Vactrols - an idea for you to grab
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SonsOfThunder
Well-known member
Umm... what's wrong with the attack time of a vactrol now? I'll have to look at the spec sheets again but I didn't think that they were so slow, not as fast as a FET, but if you wanted more speed why not use a FET?
Sorry Llamaman, my little brain is not following the purpose, though I see your thought, I think :shock:
Peace!
Sorry Llamaman, my little brain is not following the purpose, though I see your thought, I think :shock:
Peace!
By "Vactrol" I assume you mostly mean opto-resistors with a light source, not necessarily an LED.
> parallel a second resistive element of a vactrol to the primary gain reducing element
This is no different from one opto-R with a 2X resistor (except it costs more).
> keep both of the vactrols near gain-reducing state with steady CV
This is hard to do. The resistance of an opto-R is never the same twice. It is slightly temperature dependent and VERY dependent on recent history.
> control this 2nd vactrol with some short of a sharp spike derived from the SC signal..?
You can do that with one opto-R. Most opto-compressors do. Feedback opto-compressors drive high light onto the opto-R until it gains control. There are more complicated ways to get a "rate" signal to "slam" the opto-R into action.
> Calibrating this mess to something that sounds nice could be a pain
It is hard to even get it to measure "nice". The main failing of my 4-opto system was opto matching and tracking. After a few years it had drifted so bad I mostly disabled the second section.
> other end to a dc voltage to adjust how close to its on voltage you want.
DC in the audio opto-R will thump. You can use a second "tracking" opto-R, and this works well enough as a meter (see LA-2). But it does not track well enough for gain control unless you get very carefully made cells, age them in light and dark, and then match them about 6 ways. Yield would be very poor: two that match the same dark may mis-match in likght, or half-light, or end up the same but attack and decay very differently.
We probably want a single-substrate opto-R cut into 4, 6, or 8 elements and laser-trimmed. Since the biggest demand for opto-Rs was as oil-burner flame-out sensors, the opto-makers aren't too interested in our problems.
> what's wrong with the attack time of a vactrol now?
The very fast ones will drop in less than a millisecond, but rise almost as fast, so they need a full sidechain with filter. The long-release ones that can decay slow enough for good audio intrinsically have long attacks, 10-100mS. The LA-2 units seem to be a blend of fast and slow, and there does not seem to be anything quite like it in production.
Hmmmm.... a fast paralleled with a slow might give the right time-action. But it might be insane to get them to agree what level to limit to.
> parallel a second resistive element of a vactrol to the primary gain reducing element
This is no different from one opto-R with a 2X resistor (except it costs more).
> keep both of the vactrols near gain-reducing state with steady CV
This is hard to do. The resistance of an opto-R is never the same twice. It is slightly temperature dependent and VERY dependent on recent history.
> control this 2nd vactrol with some short of a sharp spike derived from the SC signal..?
You can do that with one opto-R. Most opto-compressors do. Feedback opto-compressors drive high light onto the opto-R until it gains control. There are more complicated ways to get a "rate" signal to "slam" the opto-R into action.
> Calibrating this mess to something that sounds nice could be a pain
It is hard to even get it to measure "nice". The main failing of my 4-opto system was opto matching and tracking. After a few years it had drifted so bad I mostly disabled the second section.
> other end to a dc voltage to adjust how close to its on voltage you want.
DC in the audio opto-R will thump. You can use a second "tracking" opto-R, and this works well enough as a meter (see LA-2). But it does not track well enough for gain control unless you get very carefully made cells, age them in light and dark, and then match them about 6 ways. Yield would be very poor: two that match the same dark may mis-match in likght, or half-light, or end up the same but attack and decay very differently.
We probably want a single-substrate opto-R cut into 4, 6, or 8 elements and laser-trimmed. Since the biggest demand for opto-Rs was as oil-burner flame-out sensors, the opto-makers aren't too interested in our problems.
> what's wrong with the attack time of a vactrol now?
The very fast ones will drop in less than a millisecond, but rise almost as fast, so they need a full sidechain with filter. The long-release ones that can decay slow enough for good audio intrinsically have long attacks, 10-100mS. The LA-2 units seem to be a blend of fast and slow, and there does not seem to be anything quite like it in production.
Hmmmm.... a fast paralleled with a slow might give the right time-action. But it might be insane to get them to agree what level to limit to.
SonsOfThunder
Well-known member
Might give a look at the Perkin-Elmer sheets and app notes as well.
Its cool to be talking DIY on an international basis. You are waking and I am sleeping!
Peace!!
Charlie
Its cool to be talking DIY on an international basis. You are waking and I am sleeping!
Peace!!
Charlie
[quote author="PRR"] You can use a second "tracking" opto-R, and this works well enough as a meter (see LA-2). But it does not track well enough for gain control unless you get very carefully made cells, age them in light and dark, and then match them about 6 ways. Yield would be very poor: two that match the same dark may mis-match in likght, or half-light, or end up the same but attack and decay very differently.
We probably want a single-substrate opto-R cut into 4, 6, or 8 elements and laser-trimmed. [/quote]
Dual Vactrols are not so uncommon. These have a single LDR which is center-tapped. I've played with two types of these, VTL5C4/2 and VTL5C3/2 (from memory; must look up the exact numbers to be sure).
The two halves can have slightly different absolute values (I measured this), but I expect them to track nicely (memory effect and aging included).
I have _not_ measured this, mind you, but I expect this because it's the same, center tapped, element.
So you might be able to build a servo system with these, one half for audio, other half for sensing a DC for the control loop. There might be a problem with the (small) resistance from th ecenter tap that is shared by both elements. Haven't tried this. But I know that Moog have used these things for a compander system in their classic 12 stage phaser: One half for the compressor, other half for the expander. The control signal is quite unusual in this circuit: instead of the usual fast attack / slow decay they have fast attack / some ten milliseconds of "hold" / fast decay, which creates some kind of staircase signal instead of ramps. I meantion this for one reason only: This "staircase-compressed" signal sounds rather aweful after the compressor, but very smooth again after the expander. So the two staircase operations cancel each other pretty well, which tells something about the matching of the two halves of th edual vactrol. I've built this on bread board some months ago, using VTL5C3/2s.
Another circuit worth mentioning is Don Buchla's ring modulator. He built a four quadrant multiplier with two vactrols for the audio signal, and another two vactrols to form a servo loop. AFAIK, he used VTL2C3's (same thing as 5C3, but other enclosure). And we're talking about full scale audio rate modulation here! (one audio range signal modulation another audio range signal). So I guess (I have not built this, but I want to try this some time), I guess vactrols can be made to act much faster than expected with just the right amount of cuircuitry - and not to forget: A lot of component matching and trimming. (Buchla's circuits are (in)famous for being expensive and hand-adjusted.)
JH.
We probably want a single-substrate opto-R cut into 4, 6, or 8 elements and laser-trimmed. [/quote]
Dual Vactrols are not so uncommon. These have a single LDR which is center-tapped. I've played with two types of these, VTL5C4/2 and VTL5C3/2 (from memory; must look up the exact numbers to be sure).
The two halves can have slightly different absolute values (I measured this), but I expect them to track nicely (memory effect and aging included).
I have _not_ measured this, mind you, but I expect this because it's the same, center tapped, element.
So you might be able to build a servo system with these, one half for audio, other half for sensing a DC for the control loop. There might be a problem with the (small) resistance from th ecenter tap that is shared by both elements. Haven't tried this. But I know that Moog have used these things for a compander system in their classic 12 stage phaser: One half for the compressor, other half for the expander. The control signal is quite unusual in this circuit: instead of the usual fast attack / slow decay they have fast attack / some ten milliseconds of "hold" / fast decay, which creates some kind of staircase signal instead of ramps. I meantion this for one reason only: This "staircase-compressed" signal sounds rather aweful after the compressor, but very smooth again after the expander. So the two staircase operations cancel each other pretty well, which tells something about the matching of the two halves of th edual vactrol. I've built this on bread board some months ago, using VTL5C3/2s.
Another circuit worth mentioning is Don Buchla's ring modulator. He built a four quadrant multiplier with two vactrols for the audio signal, and another two vactrols to form a servo loop. AFAIK, he used VTL2C3's (same thing as 5C3, but other enclosure). And we're talking about full scale audio rate modulation here! (one audio range signal modulation another audio range signal). So I guess (I have not built this, but I want to try this some time), I guess vactrols can be made to act much faster than expected with just the right amount of cuircuitry - and not to forget: A lot of component matching and trimming. (Buchla's circuits are (in)famous for being expensive and hand-adjusted.)
JH.
btw Mr. JH,
slightly off topic but, I saw in your site sometime ago some limiting circuit that I found interesting. Got back there to check it again and I don´t find it anymore. It´s vactrol based where the control is done by some ammount of the output from the gain block ( ssm2017 I think) thru a circuit then the resistive element between the + and - inputs... does it sound good ? If so can you post or point it?
thnx!
:sam:
Fabio
slightly off topic but, I saw in your site sometime ago some limiting circuit that I found interesting. Got back there to check it again and I don´t find it anymore. It´s vactrol based where the control is done by some ammount of the output from the gain block ( ssm2017 I think) thru a circuit then the resistive element between the + and - inputs... does it sound good ? If so can you post or point it?
thnx!
:sam:
Fabio
[quote author="Bauman"]btw Mr. JH,
slightly off topic but, I saw in your site sometime ago some limiting circuit that I found interesting. Got back there to check it again and I don´t find it anymore. It´s vactrol based where the control is done by some ammount of the output from the gain block ( ssm2017 I think) thru a circuit then the resistive element between the + and - inputs... does it sound good ? If so can you post or point it?
thnx!
:sam:
Fabio[/quote]
There's some broken links on my web site. Must fix that some day, but also had (maybe still have) ftp problems.
I'll check this as soon as I can. Circuit had an error, btw: 2x 4k7 resistors (as part of the voltage divider with the LDR) missing.
Works quite impressive, ultra-quiet, which means you can get extremely long sustain from a clean input signal. Only downside was that after a long absence of any signal, the first transient comes in way too loud. Dark adapted / memory effect stuff I assume. Could easily be fixed with a little bias current trhru the LED I guess, but never really felt enough pressure to really do this tiny modification. Always too much projects going at the same time. (;->)
JH.
slightly off topic but, I saw in your site sometime ago some limiting circuit that I found interesting. Got back there to check it again and I don´t find it anymore. It´s vactrol based where the control is done by some ammount of the output from the gain block ( ssm2017 I think) thru a circuit then the resistive element between the + and - inputs... does it sound good ? If so can you post or point it?
thnx!
:sam:
Fabio[/quote]
There's some broken links on my web site. Must fix that some day, but also had (maybe still have) ftp problems.
I'll check this as soon as I can. Circuit had an error, btw: 2x 4k7 resistors (as part of the voltage divider with the LDR) missing.
Works quite impressive, ultra-quiet, which means you can get extremely long sustain from a clean input signal. Only downside was that after a long absence of any signal, the first transient comes in way too loud. Dark adapted / memory effect stuff I assume. Could easily be fixed with a little bias current trhru the LED I guess, but never really felt enough pressure to really do this tiny modification. Always too much projects going at the same time. (;->)
JH.
Ha, I just checked my old site at T-Online, and the file is still there!
http://home.t-online.de/home/jhaible/hj_preco.gif
As I said, the drawing is missing two resistors. Add two 4.7k resistors in series with the 1uF input capacitors each.
JH.
http://home.t-online.de/home/jhaible/hj_preco.gif
As I said, the drawing is missing two resistors. Add two 4.7k resistors in series with the 1uF input capacitors each.
JH.
jhaible's opto-limiter, drawing slightly cleaned-up and resistors added.
[quote author="PRR"]jhaible's opto-limiter, drawing slightly cleaned-up and resistors added.[/quote]
Thanks for drawing the resistors in, PRR.
But before everybody is running to build this thing, be warned: As it is, it has the problem described above (dramatic overshot on first note played after a long rest), and it was never intended to work on a wide variety of audio material. It's my take on an alternative input amp for the EMS HiFli, which was used to process guitar signals. Not being a guitar player, my approach is using a physical modelling synthesizer (Yamaha VL7) to create a clean guitar sound, and then process it with the HiFli clone, of which the input part is that opto limiter. (You can see a picture of this device here: http://www.oldcrows.net/~jhaible/hifli_clone_for_sale/hifli/DSCN2336.JPG)
But the one thing that might be of general interest is the extremely wide gain reduction range that is possible with a vactrol circuit like this. I can get (almost) infinite sustain from a (VL7-created) guitar sound that decays quickly; in fact the compressor gets a clean signal even when the original signal is barely audible anymore. This is great when the input has echo and reverb effects, as the compressor also pulls the fading echoes up to normal level, until you play a new note which will push it back down. Sort of auto-ducking.
JH.
Thanks for drawing the resistors in, PRR.
But before everybody is running to build this thing, be warned: As it is, it has the problem described above (dramatic overshot on first note played after a long rest), and it was never intended to work on a wide variety of audio material. It's my take on an alternative input amp for the EMS HiFli, which was used to process guitar signals. Not being a guitar player, my approach is using a physical modelling synthesizer (Yamaha VL7) to create a clean guitar sound, and then process it with the HiFli clone, of which the input part is that opto limiter. (You can see a picture of this device here: http://www.oldcrows.net/~jhaible/hifli_clone_for_sale/hifli/DSCN2336.JPG)
But the one thing that might be of general interest is the extremely wide gain reduction range that is possible with a vactrol circuit like this. I can get (almost) infinite sustain from a (VL7-created) guitar sound that decays quickly; in fact the compressor gets a clean signal even when the original signal is barely audible anymore. This is great when the input has echo and reverb effects, as the compressor also pulls the fading echoes up to normal level, until you play a new note which will push it back down. Sort of auto-ducking.
JH.
