VTL5C3 how to role your own?

[garfo]

I'm looking at finding the right parts to simulate a VTL5C3 optocoupler to be used in a compressor.
I wouldn't mind buying these if the prices weren't as high as they are at the moment, and I might need a few.

Has anyone roled out a DIY version of the VTL5C3 that stays close to the specs? If so, which LDR did you use and LED colour and type shoukld go with that specific LDR?

Thank you in advance, hopefully someone can help!!!

By the way, I have opened an Xvive and the LED is Green, diffused. The LDR when on reads 11k and off was more than 30M.

[Aentons]

I don't have part numbers but this may be helpful to help try and figure it out.

https://sound-au.com/project200.htm

[garfo]

Thank you, I am aware of this and couldn't really find what I am looking for here, unfortunately.

[Aentons]

The SKU A-1528 LDR at Tayda is 540nm which matches green wavelength sensitivity and has a range of 10-20k

https://www.taydaelectronics.com/photo-conductive-cell-resistor-ldr-540nm-radial.html

[garfo]

Unfortunately, those are not high resistance enough when off. they should be 10M (don't mind if it's 20M. ALso, I have used a lot of LDR (I believe from Tayda) and they are way of those specs. They hundreds of ohms when on and 200M and above when off.

[matmosphere]

I've read elsewhere that the vtl5c3 is pretty difficult to emulate with an led/ldr configuration. I'm guessing you're making something like the Hollis Flatline?

I think Brian discusses it some in either the 4:1 or afterlife (both based on the flatline) build doc.

[DLW]

I'm not sure what your budget is, but cabintech has xvive VTL5C3 for <$7 a piece...
https://cabintechglobal.com/vtl5c3xv

[midwayfair]

I've never found a discrete LDR that comes close to the performance of the VLT5C1 or C3.

The XVive clones are quite good and $7 is pretty darn reasonable. Smallbear's were not (they were fine, they were just very different.)

[Aentons]

I've never found a discrete LDR that comes close to the performance of the VLT5C1 or C3.

The XVive clones are quite good and $7 is pretty darn reasonable. Smallbear's were not (they were fine, they were just very different.)

I'm curious what the performance factors are (in order of importance)? I've read others say the xvive "tail" (decay curve) is off. I've also read that stompbox applications don't typically need the extreme range and that middle of the road is good enough.

Also, I see at digikey, most of the ldrs have a green sensitivity range and there aren't many available anywhere within the red range. Could it be that we are mis-matching the reds and greens? Please forgive my ignorance

[Aentons]

I'm sure some of you have seen this, but for those that have not, this is the document referenced in the project 200 article. There is lots of good info that may be gleaned from it.

Photoconductive Cells and Analog Optoisolators (Vactrols®) - PerkinElmer Optoelectronics

https://arduino.benarent.co.uk/datasheet/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf

[danfrank]

Lag times are also important for many LDR applications. This is where a lot of the mismatches occur. There's enough LDRs out there where light/dark resistances can be matched but the lag times are different. Modulation pedals are where LDR lag times become important.

[midwayfair]

I'm curious what the performance factors are (in order of importance)?

Not in order (not even sure what that would entail):
on resistance
off resistance
total dynamic range (distance between the on/off in decibels)
current draw of the led (PE vactrols were an IR LED I believe)
time to reach on resistance
time to reach off resistance

For a compressor, the timing characteristics of the vactrol don't matter as much unless you're looking for a really fast attack (in which case, why the hell are you building an optical comp) or in a certain compressor using the VTL5C10 that basically relies entirely on the characteristics of the vactrol because the designer couldn't be arsed to put a proper detector circuit in. The timing characteristics are much more important in a modulation pedal. The dynamic range is important but unless the compressor is using ALL of the range, you can almost certainly make up for that. One of either the on or off resistance will actually be important though, because one of them is defining a ceiling or floor to the leveling.

[garfo]

Thank you guys for all the info.
What do you think about this?

I took new readings from the Xvive VTL5C3 with another multimeter. The on Resisance is spot on but the dark resistance is insane. My multimeter reads up to 200M and while taking the reading, it just stopped reading. This means that when it's dark it measures more than 200M.
Isn't this completely off specs, and if so, how is this a good quality Vtlc5 replacement?

[Zerro]

Simplyfied, if your compressor has max. ratio 1:50, your gain-segment must change only cca in the same ratio. So I would use white light LED for maximal impact in while spectrum range (maybe higher liminiscence) and ANY little speeder LDR. Maybe that will do all you need without looking for some border parametred LDRs. Depending at schematics of your device of course. Try to count gain of your compressor with parameters you can reach now and compare with those tens of megaohms ended values. Maybe mean changes are only between hundreds of kiloohms and ohms.

[garfo]

I forgot to mention that the circuit I'm basing it in is the Diamond Compressor. I have no idea what a homemade optocoupler would behave like in this circuit. For me it would be easy to use LDR's that go from hundreds of ohms up to above 200M matched with a White LED. My concern is what about the other specs, would they matter in that circuit? (The compression is gain dependent).