Vactrol T4B

Hello !

I remember reading about using vactrols as substitutes for T4B somewhere in the forum...i cannot find this anymore. Someone remember how to do it ?

Some of you guys actually used this for different time responses on your LA2A´s ?

Thanks !

Here You go!
regards
Bernd

hmm,

Is this a viable option until I can get my hands on T4Bs?

MicDaddy said:

hmm,

Is this a viable option until I can get my hands on T4Bs?

Click to expand...

Yes!

bernbrue said:

MicDaddy said:

hmm,

Is this a viable option until I can get my hands on T4Bs?

Click to expand...

Yes!

Click to expand...

Thanks a lot !

Did you (and others) tried different vactrols for shaping the time response using this option ?

Must i assume that raising the resistor´s value will change the threshold ?

Thanks a lot !

i thought t4b was a vactrol? 8) 8) 8)

Hi,
I´ve got a pair of vactrols in a relay housing but haven´t tried them yet. Some say, the attack & release time were not comparable with a real T4B. Try them out and make your own mind. Actually it´s a matter of taste.
regards
Bernd

Great! Thanks a lot for this one

Hello guys.

I think it's worth asking this question here as it'll keep the forum "cleaner" and it's related with the above Schem.

Are the Resistors and the diode an extra? Eg. protection and to bring the photocells to a more useable range? Also, what is the effect of adding a capacitor?
I've got some of these matched photocells from https://sites.google.com/site/diypartsstore/catalog/new-t4b-opto-el-panels
to build one of these, more pics/info here:
http://webpages.charter.net/altaflux/jbl_pic/t4b_unit.html

The above would not be using vactrols but silonex http://www.alliedelec.com/images/products/datasheets/bm/SILONEX_INC/70136770.pdf

Anyone can shed some more light on the above q?

jplebre said:

Hello guys.

I think it's worth asking this question here as it'll keep the forum "cleaner" and it's related with the above Schem.

Are the Resistors and the diode an extra? Eg. protection and to bring the photocells to a more useable range? Also, what is the effect of adding a capacitor?

Click to expand...

The resistors are there because the Vactrol is a photo diode so it is essentially a current operated device. The resistors effectively convert voltage to current. The original T4B had a voltage operated photo luminescent strip not a photo diode.

The extra diode is there to provide a path for the other half cycle of conduction. If you don't have it the coupling capacitor just charges up - it's a charge pump - and your compressor doesn't work.

There is little if any point or effect in adding a capacitor other than to shunt the HF response a bit. In this circuit the attack and decay time are determined entirely by the opto electronic components.

Cheers

Ian

ruffrecords said:

The original T4B had a voltage operated photo luminescent strip not a photo diode.

Click to expand...

I feel such a dork! I really wasn't understanding it! Till I read this and then it all clicked
I thought the silonex were thelight sources! Not the panel! I was finding it hard how was I going to get 2 different readouts for the meter and GR.

Now I get it!

So, cool, I don't need the resistors as I'm using the panel but will I still need the diode even in this case?

Thank you very much for your reply Ian!

jplebre said:

Now I get it!

So, cool, I don't need the resistors as I'm using the panel but will I still need the diode even in this case?

Click to expand...

If you are using the electroluminescent panel then you do not need the extra diode.

Cheers

Ian

I do not want to be a smart-arse, but isn't a Vactrol an LED and an LDR in one package?

strangeandbouncy said:

I do not want to be a smart-arse, but isn't a Vactrol an LED and an LDR in one package?

Click to expand...

Correct.

Cheers

Ian

I guess just a vactrol is way too fast to use as T4B and will add a lot of distortion in LF and no compression there, if not this guy wouldn't work http://www.xqpaudio.com/545.html. (half wave compressor, just a vactrol with the signal feeding the led and LDR as any compressor)

Maybe a bridge and a attack/release internal controls will be nice, there is an idea, to get exact values some math are needed... Release need to be limited with a series resistor to not short the input.

JS

I think I remember reading about people using night lights in place of a T4B. Photosensitive cells are fun. Consider that light is faster than an electrical signal. Its why optical Internet is faster.

buildafriend said:

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

Click to expand...

??? 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.

joaquins said:

I guess just a vactrol is way too fast to use as T4B and will add a lot of distortion in LF and no compression there, if not this guy wouldn't work http://www.xqpaudio.com/545.html. (half wave compressor, just a vactrol with the signal feeding the led and LDR as any compressor)

Click to expand...

The attack and decay times of a Vactrol vary enormously between types and is determined by the properties of the optoresistive material and not by the LED. The ones used to replace the T4B do have similar attack and decay properties. There are others with much faster attack and decay times.

Maybe a bridge and a attack/release internal controls will be nice, there is an idea, to get exact values some math are needed... Release need to be limited with a series resistor to not short the input.

JS

Click to expand...

This is the classic way of setting attack and decay times. Your circuit will need a DC buffer between the RC circuit and the LED.

Cheers

Ian

ruffrecords said:

joaquins said:

I guess just a vactrol is way too fast to use as T4B and will add a lot of distortion in LF and no compression there, if not this guy wouldn't work http://www.xqpaudio.com/545.html. (half wave compressor, just a vactrol with the signal feeding the led and LDR as any compressor)

Click to expand...

The attack and decay times of a Vactrol vary enormously between types and is determined by the properties of the optoresistive material and not by the LED. The ones used to replace the T4B do have similar attack and decay properties. There are others with much faster attack and decay times.

Click to expand...

Sorry, I was missing that, VTL5C1 has a decay of 35ms and VTL5C4 about 1.5s, turn on time is also longer but not that much, 2.5ms to 6ms.

Maybe a bridge and a attack/release internal controls will be nice, there is an idea, to get exact values some math are needed... Release need to be limited with a series resistor to not short the input.

JS

Click to expand...

This is the classic way of setting attack and decay times. Your circuit will need a DC buffer between the RC circuit and the LED.

Cheers

Ian

Click to expand...

I was hopping that there is a way with such voltage to not need it but it should be imposible to drive by the original circuit...

JS

A typical IR LED will drop about 3V at a current of 3mA so it looks much like a 1K resistor. To get a 1.5s decay time would therefore require a capacitor of about 1500uF!

Cheers

Ian