Limiters with optocouplers

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mediatechnology
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Re: Limiters with optocouplers

Post by mediatechnology » Sun Feb 22, 2015 9:39 pm

Electronic differential inputs won't allow a Vactrol based limiter to work, only with transformers, because then you put the variable resistor in parallel with the gain largest resistor.

In differential inputs it's the opposite for the Rg. Lowest resistor increases the gain.
I think you may mean instrumentation amplifier differential mic inputs.
These (like the THAT1510) as you point out increase gain by lowering the Rg resistance.
It is indeed the opposite control response.
That's because the U-pad inputs are tied to the output and wrapped around as feedback.
The shunt is Rg: Lowering it decreases NFB and increases gain.

The Texar variable gain input, which was also an electronic (active) differential input also used a U-pad.
The U-pad placement at the input, and not as a feedback network, did allow lowering the shunt to reduce gain.

So you can't exactly say electronic differential inputs won't allow a Vactrol based limiter to work.
It depends on where you put them and the topology of the differential input.

I agree that you're not going to be able to do that in a conventional topology mic preamp nor would you want to.
One thing you can do in mic preamp making it almost overload proof is vary Ra and Rb dynamically or parallel Ra and Rb with diodes to soft clip.
You can't do that with a THAT1510 or INA217 either but you can do it with and INA163, THAT1570 or an op-amp based design where you can access Ra and Rb.

But in your case stage two is probably where you need it to be.
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carlmart
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Re: Limiters with optocouplers

Post by carlmart » Sun Feb 22, 2015 10:44 pm

Can you elaborate a bit more on the 1570 and INA163 trick? I mean the 1510 circuit also has two diodes linking RG to the inputs.

Where can I find the Texar circuitry?

I've also been told that a non-RoHS approved is not allowed to be sold in Europe for instance, so a Vactrol based preamp would not be allowed.

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Re: Limiters with optocouplers

Post by carlmart » Sat Feb 28, 2015 9:23 am

Has anyone built this limiter, using Photonix (ex Silonex), using their 32SR3 optocoupler?

http://media.digikey.com/pdf/Data%20She ... NSL-32.pdf

I'm not sure if those optocouplers are RoHS approved, are they?

Some OC, like Vactrols seem not to be.

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JR.
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Re: Limiters with optocouplers

Post by JR. » Sat Feb 28, 2015 10:28 am

You should be able to ask the manufacturer... they don't say ROHS but are based in CA that has similar rules... there are probably emall or company contacts on data sheet.

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carlmart
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Re: Limiters with optocouplers

Post by carlmart » Mon Mar 02, 2015 10:05 am

Tesla is showing this limiter for their optocoupler, on their application notes.

I'm trying to get some samples for further tests.

They do not have a Western rep,, only sell direct, which is a problem.
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Re: Limiters with optocouplers

Post by JR. » Mon Mar 02, 2015 2:16 pm

If you use the other half of a 5532 for that limiter op amp with it's input stage bias current, the LDR will cause a DC shift as it changes the effective DC feedback resistance (in parallel with 22k X bias current). You can add a capacitor in series with the LDR to mitigate any such gain control feedthrough while it will need to be sized for the lowest shunt resistance for flat gain reduction.

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Re: Limiters with optocouplers

Post by JohnP » Wed Mar 04, 2015 6:19 pm

Thought I'd chime in here, since (like JR) I've used Optocouplers in a number of products over the (many) years.
Addressing the RoHS situation first. These parts contain cadmium (in the photocell); cadmium is one of the hazardous materials listed for RoHS.
AFAIK, Silonex (now Advanced Photonix) received a temporary RoHS exemption because it was shown that there were no viable (ie affordable) alternatives.
I believe that exemption was spearheaded by Marshall amplifiers; they use these optos for channel switching, tremolo, etc.
I don't know if that exemption is still in effect, but I know there are many products using these "forbidden" parts currently being sold world-wide without the "RoHS Police" taking action.

The Silonex parts are very good and readily available from DigiKey, Allied and Newark (Farnell for our oversea members).
Most useful: NSL-32SR2 and NSL-32SR3 (faster decay time).
Also available, NSL-32SR2S (note the "S" suffix) are matched devices, pre-sorted according to on-resistance at a certain LED current.
Useful when you need to control related signals (ie stereo comp).
All these parts are capable of very low on-resistance at reasonable LED currents (~150 ohms @ 5mA).

This is something the older Vactrols couldn't do, usually could get to 1K ohms or so with 40mA.
You could usually design around that limitation, but sometimes you may need a very-low on-resistance for your gain-control function,
or you have something that's battery (or low-powered) and you don't want to waste any current on the control LED.

Ref to the question of controlling the gain of a mic pre with these devices:
As Wayne stated, with certain integrated preamps (INA163), because of the pins available (SOIC-14 pkg), you have access to the circuit nodes of the internal feedback resistors (Ra, Rb).
You could parallel a matched-pair of optos across these gain resistors and reduce the overall gain of the preamp (gain decreases as opto resistance decreases).
See attached INA163 schemo with the added optos (ina163_w-opto.jpg).
I looked at this possibility (in an earlier life), but never really explored it.
You'd have to consider the various factors involved in lowering those gain resistors via ext optos (stability, front-end overload, etc).
The other preamp ICs mentioned (INA217, THAT 1510), are packaged in smaller (ie less-pins) packages, and don't provide access to those internal nodes.

Another way to control the overall gain of the mic-pre path (but not actually the gain of the pre itself) is to use an opto as the shunt-element of a U-Pad in front of the preamp.
In a typical preamp design, there are usually some low-value (10 ohms typ) resistors in each input leg to limit the clamp-current from phantom operation.
At the cost of additional (resistor) noise, you could increase those values and use one of the low-resistance Silonex parts to limit the input signal to the preamp IC.
Spec'd at 60 ohms @ 20mA LED current, the NSL-32SR3 could be used in an auto-pad to prevent clipping. For 15dB padding, your input resistors would need to be 150 ohms each.
Remember, these resistors are in-series with your input source (microphone), so the overall source impedance seen by the preamp increases.
I'll leave it to the more math-inclined to actually calculate the additional noise produced from these 150 ohm resistors vs the original 10 ohm resistors.
Don't know if I'd try to use it as a continuously-operating compressor, but with a quick-acting detector, you could prevent most clipping.

Overall, this type of (analog) opto is a wonderful design tool.
I've heard that there are attempts being made to design a RoHS-compatible part, but I don't think the market is that large these days to warrant a lot of R&D.

JP
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Re: Limiters with optocouplers

Post by JR. » Wed Mar 04, 2015 9:00 pm

Hey JP great to see you around the neighborhood.

One quick question... any thoughts on common mode issues with matching/tracking errors between the two LDRs?

JR
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carlmart
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Re: Limiters with optocouplers

Post by carlmart » Wed Mar 04, 2015 9:20 pm

Thanks for your comments and suggestions.

Many products I have looked into in the past months seemed to have abandoned limiting the first stage, and put a limiter on the following one.

Maybe it's there where the Silonex might work well. Unfortunately the only design I have seen, suggested by Silonex themselves, is a bit large and complex.

http://media.digikey.com/pdf/Data%20She ... NSL-32.pdf

So in a way it's not too much different from the one I'm gonna try in a few weeks, when I get the boards and the parts, with the THAT 4305.

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Re: Limiters with optocouplers

Post by JR. » Thu Mar 05, 2015 1:26 pm

carlmart wrote:Thanks for your comments and suggestions.

Many products I have looked into in the past months seemed to have abandoned limiting the first stage, and put a limiter on the following one.

Maybe it's there where the Silonex might work well. Unfortunately the only design I have seen, suggested by Silonex themselves, is a bit large and complex.

http://media.digikey.com/pdf/Data%20She ... NSL-32.pdf

So in a way it's not too much different from the one I'm gonna try in a few weeks, when I get the boards and the parts, with the THAT 4305.
The attraction of LDRs for limiter design is because the designs can be very simple and still sound passable. The simplest approach is to drive the light source with a rectified version of the output audio with the LDR shunting a resistor as suggested by JP. Adding gain to this simple "side chain" works as a threshold adjustment. Depending on the LDR characteristic attack and release processing may be minimal or not used at all.

This is good design to cut your teeth on, and you will end up with something unique you can brag about.

JR
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