Winding a toroidal xformer for audio impedance matching

[exe]

I'd like to add a bit more about impedance matching at sound frequencies [1].

According to Rod Elliot (he's a sound engineer, I hope I'm not misquoting him), at sound frequencies impedance matching is not needed because wave-length is too big to care about reflections and impedance discontinuities [2]. As long as the output stage can drive the load, everything will be fine. Power amplifiers do have Zobel network (a resistor with capacitor across the output) to dampen potential oscillations because loudspeakers have large inductance, but I'm not sure this can be called "impedance matching".

A rule of thumb, if the wavelength of highest frequency of interest is less than 1/4 of travel distance, then we don't need to bother about reflections, etc. So, for sound frequencies a few meters of cable is not a problem. An exception could be high-impedance valve amplifiers. If we want to get maximum power from them, then we need to match load impedance [3]. This is done with a transformer. "Solid state" amplifiers are free from that because their output impedance is low.

[1] I think we need to take into account amplifier bandwidth. Most amplifiers have 100kHz+ BW to reduce distortion. So, in reality, our frequency of interest is much higher than 20kHz if we are talking about sound amplifiers for music.
[2] Unless it's a landline or something.
[3] https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem

[SeanB]

Your earpieces and microphone will be 600R moving coil types, common in aviation as transducers, and for the microphone very likely also noise cancelling, so driving you want some small output transformers. I replaced so many of them........

https://docs.rs-online.com/7532/0900766b815288f8.pdf

as an example, pretty small, and you just have to wire all 2 sections in series and use as an autotransformer, with the earpieces across the entire winding, and feed in the audio to primary only. Not too expensive either, and in stock in the EU as well.

[Kleinstein]

Impedance matching (with a transformer) is still the way to get the most power out of an amplifier, even at low frequency. The parallel resistor is not needed - it would be more like a series resistor to avoid to low a load at the low frequency end where the transformer no longer works well.

A soft ferrite core is not that bad and may work. It is low saturation (around 0.4 T) and thus maybe more turns needed than with permalloy (around 0.7 T saturation) or iron (~ 1.4 T) or nanocrystalline (~ 1 T saturation). For a crude test one could use a CM choke (usually from soft ferrite) as a 1:2 autotransformer. It is just about finding enough inductance: 100 mH at 20 Hz would be 12 Ohms.

[dnyberg]

Maybe something like this?

https://www.mouser.com/datasheet/2/294/njrc_s_a0009983567_1-2279521.pdf

Just a note that bridged output requires individual access to both of the wires from each earpiece (no common pin). If that's a problem for a standard jack then 4 pin microphone connectors and 4 pin XLR connectors are commonly available.

...

Just a though, are aviation headsets Mono anyway?

They're wired up mono, and run out to a funky 1/4" audio plug (totally incompatible with regular 1/4" audio plugs) and for a while I thought to keep that original wiring harness, since a new replacement, should I want to restore to factory spec costs $250 (aviation prices, sheesh!), But eventually I just gave up on that, stripped out the original harness and put in all new wiring. I've brought out every available line to a large 8 pin DIN connector (like the old PC keyboard plugs), so I can build different versions of the upstream electronics, to connect to different radios, maybe at some point I'll work out a prioritizing switch, lots of options. Choosing a friction based connector lets me just yank loose in an emergency, and 8 pins lets me bring out every lead, so I have flexibility for whatever I want to do in future.

I botch things and have to go back and rework them, on a regular basis, but I'm pretty pleased so far with that connector choice.

[Gyro]

Ah good. If they're mono, there's no wiring problem with a bridge output amp (as long as there's no danger of the output jack shorting to grounded metalwork).

[dnyberg]

Well, to be more clear, the earcups hold a small PCB with 2 speakers (one used as a mic) and a small amount of analog circuitry for the active noise cancellation; only 3 solder points there: +v, gnd and signal. So I could use a bridged amp if I connected only signal and ground, giving up the anc, or single ended if I want to power the anc. So I'm pretty much limited to single ended amp, as I don't consider the anc optional.

As originally wired, though, both cups are on the same line, which is what I meant by everything being wired as mono. I've now brought out the left and right signal lines separately to the din connector, so I can do stereo from the right signal source. I've also brought out both mic lines to the connector, so the bias circuitry will be external to the helmet, most likely in the same box as the battery and switches to control anc power and mic bias independently. That'll let me depower the mic when I want, and/or to depower the anc so I can (sorta) hear ppl talking to me on the ground. If they're close and talking really loudly.