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inductors near case wall
inductors near case wall
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Old 23rd November 2017, 09:47 PM   #1
robinlawrie is offline robinlawrie  United Kingdom
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Default inductors near case wall

im fairly locked into my design now (components bought, bits cut etc..)

however ive just realised there may be an unforseen issue.

two of the output inductors on my (class d) amp boards will be within a couple of mm of the aluminium wall of the case (actually the heatsink)

i just had a realisation that the proximity to the metal case *might* alter the behaviour of the inductors (coilcraft 10uh,6 amp iron cores)


is this the case? something to worry about? anything i can do to mitigate issues?
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Old 23rd November 2017, 10:10 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by robinlawrie View Post
i just had a realisation that the proximity to the metal case *might*
alter the behaviour of the inductors
Aluminum should be ok, but not regular steel.
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Old 23rd November 2017, 10:23 PM   #3
robinlawrie is offline robinlawrie  United Kingdom
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ok thanks for the reassurance. if i do experience issues, i have a possibility to either move the inductors or add "legs" to them and mount them horizontally, but it would be a job id rather not do.

would you be able to advise what to listen out for? i assume it would change the filter slope and make the amp either more peaky at higher frequencies, or less..

on a related issue, it was necessary to rotate two other output inductors a bit to make room for a capacitor mod to the boards. they were originally parallel, vertical and a bit less than a coil width apart, now they are within less than mm of touching at one side, and 15mm apart at the other.

so: they are closer to 90 degrees offset than before, which ive read is good, but
they practically touch at one side, which im sure is bad.

again i could move one if absolutely necessary, but ive no idea if any negative effects are going to be too small to bother with, or really problematic.

in this case i imagine the issue would be one channel affecting the other via inductive coupling..?
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Old 23rd November 2017, 10:27 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by robinlawrie View Post
they were originally parallel, vertical and a bit less than a coil width apart, now they are
within less than mm of touching at one side, and 15mm apart at the other.
The iron cores will minimize interaction through the air.
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Old 23rd November 2017, 10:29 PM   #5
robinlawrie is offline robinlawrie  United Kingdom
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ok cool. i was really hoping to hear something like that. many thanks.
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Old 24th November 2017, 10:23 AM   #6
DF96 is offline DF96  England
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The aluminium may slightly reduce inductance and slightly increase HF loss. Unlikely to be a problem.
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Old 2nd December 2017, 09:14 PM   #7
robinlawrie is offline robinlawrie  United Kingdom
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Ok, another issue. If i move one output filter inductor by extending the legs 2cm, will it play havoc with btl output on a class d amp? I imagine it would imbalance the two amp "halfs" somewhat, but since the inductors only have 10% tolerances (generically, im not sure of these, cant find a tolerance rating on the coilcraft website) maybe its fine.?

How could i test this? I dont have a scope, only got a multimeter and a calibrated usb mic.

Last edited by robinlawrie; 2nd December 2017 at 09:16 PM.
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Old 6th December 2017, 10:34 AM   #8
DF96 is offline DF96  England
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The inductors are there to filter out the switching frequency and its harmonics. They should have little effect at audio frequencies.
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Old 6th December 2017, 10:45 AM   #9
robinlawrie is offline robinlawrie  United Kingdom
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Quote:
Originally Posted by DF96 View Post
The inductors are there to filter out the switching frequency and its harmonics. They should have little effect at audio frequencies.
that sounds reassuring.. which is good because ive done the mod.. not tried it yet, the psu is in pieces. i used a calculator which suggested the inductance of the extra 4cm of wire would add 0.3% to the inductance of the coil. seems a trifling amount, given usual tolerances... i was just concerned that maybe *any* imbalance between the two sides of a btl configured amp could cause problems.

anyway thanks for the guidance.
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