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The right kind of inductor!
The right kind of inductor!
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Old 13th April 2018, 04:00 AM   #1
oon_the_kid is offline oon_the_kid  Malaysia
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Default The right kind of inductor!

Hi Everyone,

I know what a basic inductor does but as things get more complicated, I am getting clueless.

I was wondering by looking at the specs, how do you know if the inductor is suitable for our usual single example class A work, example MOFO.

For example, would this inductor be suitable?

RN152-1-02-68M Schaffner | Mouser Singapore

It is listed as 68MH common mode. Assuming our current does not exceed it specifications, is this a suitable choke to use?

I know for transformers it is clear that some transformers are designed for DC to pass (As in SET types) and some are not. Does this apply to chokes?

Oon
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Old 13th April 2018, 04:59 AM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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Are you using the inductor in the power supply or the amp output ?

Copper wire is just coper wire, within reason so its the inductor core that makes it for any application.
I remember many years ago building a class d amp and just popped in an inductor off the shelf. It reached 120 degree C !
I had used the wrong core material inductor.
I changed it for a t106-2 iron powder core and it worked great and barely got warm.
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Old 13th April 2018, 06:07 AM   #3
oon_the_kid is offline oon_the_kid  Malaysia
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The conductor is used for the amp itself. Similar to the way it would have been used in the MoFo.

Oon
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Old 13th April 2018, 06:19 AM   #4
bwaslo is offline bwaslo  United States
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The right kind of inductor!
A common mode transformer might not be what you think it is. It's designed so that an AC power signal flowing through the transformer to the other side will get stripped of a any AC voltage signal the two leads have in common. It will tolerate very little DC on it's windings before it saturates. It is intended for AC, not a DC bias current.
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Old 13th April 2018, 07:46 AM   #5
oon_the_kid is offline oon_the_kid  Malaysia
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Thank you, I kinda suspected that it will not be usable....

So what must it say as description before it is considered OK...

DC -Choke?

Oon
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Old 13th April 2018, 02:14 PM   #6
6L6 is offline 6L6  United States
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The right kind of inductor!
Oon,

The inductor in the MoFo is a fairly rare part in this day and age - it's a power supply coke made for a linear PSU that was expecting to have a tube(valve) rectifier. In the early days of audio capacitors were quite expensive compared to inductors, people used big chokes, they were easy to get, but as always, time marches on, technology and manufacturing changes.

So this means that a modern catalog of components a huge single inductor will be fairly rare. For modern manufacturing they are much too expensive to build, and more importantly, they are extremely heavy. Plus, in a PSU, they can be replaced with a transistor or two and a few resistors.

SO. You are building a MoFo. Reading the article around page 10-11 (download here if you don't have it Build This MoFo! | AudioMaker ) you see that the inductor for the 19v supply needs to be able to handle 2 Amps of DC, and have a DC resistance of less than an ohm. That's going to be a big coil on a big lamination stack. Anything that has ratings that meet that spec will be a tube amp PSU choke.

You asked for specific ratings, that's simple, look a the specs of the Hammond 193T and you've got the answer;

Inductance 50mH (quite a few turn of wire)
Current 2A (big iron EI core)
DC resistance 0.7ohm (fairly thick wire, and because there needs to be many turns, the entire choke needs to be larger)

If you have a local transformer/choke winder they can most likely make something very similar that will work beautifully.

Or, just order the one the amplifier was designed around - the entire point of the Mofo is the inductor, it's the party piece, and the one that makes all the beauty in the sound.

193T Hammond Manufacturing | Mouser Malaysia

Last edited by 6L6; 13th April 2018 at 02:20 PM.
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Old 13th April 2018, 03:50 PM   #7
vdi_nenna is offline vdi_nenna  United States
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If you can find a large roll of heavy gauge enameled wire, it could work.
See the L'amp articles for an example.

Quote:
The magnet wire I used is made by Essex. Each spool is wound with approximately 1,400 feet of heavy-insulation 16 gauge with a resistance of approximately .5 ohms per pound.
http://blog.audiomaker.tech/pdf/lamp%20part%20deux.pdf
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Old 14th April 2018, 01:03 AM   #8
oon_the_kid is offline oon_the_kid  Malaysia
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Quote:
Originally Posted by 6L6 View Post
Oon,

The inductor in the MoFo is a fairly rare part in this day and age - it's a power supply coke made for a linear PSU that was expecting to have a tube(valve) rectifier. In the early days of audio capacitors were quite expensive compared to inductors, people used big chokes, they were easy to get, but as always, time marches on, technology and manufacturing changes.

So this means that a modern catalog of components a huge single inductor will be fairly rare. For modern manufacturing they are much too expensive to build, and more importantly, they are extremely heavy. Plus, in a PSU, they can be replaced with a transistor or two and a few resistors.

SO. You are building a MoFo. Reading the article around page 10-11 (download here if you don't have it Build This MoFo! | AudioMaker ) you see that the inductor for the 19v supply needs to be able to handle 2 Amps of DC, and have a DC resistance of less than an ohm. That's going to be a big coil on a big lamination stack. Anything that has ratings that meet that spec will be a tube amp PSU choke.

You asked for specific ratings, that's simple, look a the specs of the Hammond 193T and you've got the answer;

Inductance 50mH (quite a few turn of wire)
Current 2A (big iron EI core)
DC resistance 0.7ohm (fairly thick wire, and because there needs to be many turns, the entire choke needs to be larger)

If you have a local transformer/choke winder they can most likely make something very similar that will work beautifully.

Or, just order the one the amplifier was designed around - the entire point of the Mofo is the inductor, it's the party piece, and the one that makes all the beauty in the sound.

193T Hammond Manufacturing | Mouser Malaysia
Thanks, I have ordered and used them before for my amps. Have built quite a few based on them. Just looking out for cheaper alternatives. They are generally not cheap...

If one were to compare a transformer of the same size, they are a lot more.

Thanks anyway.

Oon
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Old 14th April 2018, 01:31 AM   #9
tomchr is offline tomchr  Canada
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Yeah... Inductors with any significant inductance that can handle amps of current are neither light nor cheap. There's much greater demand for transformers, which is why they're relatively inexpensive compared to inductors.

Tom
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Old 14th April 2018, 12:43 PM   #10
Michael Rothacher is offline Michael Rothacher  United States
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The right kind of inductor!
They're $35 in the US. How much are they elsewhere in the world?

And, if they are significantly more, does anyone know why? Maybe I should start an export business.
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