? You'll be better off with a simple regulated power supply. I'm not familiar with the Salas (well, with the name but not the details of the design). I'm sure it's a fine regulator. As is the voltage regulator I designed (see my website).

What matters most is that you have low output impedance and low ripple on the supply.

If you do stack the inductors, I suggest placing the inductors perpendicular to one another. That will minimize the coupling.

~Tom
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