BAJA LA2A Style Optical Limiter  [documentation]

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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paulc
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Post by paulc »

seniorLoco wrote:
I've found that running the driver at a higher voltage can help
Pfffttt ...done before !

Roland Sustainer AS-1 .... :wink:
(gut shots coming soon)

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Getting all 'surgical precision' with either a compressor or limiter for guitar use is a bloody waste of time .... imo. more for studio use maybe!

We tend to drift from the fact with these kinda of effects, it's the "feel" that matters, not much of the "hear Ye" how its squishes or sustaines!

Opto rule since they sluggish just like we humans ...aye mate !?
Ummmm O.K... Need to spend more time getting to know the guys here I guess - I can't tell if this was a pro/con/cool/shut up etc responce! Just trying to join the gang ya know?

Later, PaulC
Heritage amps/tim & timmy pedals

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Post by seniorLoco »

Hey Paul nothing like that ya .... :D

Sorry if i was rude ..... :wink:

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analogguru
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Post by analogguru »

bajaman wrote:
but IMHO it´s a cheap and poor concept which doesn´t produce any result what cannot be achieved also with VCA´s.
Tell that to Dave Gilmour and Pete Townshend :lol:
They pulled their VCA based compressors from their boards and are CURRENTLY using the Demeter Compulator in their touring Pete Cornish built pedal boards. :wink:
Sorry, but I don´t intend to teach lemmings not to follow his neighbour, and it doesn´t matter if ones name is David or Pete.
But why I should be so stubid to do it for myself.
Seriously though, I was trying to build a 9v pedal to mimic the sound and response characteristics of the famous Teletronix LA2A broadcast limiter,
Seriously... the Demeter Compulator and your concept have the same amount in common with a LA2A like a FuzzFace with a Tubescreamer - both of the last distort and both "compress".

One of the "secret recipes" of the LA-2A is the use of an electroluminescent light device with a special characteristic (and not the LDR).... without including that behaviour you are far away from an LA-2A.

So please let´s talk about a "feed-forward opto-compressor-concept" and not the LA-2A... that would be blasphemy in the ears of hardcore-studio-guys.
and it uses optocouplers and senses the input signal BEFORE the voltage gain recovery amplifier
This concept is called "feed-forward" compressor and also made with VCA´s:
http://rupertneve.com/products/portico-5043/
and very well knon since the mid-70´s:
http://www.aes.org/e-lib/browse.cfm?elib=2252

I for myself designed one with 2 OTA´s in parallel as variable resistors in the feedback loop of an op-amp in 1981.
- in this respect it operates far differently than any VCA feedback AGC based design that I am aware of on today's market.
If you are not aware of it, doesn´t mean that it doesn´t exist since more than 30 years.
Compare a 1khz square wave fed into a VCA based device such as the BOSS CS2 and you will be horrified at what comes out of the output socket.
As I wrote above:
The sh*tty "Dynacomp & Clones" concept is not all and everything in the VCA-compressor-world.... And I don´t intend to discuss about the behaviour of the worst example. Noone is using this sh*t - and paying horror prices for it - except of guitarists.

If the Attack and Release times are correctly adjusted on a (non-dynacomp)(feedback)VCA-based compressor/limiter we can compare it without any problems.
Now build my pedal - it should not take you very long - and compare the response, and you will see why I am so enthusiastic about this project. :D
bajaman
Yes, it will sound new and good.... (espacially for the guys who only know dynacomp-(clones).... and it´s ok to be enthusiastic - but please stay on the technical floor...

As you can read in long discussions about lamps/ldr´s for the univibe the response of both have an influence on the tone.... Did you ever have a look on the response curves of an LDR ? they have a very long recovery-time.....

If they wouldn´t, Demeter and your circuit would be a ringmoduator instead of a compressor due to the lack of a smoothing-cap in the side-chain. For this reason you only have two knobs instead of three... :wink: :
You can´t adjust the recovery (release) time....

analogguru


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Every (bootweak)-pedal has a purpose....
...it might be a door stop or a hammer, but every (bootweak)-pedal has a purpose.
There´s a sucker born every minute - and too many of them end up in the bootweak pedal biz.

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Post by bajaman »

Thanks for your comments and suggestions for improving this design Paul :)
In practice - yes, I have built a fully operational unit, - this is not a design concept, but a fully working 9v stompbox limiter. it most certainly is not a Rupert Neve studio designed compressor, it is a guitarist's stompbox on the floor. :D :roll: 8)
If I was setting out to design a studio quality processor with a gazillion knobs, conmpression ratios, attack and release times etc., then i most certainly would not be posting the design on this forum - OK :wink:
Sorry - i digressed a little :roll:
In practice - i have found that by using a standard 3mm red led, that I can get more than enough limiting action with the 9v battery supply.
Noone is using this sh*t - and paying horror prices for it - except of guitarists
And this is exactly why I designed (with some help from the Demeter Compulator) this optical limiter - so guitarist's, including myself, could enjoy a far better sounding pedal than all those hopeless Dynacomp clones :lol: :lol: :lol:
I certainly didn't design it for studio geeks OK :roll:
Did you ever have a look on the response curves of an LDR ? they have a very long recovery-time.....
Yes - just like our ears do too, and look ma no pumping. :wink:
You can´t adjust the recovery (release) time....
I do not want to - i am perfectly happy with it as it is - just like the LA2A :lol:
Opto rule since they sluggish just like we humans ...aye mate !?
Agreed :wink: :D

Also adding a tone control type circuit around the driver can make the comp react to different freqs
I thought about doing this but wanted to keep the design simple with just two controls - I did think about putting a similar filter as the LA2A uses in the side chain, but in practice the flat response seems to do exactly what i want soundwise. Good thinking though :wink:

The nice thing about the optos is in your audio path the whole comp element is nothing more than a resistor. This allows you to use your fav buffers/gain stages
Yes - agree 100% here too. :)

Thanks for all the feedback - now go ahead and build it like I did 8)
bajaman

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Post by analogguru »

I certainly didn't design it for studio geeks OK :roll:
But the LA-2A was designed for and/or used in studios ?
And this is exactly why I designed (with some help from the Demeter Compulator) this optical limiter - so guitarist's, including myself, could enjoy a far better sounding pedal than all those hopeless Dynacomp clones :lol: :lol: :lol:
That´s fine and good.... and maybe you will find your concept in the near future in a bootweaker´s pedal.....
but is it really necessary to make a comparision with a LA-2A..... ?

BTW, what should be so ingenious on the "rectifier"/sidechain part of the Demeter Compulator to borrow it ?

analogguru
There´s a sucker born every minute - and too many of them end up in the bootweak pedal biz.

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Post by bajaman »

BTW, what should be so ingenious on the "rectifier"/sidechain part of the Demeter Compulator to borrow it ?
Hey AG - you are the expert here - you tell me :lol:
All I know is it works in this application, but I must admit my confusion regarding the two 1N4148 diodes and the 4k7 resistor in the additional bias to the led driver stage - I think the circuit will work without these additional components - i may remove them and see - any thoughts from you on what they actually do???
If a booteeker wants to use this design, that is fine by me - I have some other stuff up my sleeve for a rainy day - stay tuned :wink:
cheers
bajaman

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Post by jakerandall »

the real question how does it sound?

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Post by paulc »

bajaman wrote:
BTW, what should be so ingenious on the "rectifier"/sidechain part of the Demeter Compulator to borrow it ?
Hey AG - you are the expert here - you tell me :lol:
All I know is it works in this application, but I must admit my confusion regarding the two 1N4148 diodes and the 4k7 resistor in the additional bias to the led driver stage - I think the circuit will work without these additional components - i may remove them and see - any thoughts from you on what they actually do???
If a booteeker wants to use this design, that is fine by me - I have some other stuff up my sleeve for a rainy day - stay tuned :wink:
cheers
bajaman
Putting a DC offset in will allow the LED to have a little "soft glow" at quiescence. This will get them started early so you don't have a dead spot in the curve while you're waiting for enough signal to get the leds to start lighting up. Gets everything in a nice working range at the start.

Later, PaulC
"bootweaker"

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Post by PurplePeopleEater »

analogguru wrote: One of the "secret recipes" of the LA-2A is the use of an electroluminescent light device with a special characteristic (and not the LDR).... without including that behaviour you are far away from an LA-2A.
A very important part of the sound of the LA2A is the transformer. It's one reason that the older units, even if refurbished completely (other than the x-former) sound better to most people than the reissues. The older units, IIRC, had a heavy duty UTC HA100X x-former that used materials that are currently not legal to use (you can thank the EPA for that). The Fairchild 660/670 are similar in that the clones being made are not true clones, because the materials of the original units are no longer legally available.

I understand that the goal of this project is to mimic some of the LA2A's distinct characteristics, but IMHO that cannot be done in stompbox form, so the name is a bit misleading. Still, it looks like a fun and easy to build project that could be especially useful to tweak and build into a bass d.i. / preamp.

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Post by bajaman »

Putting a DC offset in will allow the LED to have a little "soft glow" at quiescence
Thanks for the clarification here Paul :)
Jakerandall - how does it sound??
Well Jake, it doesn't sound!! It limits the signal fed into it and is very transparent in operation - play some single note lines quietly and it sounds the sam eon or off, but play a aggressive full chord and you will hear what it does. :wink: :wink:
I suppose you could say it evens out the sound levels and adds a subjective fatness to the sound - try it after a chorus, delay, flanger, distortion etc., and you will see why I like it - did you read about the Clay Jones overdrive pedal???
Just some food for thought :wink:
cheers
bajaman

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Post by Skreddy »

I just designed a minimal version, omitting the buffer and output stage and 1/2 9v opamp, just to use as an add-on daughter board for adding limiting to any gain stage. I'm planning to experiment with it in a delay project that I want to sound more "tubey."

I've added a parallel and a series resistor (both optional) for tailoring the LDR to your particular application.
Last edited by Skreddy on 17 Jan 2008, 15:04, edited 1 time in total.

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Post by celadine »

On the optoisolator based VCA vs. transistor based VCA thing:

- Control voltage bleedthrough is not a problem with LDRs, simplifiying VCA design.
- Optoisolators have a natural nonlinear response to control voltage which also simplifies VCA designs used for musical applications. This is a disadvantage if you want a linear responce.
- Some LDRs are fine in vacuum tube designs, with the high voltages involved. Transistor VCAs can't handle this high voltage. (yes, I'm looking at you lm13600.)
- LDR based VCAs can be designed to have exceptional DC performance (CV control, etc.).

I'm not an absolute optoisolator fan, just wanted to point out these things. Transistor-based VCAs have their uses too. (i.e., the THATco. 4320 http://www.thatcorp.com/4320desc.html is pretty sexy, despite being QSOP)

Here's a PDF with more than you wanted to know about LDRs:
http://optoelectronics.perkinelmer.com/ ... olator.pdf

And more LDR stuff; check out the 'Technical Reference' section:
http://www1.silonex.com/audiohm/principles.html

On the diode-CV thing - could be the diodes are used as voltage-controlled resistors like in this LPF:
http://files.muziq.be/schematics/paia_2720-3l.gif

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Post by briggs »

How does the "precision 1/2v supply" affect the overall design? Is it required for correct operation or can it be left off? I was thinking of using that op amp as the buffer section :)
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Post by briggs »

One more question - can you reduce the gain in the recovery stage to around 3 - 4 without it impacting on the limiting amount?
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Post by bajaman »

Is it required for correct operation
yes :wink:
can you reduce the gain in the recovery stage to around 3 - 4 without it impacting on the limiting amount
yes - but why :? - on full limit, you will not have a high enough output level :wink:
cheers
bajaman

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Post by bajaman »

I have been doing some extensive research into led and ldr home made opto couplers. Red, Yellow, Blue, white, standard, hi brite - different ldrs etc.
The best combination I have come up with so far, that most easily mimics the VTL5C10 type is a 5mm 16000Cds Hi brite White Led and a 0.5 megohm dark resistance small ldr.

VTL5C10 with 9.4v dc battery
series resistor - measured resistance
1k5 - 70
15k - 186
150k - 900
1M5 - 8k5

Home made opto coupler
1k5 - 90
15k - 330
150k - 1k8
1M5 - 15k

cheers
bajaman


ps: Steve Daniels at Small Bear has many VTL devices in his store - He can get the VTL5C10 devices too - I think he may put them in his inventory if enough folks request them :wink:

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Post by goaltoday »

Thanks for the research baja! :D

Last week I ordered the VTL5C10 from banzai.But I'm going to try some led+photocell to compare and decide what I like.

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Post by briggs »

bajaman wrote:
Is it required for correct operation
yes
Why is the buffered 1/2 v required? Is there a specific reason?

Thanks for the project baja, looks like a great effect! There's just some things that I don't quite understand there!
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Post by bajaman »

Why is the buffered 1/2 v required? Is there a specific reason
The buffered or regulated 1/2 volt supply rail is necessary because of the way that the led opto coupler works - you will notice that the led is connected between the output of the op amp (1/2 rail volts) to the 1/2 rail voltage and not to ground. i am sure AG can explain this better than I can though. :wink:

One small problem I have encountered with this pedal - when I tried to use a 9 volt regulated switch mode power pack, i got a very loud white noise. The remedy was to add a decoupling capacitor from the anode of the power input diode to ground - I used a 470uf 16v type electrolytic (positive to the anode). It is now dead quite - no problem with the battery operation though - hmmmm. :roll: :wink:
bajaman

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Post by briggs »

Interesting! I'm having a play with the envelope section of the compulator myself. It may be useful in one of my on-going designs 8)

I'm trying to get it down to one op-amp in the gain section - so I can use the other op-amp for the regulation of the 1/2v supply.

Skreddy - did you get your minimal version working?
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