Iron Powder + Resin Output Transformer

[azyx]

What about using iron powder (fiilings) in resin to diy output transformer for single ended 2a3?
Maybe mixing iron powder with copper or some other metal fillings.
The construction of such core seems easy and you could create different shapes and sizes....

[wrenchone]

If it were so easy, everybody would be doing it that way, What you describe will result in a low quality, low permeability, high loss, core not anywhere near as good as commercially available powdered metal magnetic materials, which have insufficient permeability for use in a good SE transformer. The higher permeability powder cores also tend to vary in permeability with applied AC flux, which would make the transformer nonlinear. The permeability of lamination based SE transformers is to a large extent linearized by the air gap used to define the inductance of the transformer and make it capable of supporting the DC bias current necessary for a SE amp.

[seanvn]

I doubt it would be high loss. Likely non-conductive. It is a reasonable idea to put forward. There is no need to get into a transformer rage about it. I say give it a try, you can only learn.

[jlsem]

You doubt wrongly.

John

[smoking-amp]

I tried packing powdered ferrite material (Mu 3000 matl. when in solid core form) into a donut mold once, and the best effective Mu I could get was around 7. (commercial powdered cores are extremely compressed with a hydraulic press to make the metallic powder used actually deform) Since most OTs will sorta work without a core above 1000 Hz, this might get you down to 1000/7 = 143 Hz. But with no primary inductance reserve at all.

The problem with DIY packed powdered material is that adjacent particles touch each other in small points, while air gaps of varying width separate the rest. The small contact points magnetically saturate very easily, while the air gapped portions have variable saturation effects depending on how much air gap there is. This makes for a well known non-linear Mu for loose packed powdered cores. Not so much a problem if you have a large inductance reserve, but for audio LF you won't.

How it sounds might be a different thing though, since SE xfmrs depend on non-linear 2nd harmonic distortion (due to the DC biasing tilting the perm. versus the AC swing) for some of their effect.

[Printer2]

Wonder how many psi you would need to get adequate compression.

[GloBug]

15,000 - 40,000 psi.

[ChrisA]

Originally Posted by azyx

What about using iron powder (fiilings) in resin to diy output transformer for single ended 2a3?
Maybe mixing iron powder with copper or some other metal fillings.
The construction of such core seems easy and you could create different shapes and sizes....

If you must build an OPT with found objects then look for the steel strap used to hold heavy loads to wood pallets. This is a kind is spring steel. Then you roll it around a form and make a toroid core. You can stack them and make a larger size too.

Another way to get cores is to disassemble old microwave ovens but it is much work to strip them to the bare core, cut the copper off with a hacksaw

[Sch3mat1c]

Though if you're going to use steel strap, make sure it's annealed mild steel first. That should be passable up to a few kHz (mostly depending on thickness) and 0.6 to 0.8T. Hardened shipping strap makes poor magnets and much poorer transformers!

Tim

[Printer2]

Originally Posted by GloBug

15,000 - 40,000 psi.

Well I do have a 20 ton press, too lazy to make the mold though.