Winding a toroidal xformer for audio impedance matching - Page 1
Forum section: electronics / beginners

author: dnyberg
author_rank: Contributor
timestamp: May 28, 2021, 07:35:06 pm
content: I'm adapting some headphones for use with a portable audio device, and have experienced some success: sound comes out and no smoke... but the sound is very faint. I think the reason is that portable devices assume efficient, low impedance earphones in the 15 ohm impedance range, while the ones I'm trying to drive are somewhere in the 150-300 ohm range.The solution I'm thinking of trying is to wind a couple (one per channel) of toroidal transformers with a suitable turns ratio. I understand the n^2 impedance thing, and would choose a toroid of a size I can hand wind without use of a microscope, and since we're talking tens of milliwatts, I figure the wire wrap wire I have on hand should do okay.But I have absolutely no idea what composition I should look for for use in the audio range. I'm guessing ferrites I've salvaged from PSU's were designed for much higher frequencies.Can anyone offer useful advice regarding size/composition/sources of toroidal cores I should look to for this application?Thanks!

author: Gyro
author_rank: Super Contributor
timestamp: May 28, 2021, 07:57:57 pm
content: Most audio transformers (even the small signal ones) use laminated cores, high grade Silicon steel at minimum and Mumetal for the better quality ones. The laminations need to be thin for decent high frequency response. You need to achieve a decent inductance for low frequency response. I'm not sure that you will be able to achieve useable results in a compact form factor, you probably want powdered iron cores, certainly not salvaged PSU ones (as you've already guessed).Without trying to dishearten you, you will probably achieve less frustrating results with off the shelf parts. You don't say what part of the world you're in but here are some examples:https://uk.farnell.com/c/transformers/audio-transformers?ost=audio+transformer&rd=audio+transformerAs with all things, the higher the quality, the higher the price (mainly when it comes to low frequency roll-off), but you may be able to get acceptable quality from lower cost parts.Edit: homing in a bit, maybe something similar to this one...https://uk.farnell.com/vigortronix/vtx-101-001/transformer-audio-6-3-6-3-1-1/dp/1674300The DC resistance of the 3.75R winding might not be good for portable devices without output short circuit protection though.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 28, 2021, 08:43:47 pm
content: Iron powder cores don't make good transformers, espeically not at a small size. The permeability is too low and thus too much magnetizing current needed.One more wants very high permeability. Nonocrystalline toroids would be a good option. They are found in some high performance (small size, high current) common mode chokes. Winding torroid by hand is quite time consuming.Still the transformer adds additional losses.Chances are it would be easier to buy suiteable headphones for the portable device.

author: Zero999
author_rank: Super Contributor
timestamp: May 28, 2021, 09:02:55 pm
content: Why not simply buy one? You're not going to learn much from winding a transformer.At a push, you could use a small mains transformer, run in reverse. I've tested them and they work up to around 20kHz, but I wouldn't guarantee it. 120V:24V would give a ratio of 1:5, which would give a decent match.

author: dnyberg
author_rank: Contributor
timestamp: May 28, 2021, 09:13:16 pm
content: Well, these headphones are built into an aviation helmet, so replacing them would be expensive and present some mechanical problems. Likewise, transformers perfectly suitable for a desktop audio device won't fit well into my shirt pocket. It's disappointing to hear off the shelf ferrite cores likely wouldn't work well (are we assuming I need excellent fidelity? Anything noticeably better than "voice grade" would do). Can one buy toroids made of laminated silicon steel?

author: dnyberg
author_rank: Contributor
timestamp: May 28, 2021, 09:14:51 pm
content: Oh yes, I forgot to mention one benefit I'd hoped for from hand winding (I need only 2) is I could rewind them until I found the right ratio.

author: Gyro
author_rank: Super Contributor
timestamp: May 28, 2021, 09:23:30 pm
content: Does "aviation helmet" imply that you are near a source of power? If so, an active gain stage would be a superior option, certainly cheaper. Battery power might also be an option if output power levels are low.

author: dnyberg
author_rank: Contributor
timestamp: May 28, 2021, 09:29:17 pm
content: Available source of power will be a 9V battery, but if I get 2 hours lifetime from one, that'd do, so it could be a simple amplifier and accept the inefficiencies would be a pragmatic solution. Something like this maybe (plus a vreg ofc)?https://www.allelectronics.com/item/amp-8403/3w-stereo-amplifier-module/1.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2021, 09:29:58 pm
content: Use a power transformer, say 120 to 36V. Get the smallest one you can, it'll handle more than enough voltage, even at low frequencies. Toroid is the correct choice over shell or split-bobbin type, the leakage inductance is lower meaning high frequency response will be acceptable (if maybe not ideal, YMMV).Tim

author: Gyro
author_rank: Super Contributor
timestamp: May 28, 2021, 09:36:51 pm
content: Quote from: dnyberg on May 28, 2021, 09:29:17 pmAlthough the efficiency looks high, that board is optimised for putting out a fair bit of power into a low impedance load. A better solution would be a bridged output using cmos opamps. There are plenty that are spec'd to drive 32R loads these days.Edit: A quick and lazy search on ebay (bed time), reveals a number of tiny TDA1308 based headphone amp PCBs. Typical quiescent current 3mA. Absolute max supply voltage is 7V though - I'm sure there are better boards out there.

author: penfold
author_rank: Frequent Contributor
timestamp: May 28, 2021, 09:44:38 pm
content: Quote from: dnyberg on May 28, 2021, 09:13:16 pmYes, called "tape wound cores", I've never seen them as a stocked catalogue item, probably made to order from most manufacturers, so small quantities will be pricy, same deal with small EI type cores also I guess. You could always strip the windings off a pre-wound transformer and re-wind... but I have a feeling you'd be somewhat disheartened on the idea of self-winding after that.

author: dnyberg
author_rank: Contributor
timestamp: May 28, 2021, 09:47:30 pm
content: Hm, bridged op amps. Can you suggest likely (hopefully somewhat jellybean) parts off the top of your head? Or I guess I could dig into art/electronics and cobble up something that'd run directly from the 9V supply, though that's not the direction I was originally thinking.

author: dnyberg
author_rank: Contributor
timestamp: May 28, 2021, 09:49:44 pm
content: Quote from: penfold on May 28, 2021, 09:44:38 pmYeah, I'm kinda wilting here; thought I had it largely figured out. So now I'm just looking for a practical solution, probably wildly off from my original thoughts. :/

author: bob91343
author_rank: Super Contributor
timestamp: May 28, 2021, 09:55:52 pm
content: How about a reality check? If you correct the mismatch, how much more audio power will you get? You might be surprised that the resultant volume may not be as much an increase as you hope.

author: dnyberg
author_rank: Contributor
timestamp: May 28, 2021, 10:00:25 pm
content: Quote from: bob91343 on May 28, 2021, 09:55:52 pmPossible, but how would I know until after I've tried it?

author: dnyberg
author_rank: Contributor
timestamp: May 28, 2021, 10:08:44 pm
content: Maybe something like this?https://www.mouser.com/datasheet/2/294/njrc_s_a0009983567_1-2279521.pdf

author: exe
author_rank: Unknown
timestamp: May 28, 2021, 10:17:03 pm
content: I think an amplifier is a good and simple idea. It's just a few parts: a battery, opamp, and a bunch of resistors and blocking capacitors. The only downside is you'd need to charge/replace batteries. Or find another way to supply it. There are might be suitable headphone amps on the market, I didn't check.PS why do you think impedance matching is needed? Or do you mean you need bigger output voltage swing to drive high-impedance headphones? That's imho not an impedance matching.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2021, 10:37:32 pm
content: Or this:https://www.digikey.com/en/products/detail/hammond-manufacturing/146G/454914Not particularly cheap though. If your time isn't worth much, sure, can wind it yourself, in part or in full. Iron cores aren't terribly easy to find (let alone other really nice signal cores like permalloy), but nanocrystalline is (used for EMI purposes), and has excellent permeability (in the right grades). Downside, it's even more expensive!The nearest toroid as I suggested earlier, would be something like this,https://www.digikey.com/short/797hwjh0though none of them are showing stock right now. Mouser or others may have some.Tim

author: bob91343
author_rank: Super Contributor
timestamp: May 29, 2021, 03:49:39 am
content: You can calculate the increase in power by use of a novel procedure called Ohm's law. Calculate what you have, and then calculate what you'd have with a proper match. If the difference is less than around 3 or 4 dB you won't gain enough to matter.

author: The Electrician
author_rank: Frequent Contributor
timestamp: May 29, 2021, 06:05:46 am
content: Quote from: dnyberg on May 28, 2021, 09:49:44 pmThis is the size transformer you need, and this one is near the impedance ratio you need. You could get this and try it out on one channel to see if you get much improvement.https://www.ebay.com/itm/334018573088?hash=item4dc50ef320:g:0yoAAOSwm2hgcKZd

author: exe
author_rank: Unknown
timestamp: May 29, 2021, 07:36:46 am
content: Btw, does the application needs a hi-fi transformer? I mean, it's an aviation helmet, I have doubts it was designed for listening to the music.

author: dnyberg
author_rank: Contributor
timestamp: May 29, 2021, 11:13:37 am
content: Quote from: exe on May 29, 2021, 07:36:46 amQuite right; anything better than voice grade would do. Nowdays we're spoiled in that we like to listen to decent stereo sources between bursts of radio traffic, but it's the radio that really matters, and that is most definitely not hi-fi.Thanks for the help gang; you've given me some directions to explore.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 29, 2021, 11:43:16 am
content: Its really unlikey that you power amp is going to complain about a higher load impedance. Unless its some tube madness.You can just whack 15R resitors in parallel with your phones to make them look like 15R.A long as the power amp has a reasonably low output impecance the volts across your phones wiil be nearly the same.

author: Gyro
author_rank: Super Contributor
timestamp: May 29, 2021, 11:58:21 am
content: Quote from: dnyberg on May 28, 2021, 10:08:44 pmYes, that looks like a good choice, bridged output for maximum voltage swing and nice low quiescent current. The distortion is maybe a little high for HiFi, but probably perfectly acceptable for an aviation headset.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.The little TDA1308 based boards that I mentioned as an edit to my previous post is single ended output, so doesn't have any issues with a 3 pin headphone connection, but doesn't have as big an output voltage swing as your JRC part.Just a though, are aviation headsets Mono anyway?

author: Gyro
author_rank: Super Contributor
timestamp: May 29, 2021, 12:06:01 pm
content: Quote from: Terry Bites on May 29, 2021, 11:43:16 amThe OP's problem is that the sound level is too low. Parallel resistors wont help that.

author: exe
author_rank: Unknown
timestamp: May 29, 2021, 12:30:04 pm
content: I'd like to add a bit more about impedance matching at sound frequencies .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 . 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 . This is done with a transformer. "Solid state" amplifiers are free from that because their output impedance is low. 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. Unless it's a landline or something.https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem

author: SeanB
author_rank: Super Contributor
timestamp: May 29, 2021, 01:15:12 pm
content: 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.pdfas 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 29, 2021, 01:21:57 pm
content: 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.

author: dnyberg
author_rank: Contributor
timestamp: May 29, 2021, 02:37:06 pm
content: Quote from: Gyro on May 29, 2021, 11:58:21 amThey'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.

author: Gyro
author_rank: Super Contributor
timestamp: May 29, 2021, 04:49:44 pm
content: 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).

author: dnyberg
author_rank: Contributor
timestamp: May 29, 2021, 11:49:07 pm
content: 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.