Matching Vactrols for LPGs

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fluxmonkey
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Matching Vactrols for LPGs

Post by fluxmonkey » Sun Jan 22, 2012 8:18 pm

i built a pair of the Buchla/Thomas White LPGs as a dual module, using VTL5C3 vactrols. they're great, but they sound significantly different--one is much "tighter" than the other. I'm thinking this is probably due to different characteristics of the vactrols, like the response time or on/off resistance. any ideas on how to match vactrols? with limited test equipment (a DMM and basic scope)...

thx!

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L-1
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Post by L-1 » Sun Jan 22, 2012 9:30 pm

Maybe it is not very clever advise but try to solder wires from vactrol places to IC socket, put there your vactrols and choose by ear which will sounds acceptable.

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andrewF
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Post by andrewF » Mon Jan 23, 2012 4:37 am

Put a resistor across the LED, say 2k2

connect your DMM across the LDR and measure the resistances as you feed different voltages to the LED.
Take your time, let the values settle before taking a reading

you can make plots like this one and choose the vactrols with the most similar plots (ignore the negative voltages, that is just from the circuit I was using)

for off-time, a bit harder, you could set something up with LEDs to blink on/off, best would be with an oscillscope. Really tho, if the vactrols are never switching from full-on to full-off but are prancing about in the middle somewhere, off-time shouldn't be such a big issue, or as L-1 suggests choose by ear.
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Monobass
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Post by Monobass » Mon Jan 23, 2012 7:51 am

I think choosing by ear makes a lot of sense
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Post by Funky40 » Mon Jan 23, 2012 8:21 am

Monobass wrote:I think choosing by ear makes a lot of sense
never did it, but my idea was to setup a board that can be soldered (temporary) into the pcb,
that board would have terminals to attach the leads of the vactrols with screws.

had the sam problem, built a dual LPG.
one side was my best LPG the other my worst one. the difference was astounning on some sounds, on others not really a problem.
LPGs are alive :lol:


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Luka
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Post by Luka » Wed Mar 21, 2012 5:50 pm

here is a nice page showing vactrol measurement values

https://sites.google.com/site/flofxdiy/ ... asurements

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JJ
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Post by JJ » Wed Mar 21, 2012 5:55 pm

I'm building a quad lpg and I'm hoping each channel will behave differently... Then I can choose to taste depending on what I'm after! :hmm:

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ringstone
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Post by ringstone » Wed Mar 21, 2012 6:10 pm

I think the real issue with matching Vactrols is the size of the batch you would need to buy (and therefore the additional cost) to find enough that have a similar response. Especially given the price seems to have just about doubled recently at one popular supplier...

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Luka
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Post by Luka » Wed Mar 21, 2012 6:16 pm

True

I mostly added that content to help match home brew ones to current vactrol brand optoisolators

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ringstone
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Post by ringstone » Wed Mar 21, 2012 6:19 pm

Luka wrote:True

I mostly added that content to help match home brew ones to current vactrol brand optoisolators
:tu: I think there may well be a lot more of that going on in the near future given current trends...

Cheers
Blair

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Post by gwaidan » Wed Mar 21, 2012 8:24 pm

I used VTL5C3/2s in the 5 TW LPGs I've built so far and they all seem pretty good. I wouldn't expect too much from hand-matching-ie Silonex's sorted grades only have a 10% tolerance...

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Post by Low-Gain » Wed Mar 21, 2012 8:32 pm

i like the idea of all of my lpg's being slightly different. ;)

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Post by Moog$FooL$ » Wed Mar 21, 2012 8:34 pm

andrewF wrote:Put a resistor across the LED, say 2k2

connect your DMM across the LDR and measure the resistances as you feed different voltages to the LED.
Take your time, let the values settle before taking a reading

you can make plots like this one and choose the vactrols with the most similar plots (ignore the negative voltages, that is just from the circuit I was using)

for off-time, a bit harder, you could set something up with LEDs to blink on/off, best would be with an oscillscope. Really tho, if the vactrols are never switching from full-on to full-off but are prancing about in the middle somewhere, off-time shouldn't be such a big issue, or as L-1 suggests choose by ear.
this seems like REALLY solid advice.
thanx for this af.

:tu: :tu: :tu:

myself i would have thought it to difficult to test these devices without the use of fancy gear..... i would think matching by "ear" the way to go.

af's post seems workable.... at least on paper. :despair:

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Post by emdot_ambient » Wed Mar 21, 2012 8:53 pm

Yikes, never even considered this...I've got 4 of these to build (2 dual panels)...AND I'm planning on having a switch to go between fast/slow Vactorls = Double the Vactrol matching!
:woah:

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Post by cleaninglady » Wed Mar 21, 2012 10:08 pm

Low-Gain wrote:i like the idea of all of my lpg's being slightly different. ;)
+1

That being said ; an idea Thomas White mentioned in an email was using butchered IC sockets for the vactrols so you can change them out.
This would be helpful if you wish to experiment with response times of different Vactrols , eg : VTL5C2 and VTL5C4.

But i also agree with Blair here ; they are a very expensive item to have to buy a bunch of just to match them.

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Post by tokyomatik » Thu Mar 22, 2012 12:47 am

also the idea of homebrew vactrols for me sound really interesting, could be useful also for future projects and to experiment with the photocells doesn't look so expensive:
http://www.banzaimusic.com/Photocells/
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emdot_ambient
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Post by emdot_ambient » Thu Mar 22, 2012 6:46 am

The question is, can you find photocells with the On/Off times of a VTL5C3? I looked at a few last night and they were pretty far off the specs we're used to in Vactrols...but it was only a cursory look.

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Post by CJ Miller » Thu Mar 22, 2012 1:04 pm

cleaninglady wrote:...an idea Thomas White mentioned in an email was using butchered IC sockets for the vactrols so you can change them out.
This would be helpful if you wish to experiment with response times of different Vactrols , eg : VTL5C2 and VTL5C4.
Agreed. When in doubt, socket everything and anything.

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Post by Monobass » Thu Mar 22, 2012 3:30 pm

emdot_ambient, did you do the fast/slow switch in the end? Did it work ok?
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Post by JohnP » Sun Dec 21, 2014 6:10 am

I want eight variable gain channels, all controlled from one AGC circuit.

Note 5 at the end of this data sheet shows typical matching is 25%:
http://denethor.wlu.ca/pc300/optoisolat ... uction.pdf

Has anyone tried to match Vactrols by adding a trimming resistor in series with the LED?

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Post by fuzzbass » Sun Dec 21, 2014 6:41 am

I'm with JJ here. I have the Thomas White Quad lop ass gate and each channel is different. I like it. I plan to take it apart and replace two of the VTL5C3s with VTL5C4 for slop ass gates.
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Post by JohnP » Sun Dec 21, 2014 10:32 am

I think I just came up with THE way to do it.

Use an LM3914 in the bargraph mode to drive the Vactrol LEDs.

Connect a voltage controlled current sink to pin 7, to adjust the Vactrol LED current.

A 2.49k resistor in the current sink gives 20mA LED current with 5v control voltage.

No trimmers required. The LM3914 is designed to supply the same current to all the LED's.

muuhahaha

Any one have a bunch of the cheapie Silonex parts on hand to test this?

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Post by monstrinho » Sun Dec 21, 2014 10:48 am

The easiest way is to use a breakaway SIP socket like this (just as an example). This allows you to select them by ear. It also allows you you to test "roll-your-own" vactrols (LED->LDR + heat shrink) if you want to. with prices going up again, I think I'll be trying this method for a few projects.

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Poldenstein
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Post by Poldenstein » Sun Dec 21, 2014 12:42 pm

I made sockets for the dual vactrols on my thomas white dual LPG as I wanted to try out different lengths. I used single row sockets that I cut to the purpose. I ended up with a faster pair on one channel and a slower one on the other, for the sake of versatility.
If one means to do some testing on critical components, sticking there a socket won't hurt and isn't all that bothersome.
Also, you can skip altogether the hassle of matching by getting matched pairs like the Vactec VTL5C3/2 , 4/2 etc.

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Post by JohnP » Mon Dec 22, 2014 7:59 pm

I have two of the VTL5C3's on hand.

I set up two non-inverting amplifiers with 20kΩ feedback resistors, and the CDS cells are the input resistors, connected to ground.

With 0.2vPP sine wave input and 20.0mA LED current in both:

The output from one channel was 4.00vPP, the other one was 3.78vPP.

So at a gain of 20 they are within 6% of each other.

I ordered 20 of the NSL-32SR2 and four of the dual VTL53C/2 to see what the variation is.

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