Core size matters? Microphone input transformer? - Page 1
Forum section: electronics / projects

author: mkiijam
author_rank: Regular Contributor
timestamp: July 14, 2023, 01:24:54 am
content: All other things being equal, what would a larger core/bobbin get me?same turns, same wire, same core material etc... just bigger. My wife just said "better", but I don't think that really helps here... :-(

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 14, 2023, 06:40:19 am
content: Hi mkiijam,bigger core gives you; Better low-frequency-response, lower windings per volt needed for the same inductance, lower distortions.Post some pictures please.

author: moffy
author_rank: Super Contributor
timestamp: July 14, 2023, 06:45:20 am
content: Quote from: mkiijam on July 14, 2023, 01:24:54 amYour wife is right, within reason, as wives usually are.

author: S57UUU
author_rank: Regular Contributor
timestamp: July 14, 2023, 12:45:29 pm
content: A bigger core needs less turns per volt, but how many volts does your microphone really produce?A bigger core with less turns could need a bigger magnetising current.I would be more concerned with the magnetic material, like initial permeability, if laminated, how thick, etc.Marko Cebokli

author: Terry Bites
author_rank: Super Contributor
timestamp: July 27, 2023, 02:02:42 pm
content: Roehrenonkel is right. You get may get a better LF responseUnless you have a high level of flux in the core distortion will be very low. Its hard to imagine mic signal pushing the core anywhere near saturation.Impossible I'd say.Are you being flogged a transformer idea from an Audiophool?You misheard, your wife said "you'dbetterdo ^^*^%$$9%**B* or else, you &$$£*R".Don't you ever listen!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 27, 2023, 02:57:38 pm
content: One thing I always wondered is how they manage distortion caused by hysteresis in the magnetic core.I assume they want to operate the core with a relatively strong magnetic field (but well under saturation as that causes other distortion).I'm guessing it's a delicate balance to get it right.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 20, 2023, 08:44:35 am
content: The currents in a mic transformer are minute. The mic probably generates a mV or so. Thats a few micro amps into 600Ω. The magnetic flux is way too small to come anywhere near to saturating the core. A few orders of magnitude too low. It not an issue. Line level can be a volt to two volts peak and thse transformers can overload. In accord with your preference, this can be a problem or a pleasure.

author: peter-h
author_rank: Super Contributor
timestamp: August 20, 2023, 05:20:43 pm
content: Line transformers are a different matter from mike transformers.If the txf core is big, you will get a high hysteresis loss i.e. bad distortion.Of course nowadays people pay money for 2nd order distortionWhat will they all do when Russia stops exporting ECC83s, EL84s, KT88s...

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2023, 07:41:52 pm
content: In many aspects a larger transformer is closer to an ideal one. There are however also cons with a larger transformer: effects from magnetostriction in combination with the earths field can get stronger (more material to react to stress). There is also a chance to get more eddy current loss and related noise.On the practical side a small transformer allows to use more exotic materials (permalloy, Nanoperm,..) at a reasonable price.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 20, 2023, 08:28:11 pm
content: At some point, too big of a transformer requires excessive wire length; though this could be quite large indeed, as for example doubling A_e allows halving N, while only increasing MLT by "half a doubling" i.e. sqrt(2), and this works all the way down until the smallest winding is a single turn.More to the point, core losses will limit HF for very large cores: you'd have to increase the insulation distance between laminations to keep capacitance low, or use stacks of blocks rather than ever-wider sheet/strip. Or, the insulation between lams is imperfect to begin with, and eddy currents still flow, little enough not to mind on a small scale but on a large scale it adds up.Even more to the point, we're talking specialty alloys that would simply be exorbitantly expensive (even to many audiophools) in such quantities, while acceptable performance (10s, maybe low 100s kHz upper cutoff) can be had with much smaller cores.Another practical difficulty is, no one makes cores with such a small A_W to pull off such a ratio optimally, i.e. basically a laminated equivalent of a ferrite bead. Any off-the-shelf core will have winding area balanced for general purposes, and so the l_e will be much longer (so much A_W wasted) than needed, compromising performance. Put another way, the l_e^2 / A_e ratio isn't usually very crazy (offhand, a list of some parts I've looked at, groups this in the range of 16-40; beads and balun cores can be as low as 7-10, and thin, large-winding-area shapes and toroids can be over 100), but you need an extremely low ratio to truly minimize wire length.Also also, wire length minimization doesn't really mean anything above frequencies where the core stops working; again, it needs to be laminated/blocked out appropriately to maximize HF performance, but might also benefit from hybrid construction (using some ferrite alongside), or even diplexing across multiple transformer types (which won't be easy, given the complex impedances transformers have near cutoff), but needless to say this is all well beyond 100kHz or so, which should be useless for audio purposes -- but may occasionally find application: like, ISDN was a fairly wideband communication protocol, enough that it demanded amorphous cores.I would imagine for something say fist-sized, at impedances up to a couple 10kohms, bandwidth from a few Hz up to low MHz would be juuuust achievable. The high-Z / BW tradeoff will be pretty strong, and higher bandwidth will be possible for more modest Z (say 100-600 ohms). Signal level will be quite low at the LF cutoff, of course.Tim