Power transformer as Microphone transformer

[matimatil]

I'm in the process of prototyping a DIY ribbon microphone. Living where I do, getting proper microphone transformers or even buying them overseas is pretty expensive, so I'm thinking of alternatives. From my research, a lot of ribbon microphone transformer have normal mu-metal or nickel alloy cores, similar to power transformers, and something like a 220v to 6v power transformer actually has the same turn ratio as a 1:37 audio transformer (wiring the other way around). The permeability of the core is high enough that I think that the primary inductance will be enough as to have proper low frequency response, altough I have no practical way to measure it at the moment. The transformer I got is almost as small as an audio transformer. The resistance of the coil is about 1 ohm, I am thinking that if I open the transformer up i can count how many turns it has and wire another coil to put in parallel, so I can reduce the resistance. If tests are succesful I am also thinking that I might get away with reducing the turns on the primary, and effectively make the ratio larger (my final goal is to make an active design, relying not on active amplificacion, but only buffering and impeadence matching, so maybe depending on how many turns the transformer already has I can divide it enough to make something like a 6 or 12db boost, or hopefully 18db)

Anyway, I am still waiting on the aluminum foil but wanted to get opinions on this, what do you think will happen? Will I come close to something usable or I am missing an important point?

 

[rock soderstrom]

Will I come close to something usable or I am missing an important point?

I doubt that because mains transformers are optimized for 50/60Hz and therefore the transmission bandwith will be very limited.

Welcome to the forum! Where do you come from?

 

[matimatil]

I doubt that because mains transformers are optimized for 50/60Hz and therefore the transmission bandwith will be very limited.

I heard that, altought I actually tested using a synth, conecting the side with more windings to the output, and probing the side with least. The results where good across a very large range, not much distortion on high frequency and good response. Hooking it the other way around was trickier, i suspect because the output impeadence of the synth was too high, so low frequency response was a no, but that shouldn't happen with a ribbon.

Construction wise, what makes it optimized for 50/60hz? Or in other words, if the materials and the ratio are the same as the desired audio transformer, what is different? (I have no problem opening the transformer and rewinding parts to make it work)

Welcome to the forum! Where do you come from?

Thanks! I'm from Argentina

 

[abbey road d enfer]

Construction wise, what makes it optimized for 50/60hz? Or in other words, if the materials and the ratio are the same as the desired audio transformer, what is different? (I have no problem opening the transformer and rewinding parts to make it work)

The main issue with mains xfmrs is that the primary is not too well coupled with the secondary, because of the necessary high voltage insulation.
Audio xfmrs optimize the coupling coefficient by sanwiching windings. Ribbon mic xfmrs have a very small number of turns at the primary, so it is usually a single layer, sandwiched between two half secondaries.
Now the usual M6 core material is not the most adequate for very low level applications, but some companies offer ribbon mic xfmrs with M6 core...

 

[matimatil]

The main issue with mains xfmrs is that the primary is not too well coupled with the secondary, because of the necessary high voltage insulation.
Audio xfmrs optimize the coupling coefficient by sanwiching windings. Ribbon mic xfmrs have a very small number of turns at the primary, so it is usually a single layer, sandwiched between two half secondaries.
Now the usual M6 core material is not the most adequate for very low level applications, but some companies offer ribbon mic xfmrs with M6 core...

That is good to know and already gives me something to work on for potential improvements

 

[matimatil]

The main issue with mains xfmrs is that the primary is not too well coupled with the secondary, because of the necessary high voltage insulation.
Audio xfmrs optimize the coupling coefficient by sanwiching windings. Ribbon mic xfmrs have a very small number of turns at the primary, so it is usually a single layer, sandwiched between two half secondaries.
Now the usual M6 core material is not the most adequate for very low level applications, but some companies offer ribbon mic xfmrs with M6 core...

I'm not very familiar with types of transformer constructions, like the M6. Do you have other names or terminology I can look up, or any book talking about this issues?

 

[groovyloop]

the overall tone will be not good. I spent a lot of time trying different power transformers in order to find a cheaper solution for some sound transformers and it was in vain. Overall tone suffer...

 

[matimatil]

the overall tone will be not good. I spent a lot of time trying different power transformers in order to find a cheaper solution for some sound transformers and it was in vain. Overall tone suffer...

That is a bummer, altought still hopefull: I am researching a couple of patents for transformers and ribbon microphone designs; I think that I can take advantage of the core construction and rewind the transformer to make it perform better.

 

[groovyloop]

That is a bummer, altought still hopefull: I am researching a couple of patents for transformers and ribbon microphone designs; I think that I can take advantage of the core construction and rewind the transformer to make it perform better.

if you rewind it - it will never be a power trasformer anymore...

 

[matimatil]

if you rewind it - it will never be a power trasformer anymore...

Haha, that's true

 

[groovyloop]

life is pain

 

[blakeyboy]

I once had to get a signal from the amp room in a big nightclub to the foyer.
There was no way to do this because of the layout of the building, so I dropped a cable down into the car park underneath, and ran this to the front of the building- 100 metres plus...
...noisy as hell. Tried everything- electronically 'balanced' outputs and inputs wouldn't do it.
I thought 'what the hell' and grabbed a pair of small ( 10VA ) pcb mount mains transformers,
put the signal into the mains side, the low volts side into the cable, and the reverse at the other end.
I was expecting to have to do some _serious_ eq compensation at the other end but no....!

If you are lucky and the secondary is on the outside, I would unwind a lot of it, and wind it back onto the former leaving loop tappings outside every few turns. Then you can experiment with what works best when your mic is built. I'm concerned that your ribbon will be driving what it thinks is fresh air otherwise- your bass won't suffer as in a too low impedance case, but you won't get any current into the windings, so your output will be low, so you would have to crank up the gain and hello noise....

You have a hell of a low impedance source there in a single peice of aluminium. It's difficult to imagine
a lower impedance source unless you are thinking of connecting the transformer to the output of a spot welder or something like that....

 

[chr1s]

I experimented some time ago with power transformers to be used as (cheap) tube output transformers but without much success. IIRC the measured frequency response (outside a tube circuit) wasn't very good.
I also remember reading some stuff about audio transformer design but can't remember where. There's a bit in Radiotron Designers Handbook (http://www.tubebooks.org/books/rdh4.pdf) starting at page 206 but not very detailed.

 

[Nelson Paschoal]

Hermano Matimatil, get into this project with fullpower, forget if it´s suitable or not, if can or can´t be used, just do it and if it su**s just start over, good things happen when it´s not planned, good luck!!!
PS: where did you get the aluminum foil? I´m in the same situation, I´m from Brazil and have to import oversea.

 

[matimatil]

Hermano Matimatil, get into this project with fullpower, forget if it´s suitable or not, if can or can´t be used, just do it and if it su**s just start over, good things happen when it´s not planned, good luck!!!
PS: where did you get the aluminum foil? I´m in the same situation, I´m from Brazil and have to import oversea.

I didn't order yet, but I think i'll get it from geistnote