Is a 4-20mA current loop transmitter OK for high quality audio? - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: September 06, 2018, 09:49:07 am
content: Something like the Texas XTR117http://www.ti.com/lit/ds/symlink/xtr117.pdfhas a flat frequency response over the audio range and is described in the datasheet as a 'precision device'.Outside of the telephone phone world there doesn't seem to be much information about current loop powered devices being used in audio applications.Is the phantom powered balanced type circuit a better technical solution for some reason?

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2018, 01:18:18 pm
content: There is no particular reason it cannot be done. I have used current outputs to drive audio inputs before to avoid ground loops with good results. Quality can certainly be better than necessary for most applications.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 06, 2018, 03:42:21 pm
content: Interesting question.There is no distortion figures that I could find though. And I'm tempted to think that they would not be that impressive in real-life audio applications. You'd have to define "high quality audio", but IMO, distortion could be one point where this approach could fall short.Would be interesting to build a test jig and get some measurements. "High quality audio" unfortunately needs expensive equipment to get accurate figures, especially distortion figures.Just my 2 cents.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 06, 2018, 06:23:19 pm
content: Quote from: SiliconWizard on September 06, 2018, 03:42:21 pmA sound card with 110dB SNR input and output is like $100. Not even expensive by mainstream audio standards.https://www.newegg.com/Product/Product.aspx?Item=N82E16829132100

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 06, 2018, 06:54:13 pm
content: Probably. It won't save you from induced noise, however. You need to use coax, or fully differential signaling on twisted pair, for that -- a naive implementation might keep one side at ground and suffer from common mode interference. It's common enough to implement current loops with isolation, but that introduces other opportunities for noise (DC-DC converter), plus has the difficulty of needing a linear, low noise analog isolator.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 06, 2018, 07:46:12 pm
content: Quote from: T3sl4co1l on September 06, 2018, 06:54:13 pm20mA is a small current. A very low noise and very high isolation (but very low efficiency) DC/DC converter for that application could be some LEDs illuminating a small solar panel. But it would likely make a lot more sense to just go digital for sending audio over a distance.

author: ebclr
author_rank: Super Contributor
timestamp: September 07, 2018, 01:25:56 am
content: Fiber will be much easier and better

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2018, 08:11:24 am
content: Quote from: ebclr on September 07, 2018, 01:25:56 amHow will it be easier?Optical fibre will be subject to less distortion, but there's the tooling and hassle of having to terminate the connector. With wire it's easy using terminal blocks or solder.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 07, 2018, 01:34:28 pm
content: Quote from: T3sl4co1l on September 06, 2018, 06:54:13 pmAbsolutely.The sound card approach will get you some preliminary measurements, but there are many pitfalls as you mentioned. Besides, you wouldn't be able to really publish your results.If all you intend to build is a home-made or in-house device, that's fine. But if you intend to sell a product flagged as "high-quality audio"... not so much.This kind of equipment is relatively standard for audio measurements:https://www.rohde-schwarz.com/us/product/upv-productstartpage_63493-7558.htmlMay seem overkill but it's hard to beat with a $100 sound card. Of course in a preliminary prototyping phase, the sound card will probably be fine.

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2018, 01:42:50 pm
content: Quote from: SiliconWizard on September 07, 2018, 01:34:28 pmI don't see why not. Many audio companies seem to pull figures out of their arses and they get away with it too.The specification for the XTR117 is more than good enough for audio. I doubt any double blind listening tests would be able reliably distinguish it from a NE5532 or anything else half decent for that matter.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 07, 2018, 01:47:34 pm
content: Take a look at "Holm Impulse" software for the measurements. Pretty Good...Designed for speaker and room impulse testing, but remarkably good on the bench, too...Steve

author: mikerj
author_rank: Super Contributor
timestamp: September 07, 2018, 02:01:17 pm
content: The only thing I can't see in the datasheet that would be pertinent for high quality audio is noise performance.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 07, 2018, 02:15:41 pm
content: Quote from: Hero999 on September 07, 2018, 01:42:50 pmJust because this is not a dependable and calibrated measurement equipment. A basic (IMO) metrology consideration. This would be the same, IMO, as selling power supplies that would be designed and controlled with cheap and non-calibrated $30 multimeters. If you're not in a regulated industry, nothing prevents you from selling that of course (it should still comply to basic safety regulations but that's another matter). But it's just my take on this. I don't see a reason why we should consider audio devices as not worthwhile of being treated as other kind of devices. We're talking "high-quality" here, as I understood it, and not cheap products. Companies that "pull figures out of their arses", especially in the audio field where those given figures are often not only misleading but wildly optimistic, are not dependable companies. They are hurting the business, the concept of quality and common sense. Just my 2 cents again.Quote from: Hero999 on September 07, 2018, 01:42:50 pmWell, maybe, but I don't know. Again, they don't seem to provide some key figures such as distortion figures, and given the nature of a typical voltage-to-current converter, I'd suspect potential issues with that.Again, that's probably "good enough" as you said (which would be a pretty vague concept), but somehow, I have a hard time seeing "high quality" (proven with correct measurements) and "good enough" (being acceptable to the listening ear) in the same area. Even so, I'd be interested in listening to the result.

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2018, 03:09:44 pm
content: Quote from: mikerj on September 07, 2018, 02:01:17 pmThe bandwidth and slew rate are ample for audio and the non-linearity is low enough that it should translate into respectable THD figures.Of course it should be tested, especially as it's being used for an application it was never intended for, but there's no point in going over the top.