[HN Gopher] Audio Optocouplers
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Audio Optocouplers
Author : brudgers
Score : 29 points
Date : 2022-09-24 21:03 UTC (1 hours ago)
(HTM) web link (learnabout-electronics.org)
(TXT) w3m dump (learnabout-electronics.org)
| exmadscientist wrote:
| Be aware that these things have serious linearity (signal
| quality) issues, to the point where us "mere mortals" who design
| analog circuitry for T&M equipment try to stay away from them
| wherever possible. Audiophiles, of course, can hear the
| difference between copper, silver, gold, or platinum plating on
| the cable shield's drain wire in their neighbor's washing machine
| when he's on vacation in Hong Kong, so, you know, use with
| caution, you don't want to give them a heart attack or anything.
|
| (Unless you like the particular flavor of distortion these guys
| give. Then, by all means, help yourself!)
| melony wrote:
| Does the non-linearity come from the diode or does it come from
| the resistor?
| [deleted]
| klodolph wrote:
| Resistors are generally very linear. They're put in the same
| category as transformers, capacitors, and inductors, where we
| just ignore the non-linearities--they're usually too small to
| measure, although reverse-biased electrolytic capacitors are
| definitely nonlinear and I've heard you can be affected by
| nonlinearity on carbon-composition resistors if you put
| enough voltage across them--like, 100V--don't know if it's
| true, just "heard it somewhere".
|
| Diodes are incredibly nonlinear, that's the whole point of
| having diodes.
| melony wrote:
| Isn't the diode linear only within a certain part of the VI
| graph?
| jupp0r wrote:
| No, it's exponential all around. For any infinitesimal
| small delta V you can assume it to be almost linear. This
| works for amplifying really small signals.
| klodolph wrote:
| It's not really linear anywhere, except in the sense that
| it's got a smooth VI graph, so you can pick a point on
| the curve and draw a tangent line if you want. That's not
| really linear, unless you have small enough signals or
| bad enough oscilloscopes.
|
| It's much closer to exponential, which is Shockley diode
| equation / "diode law" which is the simplest model you
| would use besides the "diode is a one-way switch with a
| voltage threshold" model.
|
| The diode law is I = Is[ exp(VD/nVT) - 1 ].
|
| There are more sophisticated models of the diode that use
| multiple exponentials. Remember that these are all just
| approximations of reality based on some simplifying
| assumptions, the only real question is "how much do you
| want to simplify", and linear models are very rarely
| useful for modeling diodes.
| GeneT45 wrote:
| Your comment can only be truly appreciated by plumbing the
| bizarre depths of "high-end audio" where nitwits (errr...
| customers) will pay $700 for a 6' Kapton-insulated power cord
| for their stereo. You know, to plug into the wall where it
| connects to 100 ft of Romex that cost $0.20/ft...
|
| Looks like my favorite example has disappeared, but there are
| always these: http://www.audio-consulting.ch/?Parts:Woodlenses
| dylan604 wrote:
| connect it to a foot pedal so a guitarist can use it to create
| a unique sound. it'll sell 10s of dollars!
| xani_ wrote:
| Unironically diode-to-photoresistor optocouplers are used in
| some compressors and filters to get that soft analoguey sound
| (as they are plain slow to react to signal). Which is kinda a
| supply problem as material they were made with was banned for
| toxicity
| squarefoot wrote:
| True. LDRs (photoresistors) are however still very easy to
| find on the usual online suppliers (Ebay, Aliexpress etc.)
| although they're banned in some areas for containing
| cadmium. I've used them successfully in LED+LDR
| arrangements to emulate Vactrols in vintage-style audio
| compressor circuits. To replicate the vintage compressor
| slow attack, in theory one should use a filament lamp+LDR
| pair so that the lamp slower attack and very slow decay,
| compared to a LED, would give that particular curve.
| However this is something that can be easily replicated
| using a RC network within the circuit that drives the LED.
| jeffbee wrote:
| Why would anyone choose these over galvanic isolation?
| klodolph wrote:
| It's _way_ cheaper. Like, it 's not even close.
|
| (Note that this IS galvanic isolation--I assume you are
| talking about transformers, which is the other common way to
| galvanically isolating circuits.)
| MisterTea wrote:
| I've used the IL300's to interface signals in a legacy machine to
| a PLC and found they were not very linear. Don't see how these
| 4N25's will be any better. Wound up using texas instruments/burr
| brown isolation amplifiers instead (ISO122 or something).
| exmadscientist wrote:
| Those are probably great if you can afford them. I've never had
| the budget for them in a production design, so haven't played
| with them much. Standard practice these days is just to toss a
| full ADC on the other side of the barrier and then isolate the
| SPI bus (or what-have-you). A few ADCs have the isolation built
| in, but for others there's stuff like TI's ISOW7841, which have
| 4 isolated digital lines plus isolated power in one package.
| It's all kind of expensive when added up, but it's cheaper than
| the old style analog isolation amplifiers, and you'll usually
| get more than one ADC channel once everything's together, so
| you win as soon as you want two signals to cross the barrier.
| MisterTea wrote:
| This was 10+ years ago and was a smaller internal project for
| two older machines so we didn't care they were $20 or $30
| each.
|
| And the isolated digital trick is used in the new industrial
| isolators. The neat part is since there's a micro in them
| they add in scaling and conditioning options which is nice.
| xani_ wrote:
| Did you put one of the diodes in the feedback loop to linearize
| them ?
| amelius wrote:
| Feedback is frowned upon in audiophile circles:
|
| https://www.audioholics.com/audio-amplifier/amp-myths-
| negati...
| MisterTea wrote:
| Yes but there were still linearity issues. The senior
| engineer played with it for a bit until he said screw it,
| spend the money and be done. Project was small enough to
| justify the cost.
| aidos wrote:
| I had a brief foray into electronics a few years ago hacking
| about with making a really crummy arduino synth. Part of that
| involved wiring it up to the midi output of a controller.
|
| The concept of an optocoupler is just beautiful - isolate the
| circuits from each other by transferring your signal using light!
|
| Looking at the diagrams here of how to increase the bandwidth
| through the device, electronics is all still total voodoo to me.
| klodolph wrote:
| The second circuit is actually really boring once you recognize
| the parts. From left-to-right, it's:
|
| 1. A voltage to current converter
|
| 2. An optocoupler
|
| 3. A buffer
|
| 4. An amplifier
|
| Stage #1 drives the optocoupler, since current is what gets
| turned into light inside the optocoupler.
|
| The output is high-impedance, so you add a buffer, which is #3.
|
| The output has a low-level, so you amplify it, which is #4.
|
| If you work through a chapter of an EE book that explains op
| amps, and you're not afraid of algebra, you'll find this stuff
| becomes approachable very quickly. Designing circuits like this
| is a bit more challenging, but reading them is very
| approachable.
| achr2 wrote:
| Low cost ethernet transformers have a wider bandwidth and better
| linearity. And would work with similar topologies.
| jleahy wrote:
| The core might saturate at these low frequencies?
| squarefoot wrote:
| If you need to electrically isolate an audio signal path, decent
| audio transformers aren't that expensive these days. A Triad
| Magnetics TY-250P is less than EUR6 at Mouser in single quantity,
| and is for sure a lot more linear than most optocouplers.
|
| https://eu.mouser.com/datasheet/2/410/TY_250P-1892894.pdf
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