Vactrol T4B

[buildafriend]

Consider that light is faster than an electrical signal. Its why optical Internet is faster.

??? Consider, it's not. In your terms, you need an electrical signal to turn the light on, so signal is first. When electrical signal stops, being first again, your light takes some time to stop shining. Transport of light is faster though, but distance inside your T4B isn't miles/continents apart.

Gotcha. Thanks harpo, you have helped me a lot on the past.
light = 3x10^8 m/s
electrical signal is approx 2/3 that

but,

it go's electrical signal into light signal therefore the electrical signal is dominant.

Please excuse (my dear aunt sally) my deathly annoying newbness but I'm wondering why the T4B was used. I read here http://www.kenetek.com/T4AT4BUnits/AboutT4AT4BT4CElectroOpticalAttenuators/tabid/201/Default.aspx that it has a big impact on the sound (in terms of "smoothness") but is that it's most important attribution to the unit?

if this is annoying question free feel to ignore it. thanks regardless.

 

[ruffrecords]

Please excuse (my dear aunt sally) my deathly annoying newbness but I'm wondering why the T4B was used.

At the time it was designed, just about the only other form of gain reduction was vari-mu. The T4B led to a somewhat simpler circuit compared with, say, the Fairchilds of the time.

Cheers

Ian

 

[Harpo]

Ian, thanks for jumping in.
@buildafriend, you might find this thread useful in answering your question.
There are other T4B threads worth reading.

 

[AUDIO FREQ]

This is a 10 year old thread, but I think I want to try and make a T4B with some fast vactrols using the schematic posted by bernbrue. Should I try some of the newer optocouplers like https://www.mouser.com/datasheet/2/1418/DS_NSL_32SR2-2950649.pdf

I could get 2 of them of course. But how would I match them or test them?

 

[Zhyyra]

Glad you "bumped" this AF!

I'm planning a similar route myself. I'm also thinking of a couple of LRD's closing off a little plastic tube with LED's inserted in at the opposite ends (effectiveley building a vactrol). Surely using LEDs instead of the EL phosphor strip will be advantageous due to the decade performance of the EL strip from use and aging.

Zhy

 

[AUDIO FREQ]

Glad you "bumped" this AF!

I'm planning a similar route myself. I'm also thinking of a couple of LRD's closing off a little plastic tube with LED's inserted in at the opposite ends (effectiveley building a vactrol). Surely using LEDs instead of the EL phosphor strip will be advantageous due to the decade performance of the EL strip from use and aging.

Zhy

I think I want to try the LED method as well. Which LED's and LDRs are you looking to try?

 

[Zhyyra]

I have not really decided yet AF but, have been looking for some LDRs that would emulate similar characteristics to the regular T4Bs.
something like the ones described in the attached PDFs (look lower down the list for GL20537 in the GL series doc).

The optocouplers that you refer to also look like a viable option though I do note a typical 5ms rise time which would give the compressor a faster attack the LDRs commonly used in the LA-2As of the past had a typical response time of 10ms. Whether this would result in any noticeable difference or be insignificant, I do not know.

The LEDs that I would use I have not decided on yet but, I would imagine that it is just a matter of matching the LED output wavelength with and LDRs response wavelength quite closely.

I (no doubt) have a bit of researching to do.

Zhy

 

[punk]

the t4b i use is with the VTL5C4/2 vactrol. i had no luck with any other vactrol in trying to match a t4b. i use 115k resistors. most any vactrol should work but getting the slow release of a real t4b is hard to find. i did not use the diode but probably a good idea. i had to go through a number of vactrols to get one or two that would match a t4b. but i actually found vactrols with a somewhat faster release more useable for my purposes. kenetek did tests on this for me and the recovery graphs were almost identical to a t4b. sorry i no longer have the graphs.

thanks to kev for drawing the schematic that bernbrue posted.
byron

 

[Zhyyra]

Thanks for your input punk.

The attack is still slow at an effective 6ms and I do note the (relatively) fast release of the VTL5C4/2 at only 1.5 seconds.
mmmm I see experiments on the horizon.

Zhy

 

[AUDIO FREQ]

the t4b i use is with the VTL5C4/2 vactrol. i had no luck with any other vactrol in trying to match a t4b. i use 115k resistors. most any vactrol should work but getting the slow release of a real t4b is hard to find. i did not use the diode but probably a good idea. i had to go through a number of vactrols to get one or two that would match a t4b. but i actually found vactrols with a somewhat faster release more useable for my purposes. kenetek did tests on this for me and the recovery graphs were almost identical to a t4b. sorry i no longer have the graphs.

thanks to kev for drawing the schematic that bernbrue posted.
byron

Thank you for this. I actually would like a faster release. Which faster vactrol did you come to like?

 

[punk]

the VTL5C4/2 was the only vactrol that would work for me. about 2 or 3 out of ten would work like the original t4b. the rest would release somewhat quicker. i'd call it 'medium' release compared to the slow release of the t4b. but for tracking i found it more useable.
all other types of vactrols that i tried released very fast. that was about 20 years ago. you may have different experiences today. i'm pretty sure i got all the vactrols i tried from allied electronics. i built one la2a as close to the original using jahnsen's book with a real t4b and utc iron. i built another with jensen iron and my vactrol t4b and probably use it more. ymmv

and it wasn't long after i'd done this and some clever folks here sourced the original ldr's and light panels to make an exact copy. lots of options.

byron

 

[Zhyyra]

Thanks again Byron. Lots of options indeed.
I'm sort of sure (and hoping) that some LDRs with longer release times are available now (20 years on) and that the right combination of LDR and LED will give a satisfactory result. I do acknowledge that "satisfactory" would be application and person dependent.

AF, I've just noticed that the release (decay) spec on the DS_NSL_32SR2-2950649 vactrol component that you provided the link for has a release of 80ms on the spec sheet. That's the slowest I've come across by far. Even the stand-alone LDRs that I've looked at average about 10 to 20ms.

Anyhow here is an article by Rod Elliot on DIY vactrols that I found quite interesting and helpful.
DIY Vactrol

Zhy

 

[AUDIO FREQ]

Here You go!
regards
Bernd

If using variable resistors in place of the 220K, would that adjust the attack and release times?

 

[AUDIO FREQ]

Just wanted to report, I bought a Vactec Vactrol VTL5C10, and boy is it sensitive. On plugin doctor, it shows it compressing sooner, and harder (knee) than the VTL5C2. I am going to buy a C9...trying to find one with the shallowest and softest knee (slope).

Any ideas anyone?