Limiters with optocouplers

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carlmart
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Re: Limiters with optocouplers

Post by carlmart » Fri Mar 06, 2015 6:27 am

I'm not so convinced putting an LDR at the input it's the right move.

That it's worth trying and listened to, that is certainly something to try.

I'd like JP suggesting, if possible with some sample circuits, a limiter on the second stage.

I'm curious about simpler ways to implement an optocoupler (Silonex or Vactrol) in the second stage, where it seems to be less harmful and less distortion prone.

Right now I was checking the Shure FP33 schematic, and I was wrong: the limiter is on the second stage. And it's not using a Vactrol. The schematic is copyrighted, and I don't think I'm allowed to put it here.

We can't forget that the limiter should be an "emergency lifesaver". A take would be ruined if it wasn't there, and you will be able to use it. The rest of the time, the limiter should invisible and non-existent, not adding any distortion or anything.

That's why I distrust any limiter circuits where the audio goes through it, instead of being parallel to it, like a Vactrol in the gain resistors.

Silonex suggests a limiter with the optocoupler at the input, and I don't think that's effective or the way to go.

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mediatechnology
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Re: Limiters with optocouplers

Post by mediatechnology » Fri Mar 06, 2015 4:59 pm

Thanks to everyone for contributing to this thread.
We haven't heard from JP in awhile...
One quick question... any thoughts on common mode issues with matching/tracking errors between the two LDRs?
I realize the question wasn't posed to me but I don't think the CdS cells impact common mode rejection since they will change differential gain but CM gain will remain unity until CM is rejected by A3 (INA163).
But, I-led vs. gain might not be very linear due to mistracking.

What I have heard suggested is to make the Cds resistive elements in JP's drawing back-to-back diodes to provide instantaneous cycle-by-cycle limiting to prevent severe overloads.
The instantaneous distortion on overload is high but recovery is instantaneous.
Sort of a smooth clipping.
How the INA163 or 1570 handle that is something I've never tested.
I'm not so convinced putting an LDR at the input it's the right move.
I tend to agree with that in the context of a mic preamp for the reasons JP gave.
The Texar example I gave was for a line input with a U-pad.
Unfortunately the schematics are not available for that unit.
It was a nice-sounding multi-band processor.
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Re: Limiters with optocouplers

Post by JR. » Fri Mar 06, 2015 8:04 pm

yup... never mind...

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mediatechnology
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Re: Limiters with optocouplers

Post by mediatechnology » Fri Mar 06, 2015 9:10 pm

BTW I see "RoHS By Exemption" on a lot of items listed at Mouser.
They may have gotten an exemption for CdS cells.
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Re: Limiters with optocouplers

Post by JR. » Sat Mar 07, 2015 10:45 am

ldr_comp.jpg
ldr_comp.jpg (101.06 KiB) Viewed 13872 times
Coincidentally last night I was going through some old note books looking for stuff to throw away, like most of them, and found this.

This is from 1977 and some bench testing I did with a center-tapped LDR... By grounding the mid point I could use one half to linearize the gain control of the other half.

This is configures as a 2:1 compander. Of course for a simple limiter you don't need to know or control the amount of attenuation so precisely.

FWIW I dismissed this approach in favor of solid state gain elements which back in the '70s were still pretty crude compared to today's technology.

Note: this schematic is offered just for chuckles, I don't know if the parts are even still made.

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mediatechnology
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Re: Limiters with optocouplers

Post by mediatechnology » Sat Mar 07, 2015 11:13 am

Thanks for sharing this.
Your file cabinets are a treasure-trove.

Though the center-tapped LDR might not be available (VTL5C?2) I think you could get pretty good linearity by using two LDRs with series-connected LEDs.
I've seen some modern optoisolator linearization techniques based on your circuit.

I'm reminded of the LA-3 using the second LDR for metering - this is better because with control linearization the attack/release is more predictable.
Without linearization the control gain is all over the place.
I wonder if with linearization if a feed-forward topology is possible.
The compressor's errors in feed-forward might be an interesting effect.

I realize this was a bazillion years ago but for someone doing it today...
Would keeping the L-pad DC-coupled and then AC-coupling the L-pad output to the OA input be better?
The gets the coupling capacitor out of the attenuation shunt.
Did I miss something?

I have a bunch a VTL5Cs and am tempted to try this.

FWIW there is a Clairex catalog here: viewtopic.php?f=12&t=431
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JR.
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Re: Limiters with optocouplers

Post by JR. » Sat Mar 07, 2015 11:23 am

Maybe I'm just an old curmudgeon but I have little interest in old technology. I do not understand the modern revival of vinyl. No it isn't better than digital.

Sorry for the rant.. I am more excited by applying modern digital pots to do audio tricks and stuff like that.

JR

PS I just got an order of new parts in today including neon lamps So I guess I still dabble in "some" old technology. :lol:
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mediatechnology
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Re: Limiters with optocouplers

Post by mediatechnology » Sat Mar 07, 2015 12:00 pm

I do not understand the modern revival of vinyl. No it isn't better than digital.
Thread veer alert:
I have a hunch that vinyl masters aren't clipped to death like CD and iTunes releases of the same material and therefore actually sound better despite the medium's limitations.
I'd like to test that theory.
Maybe I should visit Best Buy and buy a record, CD, then download the song.
Other than DVD Blu-Ray I haven't bought physical media in years.

Farther off-topic alert:
I just saw a press release from Roland in cooperation with Itron? where they are making planar low voltage audio tubes (triodes) in what appears to be a modified fluorescent display technology.
IIRC plate voltages are in the 35-50V range.
I suppose I could fire up an old display and have a seven plate (seven segment) triode to play with.

Back to OP:
For the resistive L-pad impossible-to-overload line input I don't know what's out there that would be an equivalent technology to the CdS cell.
https://ka-electronics.com

Covidland (The Movie) Part 1: Lockdown https://banned.video/watch?id=615f96b3aa816336dbd21792

A voice-over with Orson Welles: "What Is It That You Want?": https://waynekirkwood.com/content/Orson ... u_Want.mp3
Americas Frontline Doctors Lawsuit Motion For Preliminary Injunction Of Covid-19 Vaccine Emergency Use Authorization: https://waynekirkwood.com/images/pdf/Am ... zation.pdf

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JR.
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Re: Limiters with optocouplers

Post by JR. » Sat Mar 07, 2015 12:51 pm

mediatechnology wrote:
I do not understand the modern revival of vinyl. No it isn't better than digital.
Thread veer alert:
I have a hunch that vinyl masters aren't clipped to death like CD and iTunes releases of the same material and therefore actually sound better despite the medium's limitations.
I'd like to test that theory.
Maybe I should visit Best Buy and buy a record, CD, then download the song.
Other than DVD Blu-Ray I haven't bought physical media in years.
I actually wrote magazine columns about the comparison between CDs and vinyl back in the '80s and back then vinyl had the extra processing ("sweetening". post-processing) that pretty much resulted is a better version of a given master. )

There may be a lighter touch practiced with the new vinyl releases but this is kind of backwards since on paper digital promises more dynamic range. The modern consumers seem to be inured to lots of clipping.
Farther off-topic alert:
I just saw a press release from Roland in cooperation with Itron? where they are making planar low voltage audio tubes (triodes) in what appears to be a modified fluorescent display technology.
IIRC plate voltages are in the 35-50V range.
I suppose I could fire up an old display and have a seven plate (seven segment) triode to play with.
yup, not a new concept but repurposing of quiet production lines.. There is still some discussion of IC vacuum tubes but that too has been kicking around for decades.
Back to OP:
For the resistive L-pad impossible-to-overload line input I don't know what's out there that would be an equivalent technology to the CdS cell.
This stuff is still around in large production... My cheapo driveway lamps use separate Cds(?) cells to turn the lamps on off, even though the solar cells already there seem logical to probe their output to determine ambient light level. Apparently the separate light level detector works better, easier, and is still only pennies.

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ricardo
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Re: Limiters with optocouplers

Post by ricardo » Sat Mar 07, 2015 4:23 pm

mediatechnology wrote:I have a hunch that vinyl masters aren't clipped to death like CD and iTunes releases of the same material and therefore actually sound better despite the medium's limitations.
Nothing as crude as that!

It's all compressed :( using vintage 1960's gear but to an extent unrecognisable by the original designers of the gear. Just look at any Top 10 hit today on Audacity or your favourite DAW.
Maybe I should visit Best Buy and buy a record, CD, then download the song.
Other than DVD Blu-Ray I haven't bought physical media in years.
It's happening to CD too :o

One of Bob Cordell's demos for Dynamic Range is Rickie Lee Jones' Ghetto of my Mind from Flying Cowboys. Present issues have about 10dB less Dynamic Range than the original issue from the previous Millennium.

A friend is ex-EMI and was involved with one of the most famous recordings of all time, the 1965 du Pre, Barbirolli Elgar Cello Conc. His signature is on the master. He gave me a digital copy (purely for educational purposes of course) It's the first time this Millennium I regret no longer having 2x200W/channel and the speakers that can take full advantage of this.

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