VACTROL VTL5C3

ruffrecords · Feb 1, 2012

I have been testing a VTL5C3 low cost Vactrol. The spec says its resistance should be around 30K ohms at 1mA current but I measure about 7K. I have checked at a range of currents and in all cases the measured resistance is less than a quarter of what it should be according to the data sheet. I am confident the currents are correct as I am feeding the LED via a 10K resistor from a variable supply and measuring the voltage across the 10K so 10V = 1mA. I am using a regular meter to measure the cell resistance and I have checked the meter on fixed resistors of similar values to verify it is reasonably accurate.

So I am puzzled as to why I get consistently wrong values. Anyone had experience of these or can how any light on the subject?

Cheers

Ian

Edit: I bought two of these to play with so I just connected up the second one to make sure I did not have a broken one but I get very similar readings.

shabtek · Feb 2, 2012

only 1,could it be a dud

ruairioflaherty · Feb 2, 2012

Is the 10K resistor definitely measuring 10K?

My understanding of Vactrols in general is that they have fairly wide deviations from spec and in order to make use of them in dynamics circuit heavy feedback is used to regulate their behavior.

bruce0 · Feb 2, 2012

I have a bunch of these VTL5C3... "harvested" from old altec and rtl equipment. I can check one but not till Feb6 (when back in town) if this is not solved by then I will check to see how "out of spec" they are.

I have checked VTL5C3 and 5C2 before and found them relatively linear, but I have never checked for absolute resistance (I adjusted current to center the compressor in the linear region).

ruffrecords · Feb 2, 2012

shabtek said:
only 1,could it be a dud

That's what I thought but I have now tested another and it is the same.

Cheers

Ian

ruffrecords · Feb 2, 2012

bruce0 said:
I have a bunch of these VTL5C3... "harvested" from old altec and rtl equipment. I can check one but not till Feb6 (when back in town) if this is not solved by then I will check to see how "out of spec" they are.

I have checked VTL5C3 and 5C2 before and found them relatively linear, but I have never checked for absolute resistance (I adjusted current to center the compressor in the linear region).

That would be good - I am not in a rush so a few days wait is fine. I suspect most applications need some sort of calibrate procedure which may explain why this has gone relatively unnoticed.

Cheers

ian

abbey road d enfer · Feb 2, 2012

ruffrecords said:
So I am puzzled as to why I get consistently wrong values. Anyone had experience of these or can how any light on the subject?

Absolutely standard with Vacrol's. Their actual tolerances are ocean-wide.
I used thousands of them and I always had to cater for these large variations in the design (trimpots are mandatory except in switching applications).
Silonex are a tad better in that respect, but still they need adjustment.

ruairioflaherty · Feb 2, 2012

abbey road d enfer said:
Absolutely standard with Vacrol's. Their actual tolerances are ocean-wide.
I used thousands of them and I always had to cater for these large variations in the design (trimpots are mandatory except in switching applications).
Silonex are a tad better in that respect, but still they need adjustment.

Do you find that they drift over time or is the deviation from spec set in stone? I've read about again in T4B but my impression was that it was more EL panel than the LDR.

gemini86 · Feb 2, 2012

I've read that the optos have a huge variance. There was a discussion on the matching process in the t4 clones being offered currently. Wouldn't want that job.

ruffrecords · Feb 2, 2012

abbey road d enfer said:
ruffrecords said:

So I am puzzled as to why I get consistently wrong values. Anyone had experience of these or can how any light on the subject?

Click to expand...

Absolutely standard with Vacrol's. Their actual tolerances are ocean-wide.
I used thousands of them and I always had to cater for these large variations in the design (trimpots are mandatory except in switching applications).
Silonex are a tad better in that respect, but still they need adjustment.

Thanks for that. It is encouraging in the sense I am probably not doing anything silly but disappointing from the design point of view. Thanks for sharing your experience.

Cheers

Ian

abbey road d enfer · Feb 2, 2012

ruairioflaherty said:
Do you find that they drift over time or is the deviation from spec set in stone? I've read about again in T4B but my impression was that it was more EL panel than the LDR.

They do drift somewhat over time, but not in an alarming way for common audio applications. Apparently, heat has a cumulative effect on photocells - think it has to do with the mobility of electrons, that make them sensitive to photons, but also to thermal energy.They have a rather high tempco too (about 1% per °C). That's why they are almost always used in a feedback detector configuration for comp/lims.

ruairioflaherty · Feb 2, 2012

abbey road d enfer said:
They do drift somewhat over time, but not in an alarming way for common audio applications. Apparently, heat has a cumulative effect on photocells - think it has to do with the mobility of electrons, that make them sensitive to photons, but also to thermal energy.They have a rather high tempco too (about 1% per °C). That's why they are almost always used in a feedback detector configuration for comp/lims.

Thanks for the info, I've been wanting to try a hi frequency only version of Fred Forssell's opto compressor if time allows so hopefully I can get my hands dirty with some optos this year.

Cheers,
Ruairi

okgb · Feb 5, 2012

excuse the slightly off topic question , but what does the C # after the 5 designate ?
I see vactrols in gtr amp switching circuits , tia

gemini86 · Feb 5, 2012

Why is this thread tagged "VCA"?

ruffrecords · Feb 5, 2012

gemini86 said:
Why is this thread tagged "VCA"?

Voltage Controlled Attenuator

Cheers

Ian

gemini86 · Feb 5, 2012

ruffrecords said:
gemini86 said:

Why is this thread tagged "VCA"?

Click to expand...

Voltage Controlled Attenuator

Cheers

Ian

Doesn't VCA traditionally mean "voltage controlled amplifier"?

ruffrecords · Feb 6, 2012

gemini86 said:
ruffrecords said:

gemini86 said:

Why is this thread tagged "VCA"?

Click to expand...

Voltage Controlled Attenuator

Cheers

Ian

Click to expand...

Doesn't VCA traditionally mean "voltage controlled amplifier"?

Traditionally? Well, an attenuator is simply an amplifier with a gain of less than one. Stick a voltage controlled attenuator on front of an amplifier and you have a voltage controlled amplifier.

It was the best tag I could think of and you are allowed only one. Do you have a better suggestion?

Cheers

Ian

gemini86 · Feb 6, 2012

Sorry I didn't mean to start anything... just trying to make the tags more helpful. maybe Opto?

JohnRoberts · Feb 6, 2012

ruffrecords said:
gemini86 said:

Why is this thread tagged "VCA"?

Click to expand...

Voltage Controlled Attenuator

Cheers

Ian

I recall back during the VCA wars a few decades ago, there was one guy pushing his attenuator only version as a VCAT, or something like that.

JR

ruffrecords · Feb 6, 2012

gemini86 said:
Sorry I didn't mean to start anything... just trying to make the tags more helpful. maybe Opto?

As far as I am aware you can only assign a single tag to a thread. I did not want to spend too long on it so I chose one that I thought described what it is often used for in this forum. I am not sure how the tag thing is supposed to work anyway but I assume it is intended to help searches. Opto VCA might have been more meaningful but I don't think you are allowed to use more than one word.

Either way, I agree it is not worth arguing over

Cheers

Ian

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