winding toroidal trafos

[tmbg]

Anyone have any experience just up and making their own toroidal transformers? I used to do a bit of dabbling with radio circuits, and it was very common to wind simple transformers on toroid cores, like 50 turns primary and a couple of turns secondary... I wonder what sort of audio response you could get out of using a biggish ferrite core and winding your own input or output trafo...

 

[PRR]

Toroid or ferrite?

Iron toroids work fine for audio if you avoid DC.

Classic ferrites have about 5% of the inductance of iron, so bass response is a Big Problem. Improved ferrites exist and I have seen them used, but I don't see what the advantage is.

 

[tmbg]

hm well, I think I have both iron and ferrite cores in toroidal form... I'll have to reexamine my stock. Most of them are pretty tiny.

as for avoiding DC, for an output trafo this seems easy to me, just use a decoupling cap before it. For an input trafo, would a microphone ever put out DC? I guess it wouldnt really hurt to stick a decoupling cap before that too...

or wait a sec, L and C in series is a resonant bandpass, isnt it :/ haha

I willfully make a fool of myself to allow the wisdom of the masters to rain upon me

 

[PRR]

> I have both iron and ferrite cores in toroidal form

Sorry, I forgot that there is a powder-core called "iron". It is small bits of iron in a non-conducting binder. This is "not as good" as solid strips of iron when you want audio frequencies.

And yet there are some pretty exotic cores in audio today. I doubt they are radio-cores, they are probably bleeding-edge new-product.

> wait a sec, L and C in series is a resonant bandpass, isnt it :/ haha

Well, sure. But you always have resistance too. If all the resistances and reactances are similar at resonance, the Q is about 1 and the response is flat to a point and then falls 12dB/octave. That can be a perfectly acceptable response shape. You can fudge things so Q is about 0.7 and get a Butterworth maximally-flat response, go Q=0.5 for a more Bessell linear-phase response, or run Q=1 or 1.4 to get a several-dB bump that cancels other fall-offs in the system.

But you sure can get in trouble if you forget this. A classic case is a capacitor and transformer off a low-impedance source (op-amp) and no or hi-Z load. You can end up with a huge 2Hz resonance, which you may not notice in testing, except all the music sounds unsteady from all the subsonic noise and garbage straining the system.

 

[tmbg]

hm, so solid iron toroids would work well....

dumb question. What about buying a stick of iron pipe from Home Cheapo, and cutting rings off it, then die-grinding the corners smooth? Would those make decent cores for audio trafos?

 

[mcs]

No, I don't think so. You don't need solid iron cores, but cores wound from thin strips of iron.

Best regards,

Mikkel C. Simonsen

 

[tmbg]

its always gotta be something complicated, doesnt it

I'm just weird and obsessive about figuring out how to make everything from scratch. The thought of paying $50 for transformers makes me a bit ill.

 

[CJ]

i wouuldn't mess around with diy power transformers. there are some saftey issues there. most power transformers are hi-potted to make sure they will handle say, 1.5 kv dc between windings so they meet UL, CE approved and all that.
no big deal if a line in transformer pops, but something sitting on the power line might fill your room with acrid smoke, probably when you are not at home to extinguish the flames due to Murphy's law.
cj

 

[tmbg]

Don't have any desire to do power transformers... right now just matching transformers for like preamps and such, maybe tube amp output transformers at some point...

 

[rafafredd]

There´s some easily obtainable mumetal tape. I think that´s what they call it. It´s like a big strip of mumetal. Would this work as a toroidal core, roled? Like a roll of tape.... you know.

 

[CJ]

the mu metal, unless it is coated with a thin insulator, will short between lams turning it into a big block of useless steel.

i added this to the tranny meta last friday:
http://www.ee.surrey.ac.uk/Workshop/advice/coils/power_loss.html#lams

here is another link I added, short and sweet articl on Nickel lams:
http://www.protolam.com/page8.html

look in the phone book for a transformer shop near you. then snoop around the dumpster area. they recycle transformer iron. i used to fill up a dumpster every 6 months or so. probably won't find any nickel.
or just ask the owner if he has any scraps for a poor diy'er. we had all kinds of useless crap hanging round for years and years.
cj