ribbon mic transformer primary impedance / inductance

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rafafredd

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Jun 3, 2004
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Rio, Brazil
I´m trying my first transformer winding.

It´s a ribbon mic transformer in a permalloy toroidal winding. I´ve succeed in winding 865 turns into a toroid (Yes, hand random winding into a toroid, I did it) and I would like to know what should I use as a base for calculating the primary.

865 turns gives me a DCR of 16ohms and a little more than 2H inductance. Just perfect for a 150ohms microphone winding.

So, I can wind 29 turns secondary for a 1:30 transformer, or maybe go higher ratio and wind a 18 turn primary for a 1:50 ratio, or 1:70 who knows...

There is space for very large AWG wire for a very low DCR winding.

I would rather calculate how many turns would be good for the ribbon and let the ratio be a result of the needed primary vs the already winded secondary. It would be better than choose whatever turns ratio and end up with a too large or too small inductance primary winding.

So the simplified question:

What impedance / inductance does a regular ribbon wants to see at the primary of a ribbon mic transformer?
 
Marik is the man to answer this question as he seems to know lots about transformers too!

My suggestion is low DCR as you have already suggested and high inductance.

The thing is though, you do need to plan the ratio, as this will define what is impedance is reflected back to the ribbon when connected to a particular preamp input, plus you will have to consider noise / signal to noise ratio....
 
I just did a 13 turns primary eith very big wire, and I get 0.455 mH inductance and near zero DC resistance, but the turns ratio is probably too high???

~1:67...

haha. What do you think about this inductance for a ribbon? 0.455mH? Too low?
 
[quote author="rafafredd"] 0.455mH? Too low?[/quote]
Yes, too low.
some around 5 mH, 25 miliohms maybe good for 5 cm length ribbon.
But at what frequency have you measured?
permalloys have some 200 Hz critical frequency.

xvlk
 
Xvlk, remember ribbon width, not only length!

Rafa, we want probably between 1:25 and 1:40 for ratio, but this depends on the ribbon material and dimensions.

What sort of ribbon / microphone are you thinking of?

PS. I am glad you are doing this!

Roddy
 
Oh, it seems I´ll need a big motor, with big magnets and big ribbon... Well, I´ll try 30 turns for the primary and see how it comes along.

Now I know why ribbon mics have generally a transformer secondary of 30-50ohms instead of 150ohms. More space for the primary, ence more inductance possible.
 
You calculate the 1:67 as an "ideal" ratio. In practice it will be less and depends on primary/secondary coupling. Did you measure it with a signal?

I never tried toroids and did interleaving on E and U hi Nickel cores. The best coupling was on U core, with primary wound with a foil.

Also, you want a secondary with a low DCR, as well.
 
[quote author="CJ"]How many volts at 20 hz will a ribbon crank out?
[/quote]

Nothing... or almost. Large ribbons are tuned somewhere around 25Hz, so below that they are stiffness conrolled, with 12db per octave fall. Also, because of air viscosity in the gap between ribbon and pole pieces, on those frequencies the system is already resistance controlled, reducing the output even further. So, on 20Hz it is close to nothing.
 
[quote author="CJ"]
OK, what is a nominal ac voltage from a typical ribbon at 1000 Avis?
[/quote]

Somewhere in between 0.03-0.1 mv/Pa, unloaded. A source impedance (resistive) can be anywhere from 0.1 to 1 Ohm, depending on the ribbon.
 
Pa?

Just tell me the voltage on a typical pri so I can calculate the dang flux already!

I think Raf's core is going to swallow the signal.

Also, he might need to wind the pri first so he can magnetize the core better.

What is the core like on the best ribbon transformer?

Small, I bet.
 
I don´t know the exact size of the cores. it´s like 3 cm diameter. I´ll have to measure later and post back.

But, from what you are saying, I´ve probably wasted my time, as there´s no interleaving. There´s just a secondary of 865 turns in the core, and I thought I could just wind a primary on top of it, but it seems that it´s not a good idea... Primary should be inner, no? Or interleaved?

Anyway, it´s a great thread. I can use this one as a EQ inductor and try to wind it again, starting with the primary. I have 50 pieces of these cores.

My secondary has only 16ohms DC resistance. I think it´s really low enought for a 150Z winding, no?

I´m had a look at Lundahl version of the ribbon mic transformer, and their secondary has 59ohms DCR, but the transformer is probably fantastic. the fact that there are four separate primary windings, for matching many kinds of motors, makes me think that probably has four interleaving sections between primary and secondary.

http://www.lundahl.se/pdfs/datash/2911.pdf
 
Holy Cow how did you get that many turns?
You must have spooled up some wire on something you could pass thru the hole.
I bet you have a splice or two.
You might be able to pull the pri into the sec turns, use heavy wire, you want low dcr anyway, plus, you do not need many turns.
Imbed the pri into the sec. Good coupling.
You do not need interleaving with a mic xfmr, becuase a little roll off can be a good thing.

I had bad luck with capacitance stealing all my high end on a torroid I wound.
So I did some research and found that if you wind in "pies", you can reduce capacitance.

The theory behind this is that you are putting the turns with the most voltage between them on opposite sides of the core, thus reducing stray C.

The prefered method is North West East South.
You want the pies opposite each other as you wind. This is, of course, impossible to do for all windings, but this is better than nothing.

Here is an exagerated sample of what I mean.
I kind of just squeezed the turns into 4 quadrants.
Your actual coil should not have those big divisions between the sections, you should gradually expand the pie as you add layers to creep out to the boundry of the adjoining pie.

nwes_torroid.jpg


Not to make you want to throw up and stab me, but I wound 750 turn torroids in about two minutes a piece for the engineering staff here.
A happy coincidence, them buying a torroid winder and me being into coils! :thumb:
 
[quote author="CJ"]
What is the core like on the best ribbon transformer?
[/quote]

CJ, I think I may have some spare ribbon transformers which could be useful for, err.... examining.

Sone of them are high-ratio, Hi-Z models, but they may still be useful as the basic costruction is often the same as their Low-Z counterparts.

Should I look them out and send them over?

Roddy
 

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