How can I determine if this audio transformer is suitable? - Page 1
Forum section: electronics / projects

author: Starlord
author_rank: Frequent Contributor
timestamp: January 26, 2015, 09:34:33 am
content: I have a ground loop in my circuit that's messing up my audio. I posted about it here before, if you care to see the PCB layout and such.Up until this point I've been correcting the issue with ground loop isolators. Essentially they're just transformers in a box with cables attached so you can place them between your amplifier and your audio source to break the ground loop. Works great.But it's a costly solution, and it's embarrassing that my boards don't work without it in place.I'd avoided transformers because when I searched for them on Digikey in the past all I found was these big ugly looking things like you'd find inside a rotary phone from the 1980's. No way was I going to stick one of these on my beautiful surface mount PCB. Besides, at like $6 a pop they were hardly any less expensive than the isolators.But I decided to look again today because I figured someone must have surely invented a smaller surface mount transfomer by now, and lo and behold I found this offering:http://www.murata-ps.com/data/magnetics/kmp_da100.pdfIt's not quite as miniaturized as I'd like, but it's pretty small. And it looks nice. And best of all, it's fairly cheap. Cheaper than that $3 headphone chip I was looking at made by TI.Only one problem. It is my understanding that not all transformers are created equal. If you search on Mouser for example, many list a frequency range they're good over. And this particular one just says "60hz". Is that a minimum? Perhaps. In which case it would be suitable. Except there are some other transformers which say they're good from 60hz-500hz. That's bad. A transformer that removes all frequencies above 500hz is of no use to me. My ideal transformer would pass all frequencies from 20hz to 20,000hz.So how can I tell what frequencies this transformer would pass, and which it would filter? And while I know what inductance is, what's the max leakage inductance? And what's the min volt-time product? And what's return loss and common mode rejection? And why is the DB value of them so high? Are they saying it's not lossy at those frequencies? Cause like -20DB on a low pass filter would indicate how much certain frequencies are filtered out, but that ain't negative.And what does suitable for 75 and 110 ohm circuits mean? Are they referring to the line out impedance or something? Like I have a 107 ohm I think it is resistor between my DAC and my DC blocking capacitor. So would that be the value they're referring to and that that shouldn't be below 75 ohms?

author: nfmax
author_rank: Super Contributor
timestamp: January 26, 2015, 09:38:17 am
content: This transformer is not suitable for isolating audio signals. It is designed for isolating (fast) digital signals which happen to carry audio information - not the same thing at all!

author: Starlord
author_rank: Frequent Contributor
timestamp: January 26, 2015, 10:15:56 am
content: Quote from: nfmax on January 26, 2015, 09:38:17 amWhy is it marketed as an audio transformer then? What makes digital audio data different from any other digital data? Might the transformer introduce noise into the analog circuit and this prevents it?I'd still like to know how to determine what frequencies a particular transformer will pass. Otherwise I'll never be able to find a suitable transformer... assuming one even exists. :/

author: Starlord
author_rank: Frequent Contributor
timestamp: January 26, 2015, 10:18:17 am
content: Hm, I should have read the datasheet more closely, my eyes just glossed over after I looked at the tech specs and I didn't understand them. I see some stuff in there now mentioning digital signals and what some of those parameters mean.Well, it says what the isolation voltage is anyway, but I kinda guessed that one.

author: jaxbird
author_rank: Frequent Contributor
timestamp: January 26, 2015, 10:47:38 am
content: Quote from: Starlord on January 26, 2015, 10:15:56 amLine level audio transformers are not very common, but they do exist, look at something like Lunddahl:http://www.lundahl.se/our-products/line-input/But expect to pay an arm and a leg.Would probably be cheaper to convert to digital, isolate and back to analog.

author: bktemp
author_rank: Super Contributor
timestamp: January 26, 2015, 11:07:39 am
content: Maybe you can replace the transformer using an operational amplifier with a differential input. This solution is much smaller and can provide a much more linear frequency response. If the interference is low in frequency, this should work quite well.

author: Starlord
author_rank: Frequent Contributor
timestamp: January 26, 2015, 11:14:14 am
content: Quote from: bktemp on January 26, 2015, 11:07:39 amI don't know how to do that, and when I looked into the general cost of op amps, they were like $5 ea in low quantities, versus $3 for TI's headphone amplifier IC that they recommended in their ground loop break circuit notes. If I could find a solution that would be like $1 in quantities of of 25 or so that would be great.

author: Kalvin
author_rank: Super Contributor
timestamp: January 26, 2015, 11:59:01 am
content: Check some chinese PCB audio transformers. You might find something cheap but useful, if your requirement is "cheap but not hifi quality". You might get lucky and find some audio transformers used in transistor radios.