Winding a ribbon mic transformer

[Deactivated-1]

Do I need to wind the secondary on the inside, and primary on the outside, like most ribbon mic transformers that I've seen are wound?
Is there any benefit to layer the windings like is done in some hifi tube output transformers? (having the primary windings both on the inside and outside sandwiching the secondary)
Does around 30 turns for the primary and 1000 turns for the secondary sound ok for a standard balanced output?
Do I need a center tap to make it balanced? wouldn't it balance by itself if I just don't connect anything (or rather just connect the case for shielding) to the ground pin of the balanced connector? and if I do need to wind a center tap, is it super critical to get it exact? I know how a balanced input works but I just don't have much experience with how much impact on noise is an imbalance in the signals.

[Kleinstein]

Having the primary on the outside is not really needed. It may help to have the larger bending radius for the thicker wire though and less of the thinner wire. The difference which winding is inside is not that important. I would even prefer the primary inside, as winding normally drives the magnetization of the core. Having the primary (or secondary split increase the couling). HIgher parasitic capacitance should not be a problem here.

How many turns are needed really depends on the core size, source impedance and resistance for the wires (including the part between mic and transformer). The larger the transformer the fewer turns are needed. 30 Ohms primary look like a lot as it would likely be way more resistance than the source.  A ratio of some 1:30 seem resonable, though maybe a bit on the high side as the impedance transforms like the ration squared. The point is to keep the magnetization well away from saturation and the inductance well higher than the nominal signal impedance.

Wether one needs to center tap depends on the way the input circuit is made. Some get away without and some need it. The symmetry should no be that imortant - it again depends on the counter part side.

[Kleinstein]

One can calculate the required magnetization from the core coss-section area and voltage per turn. The maximum usefull core magnetization depends on the materail (e.g. some 400 mT for permalloy or cobalt based amphous, some 600 mT for iron or iron based amorphous and maybe a little more for nonoperm. Ferrite would be good for maybe 200 mT, but would need to be right type with high mu.

Another hint for the number of turns is the inductance of the core. The secondary inductance (no primary connected) should give an impedance well higher than the usual 600 ohms typical for a banced mic at reasonable low frequencies (e.g. 20 Hz). So one can also calculate a minimum mumber of turns from this side. Getting to low in the inductance would get nonlinearity from the core magnetization.

The use of a bobin suggests that the core is a 2 part construction. In this case it really helps to get good contact to avoid any extra air gap.

[Conrad Hoffman]

If you aren't already there, be sure to check out the micbuilders on groups.io- https://groups.io/g/MicBuilders/messages

[NBryanBlack]

Your transformer does not need a center tap to be balanced.  If this winding is to be connected to a normal mic circuit via XLR, I would recommend against putting in a center tap, or, if you do, do not ground it via pin 1!  This would put 48 volts through your transformer winding if phantom power was turned on, even by accident.  Not worth the risk in my opinion.