Does anyone know of a good replacement for the VTL5C9 optocoupler?

[JMFahey]

Sweet suggestion, I’d love to build my own! I’ll have to take a look at cells. As I mentioned earlier, I was told that the specs of the optocoupler were pretty important. Being that the cells you linked have an on resistance of 1k and off of 10kohm, I wonder how that would affect the circuit with the originals having an on of 1k and off of 2 megohms.

That 10k spec is nonsense, or better said, must be the LDR resistance when passing "X" milliamperes (or microamperes) through a LED or at some specified LUX level.
ALL LDRs I bought in the last 50 years, from these tiny 5mm ones to expensive Philips ones inside a glass vacuum tube bottle worked about the same, all went >1M in absolute darkness.
I´ve used thousands of them, for various purposes: limiters - compressors - voltage controlled filters - Wah Wah - remote attenuators/volume controls -channel switching in Tube Amps - you_name_it.

I’m guessing rise and fall time would be dependent on the LED?

Not on the LED but on LDR itself.
Reasonably fast attack, slow release.

I’d love to see some pics.

Here:
A Milion Dollars worth of Optocouplers (ok, 200 of them 😉 ) for about $40-50 in parts, what´s not to like?
As said before, I use LOTS of them:

I grind LED lens/dome flat.
Not strictly necessary but it improves mounting and makes Opto slightly shorter.
I align them on a small piece of tape, then add a drop of cyanoacrilate glue between them, or clear/translucid Epoxy.

Then fully wrap them in tape, push tape ends inwards with my nail.

Then heat shrink it.
While shrink is still hot and soft I press it flat with long nose pliers.
You can trim excess later.

I used red shrink because I had it on hand, fine unless LDR sits in direct sunlight or under a strong lamp (not a problem inside a chassis) but for peace of mind or OCD use black shrink.

Looking at the schematic above, what’s your opinion of the changes made? I was told by two separate individuals that the optocoupler used was a pretty special device. Maybe not???

Dan

Trim excess shrink tubing and if needed, bend leads.
Leave LED ones straight , LDR ones are way more flexible.

Specs roughly match your first datsheet, these work anywhere an original VTL does.

Are these closely matched to build a Hi Fi class voltage controlled stereo volume pot?
Doubt so (doubt originals do by the way), but that´s not the point.

Your Power Guard is strikingly close to my own Bass/Keyboard/PA Power Amp limiters, whose job is to attenuate input when clipping is detected, and there they work very well.
´Nuff said 😉

 

[sawyers]

Just for laughs I bought a XIVE VTL5C9. The Chinese knock off that seems to make noses turn up. XIVE being SHENZHEN FZONE TECHNOLOGY CO.,LTD https://xvive.com/audio/about/

Dark adapted the resistance of the photoelement was 5 gigaohms. So high it was difficult to measure and to all intents and purposes open circuit. At 1mA diode current the photoelement resistance was 430 ohms. When the LED was turned off, it went back to multiple Gohms within a second,

430 ohms at 1mA LED current is pretty much bang on for a VTL5C9.

I bought mine from https://www.thonk.co.uk/ , and it was a "gold star" because there was a gold dot on it. https://www.thonk.co.uk/shop/vactrols-vtl5c/

So whatever you think about the Chinese Vactrols, they do what they are supposed to.