Pro Audio Design Forum

In the past limiters with Vactrols were the norm. But the politically correct present seems to have make them almost disappear.

Actual VTL parts are hard to find, or every expensive (Farnell) or rather suspicious to buy (Ebay or other China suppliers).

But I found these guys in Czechoslovakia that seem to be resurrecting the part:

http://www.tesla-blatna.cz/en/products- ... c-elements

They even suggest a limiter circuit using it.

http://www.tesla-blatna.cz/_soubory/opt ... optron.pdf

Now the question is price and availability.

Or maybe they went away for good reason.

JR

The reason seemed to be that they were made with cadmio or lead, if I am not wrong. It was not a technical or quality reason.

I believe it was the Cadmium.

Silonex is now Advance Photonics http://advancedphotonix.com/optosolutio ... ocouplers/

Available from Digi-Key: http://www.digikey.com/product-search/e ... gory&chp=0

Also at Allied: http://www.alliedelec.com/search/result ... photocells

Allied stocks the Silonex and Excelitas. The Excelitas are the original Perkin Elmer/Vactrol line.

I've also used the H11F1 LED/FET opto for gates and limiters. It has the usual limitations of JFETs regarding THD and level.
The original H11F1 got replaced with a near identical part number.
I don't know what the status is of the current part.

Here in the US photocell-based products are still sold.

Not to be contrary just to be contrary but sometimes these are embraced just because they are old and perhaps their speed limitation inadvertently delivered transfer functions that people liked.

While at Peavey I participated in a tube-LDR based limiter (VCL), and it was well liked but in my judgement one could mimic the transfer function and even non-linearities of such approaches using modern DSP platforms if it really was worthwhile.

I guess I shouldn't complain too loudly as a new generation of hipsters embrace vinyl for no practical justification.

JR

Thanks for the update.

I composed a drawing, comparing the circuits both manufacturers propose.

I asked for help from support on both manufacturers. Let's see what they suggest.

The idea is to protect the signal from clipping peaks, which might save a recording in unexpected circumstances.

Apparently Vactrol like based circuits proved more transparent and having less distortion than others.

Did the design with optocouplers I attached showed well? On mine Firefox it doesn't.

Here's a simpler design, but I'm afraid every design works with its specific optocoupler, right?

Back in the late '70s I did a bench investigation into alternate gain elements, in light of the crude and expensive VCAs back then.

I sourced a center-tapped LDR and with the center-tap grounded I could use one half to linearize the gain law of the other half for use as an accurate gain law audio attenuator. I eventually abandoned it for even the old VCAs that were more flexible thanks to log gain control.

JR.

The drawing renders fine.

One clever application I saw using a Vactrol that couldn't be well-duplicated was in the Texar Audio Prism broadcast processor by Glen Clark.

At the input to the processor a Vactrol was used as the gain element at the center of a passive U-pad.
The input diff-amp followed the U-pad.
No matter how hard the jock pushed the console level it was impossible to overload the Texar's input.
You could clip the console to square waves (common with the overnight dudes) but you couldn't overload the processor.

Vactrols are still very popular in electronic music in phasers.
I would hate to see them completely obsoleted.

I think you could probably come up with a better limiter for this project though.
The attached design only detects one polarity.
Maybe clone a mini LA-4?

Unfortunately I won't be able to use the limiter at the input, as it should, because the chip I'm using (THAT 1510) won't allow it. That or any chip like it.

So I will be using one in the second stage. Many people, like Tascam, are doing that now.

Electronic differential inputs won't allow a Vactrol based limiter to work, only with transformers, because then you put the variable resistor in parallel with the gain largest resistor.

In differential inputs it's the opposite for the Rg. Lowest resistor increases the gain.