The right kind of inductor!

[oon_the_kid]

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

 

[nigelwright7557]

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.

 

[oon_the_kid]

The conductor is used for the amp itself. Similar to the way it would have been used in the MoFo.

Oon

 

[bwaslo]

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.

 

[oon_the_kid]

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

 

[6L6]

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

 

[vdi_nenna]

If you can find a large roll of heavy gauge enameled wire, it could work.
See the L'amp articles for an example.

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 part deux.pdf

 

[oon_the_kid]

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

 

[tomchr]

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

 

[Michael Rothacher]

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.

 

[sippy]

UK prices:
193T - £34
193V - £71

 

[Soundhappy]

France Mouser prices are :
193T cost 37.95 € ~ 46.82 US$
193T Hammond Manufacturing | Mouser France
193V cost 79.29 € ~ 97.82 US$
193V Hammond Manufacturing | Mouser France
I build Big 20 watts MoFo and imo cost & Diy project ratio is excellent.
Very affordable if compare with more fancy amplifiers.
WAF can be good as well. Kind regards

 

[oon_the_kid]

The reason I am asking this topic is, the inductor I used as an example is only USD $4.

So it makes me wonder if other inductors can be used. After all Mouser stocks 20,000 inductors.

I have used a smaller inductor for making a tweeter amp. 15mH by bourns.

Another question that puzzles me is, we know the inductor typically has to be between 50-100 mH. But if a smaller one is used, which would degrade, the gain bandwidth or the power bandwidth or just an increase in distortion?

Oon

 

[6L6]

For the application you are asking about the inductor needs to;

1) have some reasonable inductance for the job, say 25-50mH

2) be able to withstand the DC current that the circuit requires. If it's too small it will saturate and lose most of it's inductance

The DC aspect is what will make it physically large and therefore expensive.

 

[oon_the_kid]

I used this once on an amp. I worked quite well.

1140-153K-RC J.W. Miller | Mouser Singapore

I didn't measure freq response etc. So I suspect it doesn't go all the way to 20Hz. I was only hoping for a hundred hertz. And the power was much lower.

Looking at the data, one thing that indicated that it was suitable was it listed Isat which is twice of that of I rms.

I think this one is also suitable.

C-56U Triad Magnetics | Mouser Singapore

From what I gathered so far, this seems to be what would make it suitable.

Air core. PE has aircore inductor up to 16mH.

DC choke.

Any other ideas?

Oon

 

[oon_the_kid]

I had some thoughts. What if I used an AC choke. But kept the current low, say half of the max current. That would ensure the core does not saturate.

Say for example, we use a inductor designed for 4A, if we use about 2A DC.

Oon

 

[Michael Rothacher]

The C-56U borders on adequate, especially if you don't need the bandwidth.

 

[oon_the_kid]

The C-56U borders on adequate, especially if you don't need the bandwidth.

Hey Michael,

I think you might one of those most experienced in this field. Is it the frequency response bandwidth or the power bandwidth is affected if the inductor is too small?

I am going to give this one a try..

PA4415NL Pulse Electronics | Mouser Singapore

The DC current I am going to use 1-2A is much lower than the peak currentof 5A. Will see it works out..

Oon

 

[Zen Mod]

that's AC inductor

you'll have bloody nuttin' in speakers

 

[6L6]

Oon -

An externally hosted image should be here but it was not working when we last tested it.

Wrong tool for the job.

Why the reluctance to use the proper parts? Budget constraints or trying to use a common part in an uncommon way?