BAJA LA2A Style Optical Limiter

bajaman · Post by **bajaman** » 25 Nov 2007, 09:52

Hello my friends
I am particularly proud and pleased with this new project - I have just completed the working prototype - pictures to follow

I started this project by trying to reverse engineer the Demeter Compulator from pictures posted here by some of you folks, but I could not see the solder side of the board and despite numerous requests, nobody posted it

So, I quietly shelved this project and did some work on other projects, but yesterday a friend kindly provided me his unit to open up, and the rest as they say is history.
I got my trusty digital camera out and got some lovely pictures from all angles - I will post these soon, so do not panic.
Then I traced the schematic and was a little disappointed, because it did not seem to function like the LA2A limiter that I believed it was modelled after

Hmmmmmm, I thought - i can do better than this, i thought to myself, so I set to work and this is what I came up with. Although it shares some similar features and parts with the Demeter Compulator it functions completely differently - in fact it functions just like the Teletronix LA2A Limiter

THIS IS NOT A COMPRESSOR like the Demeter it is a LIMITER

The Demeter Compulator derives it's control signal from the OUTPUT of the unit. My Optical Limiter derives it's control signal from the INPUT of the unit and therein lies the major operating difference.
I have played some guitar through the Demeter Compul;atorand although it is a very nicely made unit I was less than impressed by - no indicator to show how much compression and a tendency to release erratically on high compression settings - a bit like an underdamped swing door

Nevertheless, Pete Townshend and Dave Gilmour both use one in their Pete Cornish rigs, so it cannot be that bad then can it

Anyway - I have a board layout and parts placement overlay all done for the compulator - I will tidy up the schematic and post it in the next few days, but I think you will like my Limiter better. Anyway I will post them both as projects and let you folks decide which one to build, or you can build them both if you are keen

Enough tlk - here is the first instalment in this project - the Baja Optical Limiter schematic - board layout and PCB will follow very soon , but as it is my Birthday tomorrow, I may be a wee bit incapacitated

enjoy
bajaman
BAJA Optical LA2A Style LIMITER

Aharon · Post by **Aharon** » 25 Nov 2007, 15:13

Happy birthday Baja!!!!
Hey,your generosity is limitless...another great project,always original ,always useful................
I don't even look at the Bootique boxes disected section!!
YOU RULE!!.
Thanks
Aharon

Stephengiles · Post by **Stephengiles** » 25 Nov 2007, 18:16

Happy birthday indeed. I was discussing with my wife only this morning the downside of a heavy drinking session - that is to say, how your head feels the next morning!! Anyway as it is already tomorrow for you in NZ (I'm in England) I trust your head is not feeling too bad.

The worst ever for me was New Years Day around 1980, after a night of much whiskey, cards and rich food, I was faced with a drive home of some 40 miles whilst hoplessly over the drink/drive limit.

I have some thoughts on your wonderful circuit which I'll commit to words anon........but now to the Italian Restaurant!

goaltoday · Post by **goaltoday** » 25 Nov 2007, 19:48

Happy Birthday!![smilie=a_partyguy.gif]

What Light/Dark Resistance would you recommend for the ldr? Thanks.

DWBH · Post by **DWBH** » 25 Nov 2007, 19:58

What's does a limiter do?

bajaman · Post by **bajaman** » 25 Nov 2007, 20:40

What Light/Dark Resistance would you recommend for the ldr?

I used one of the small ones available from the local electronics store - I think it is a Philips, but not sure. I will try and get some specs for it for you - feel free to experiment OR you could just use a Vactec VTL5C9 or VTL5C10 device - should work okay.

What's does a limiter do?

from wikipediaLimiting: Any process by which a specified characteristic (usually amplitude) of the output of a device is prevented from exceeding a predetermined value.

Limiting can refer to non-linear clipping, in which a signal is passed through normally but "sheared off" when it would normally exceed a certain threshold. It can also refer to a type of variable-gain audio level compression, in which the gain of an amplifier is changed very quickly to prevent the signal from going over a certain amplitude.

Hard limiting (not to confuse with "clipping") is a limiting action in which there is
(a) over the permitted dynamic range, negligible variation in the expected characteristic of the output signal, and
(b) a steady-state signal, at the maximum permitted level, for the duration of each period when the output would otherwise be required to exceed the permitted dynamic range in order to correspond to the transfer function of the device.
Soft limiting is limiting in which the transfer function of a device is a function of its instantaneous or integrated output level. The output waveform is therefore distorted, but not clipped

cheers
bajaman

goaltoday · Post by **goaltoday** » 25 Nov 2007, 21:09

Thanks bajaman.

bajaman · Post by **bajaman** » 26 Nov 2007, 19:47

Here is the complete project
hope you enjoy it as much as I do

bajaman
PCB
LAYOUT
SCHEMATIC

bajaman · Post by **bajaman** » 27 Nov 2007, 00:05

I just modified a customers BOSS DD2 (DD3) to give the TES sound (tape echo simulator) - the echo is less high treble and the repeats are progressively less bass, and a couple of small mods to increase the number of repeats available etc. I will post details soon for those who wish to improve their BOSS pedal sound.
ANYWAY - I connected my BAJA Optical Limiter AFTER the BOSS modded DD2 and was very pleasntly surprised at how it fattened and warmed the sterile digital sound - now it sounds much more like a tape echo.
SO - build this project - you will not regret it, I promise you

bajaman

bajaman · Post by **bajaman** » 27 Nov 2007, 10:19

Here is a label - it fits a standard Hammond 1590B case.
The controls and the led pilot light indicator will fit to the pcb perfectly.
I used Alpha 16mm spline shaft pots with 30 amp fuse wire to extend the legs. The pots are mounted on the solder side of the circuit board and a 3mm self adhesive bumper is attached to the rear of the pot case to prevent it shorting against the solder side of the board. Once the pots are attached to the case they hold the circuit board and pilot indicator light firmly in place too.

cheers
bajaman

Baja Optical limiter label

MoreCowbell · Post by **MoreCowbell** » 27 Nov 2007, 17:13

Cool project Baja...I'm gonna take a "building break" for a bit, but I might find time to make this one in the next couple months. Anyway, Happy Birthday...and Happy Holidays !

Skreddy · Post by **Skreddy** » 27 Nov 2007, 19:31

Wow; looks great! Nice project. Bajaman!

bajaman · Post by **bajaman** » 27 Nov 2007, 20:17

I might find time to make this one in the next couple months

Don't wait that long Andrew

This sounds really really nice after any fuzz or distortion pedal - fattens the sound, warms it up, and generally makes it sound more tube amp like.

cheers
bajaman

bajaman · Post by **bajaman** » 27 Nov 2007, 20:21

Designing (with some help from the Demeter Compulator - do a search on this site) this effect has completely changed my outlook on compressors / limiters. i used to think they sucked tone big time - Boss CS2 for example, but after building this project I am a true believer in their real worth (optical based devices - NOT VCA)
bajaman

analogguru · Post by **analogguru** » 27 Nov 2007, 21:16

i used to think they sucked tone big time - Boss CS2 for example, but after building this project I am a true believer in their real worth (optical based devices - NOT VCA)
bajaman

bajaman.... I can understand your enthusiasm, but you are technician enough to explain me what an opto based compressor should be able to do, what a VCA-based can´t....?

The CS-2 is "dynacomp based" the same concept most of the compressors (including keeley) use.... but only in the "guitar world".

Sorry, but the "guitar world" is not identical with the whole compressor world. And even in the "guitar world" are exceptions:
for example the DOD 280 or the Boss CS-1 are opto-based... so opto is nothing new - but guitarist only ask for Dynacomp-clones.

This doesn´t mean that the Demeter concept is the last wisdom of the world... it sounds maybe better than the well-known dynacomp-world... but IMHO it´s a cheap and poor concept which doesn´t produce any result what cannot be achieved also with VCA´s.

If you don´t believe it, have a look to find such a unit for comparision:

It´s VCA based (CEM 3381) and should deliver at least the same results.

analogguru

bajaman · Post by **bajaman** » 28 Nov 2007, 00:11

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

They pulled their VCA based compressors from their boards and are CURRENTLY using the Demeter Compulator in their touring Pete Cornish built pedal boards.

Seriously though, I was trying to build a 9v pedal to mimic the sound and response characteristics of the famous Teletronix LA2A broadcast limiter, and it uses optocouplers and senses the input signal BEFORE the voltage gain recovery amplifier - in this respect it operates far differently than any VCA feedback AGC based design that I am aware of on today's market.
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. 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.

bajaman

Post by **modman** » 28 Nov 2007, 00:24

analogguru wrote:
i used to think they sucked tone big time - Boss CS2 for example, but after building this project I am a true believer in their real worth (optical based devices - NOT VCA)
bajaman
bajaman.... I can understand your enthusiasm, but you are technician enough to explain me what an opto based compressor should be able to do, what a VCA-based can´t....?

The CS-2 is "dynacomp based" the same concept most of the compressors (including keeley) use.... but only in the "guitar world".

Sorry, but the "guitar world" is not identical with the whole compressor world. And even in the "guitar world" are exceptions:
for example the DOD 280 or the Boss CS-1 are opto-based... so opto is nothing new - but guitarist only ask for Dynacomp-clones.

This doesn´t mean that the Demeter concept is the last wisdom of the world... it sounds maybe better than the well-known dynacomp-world... but IMHO it´s a cheap and poor concept which doesn´t produce any result what cannot be achieved also with VCA´s.

If you don´t believe it, have a look to find such a unit for comparision:

It´s VCA based (CEM 3381) and should deliver at least the same results.

analogguru

If it would be revolutionary he wouldn't be posting it, I know him, he's an intelligent guy. I'm sure he's got much more convoluted stuff up his sleeve.

If you can provide schematic and layoout we could put it in a different folder and then the challenge is on.

We can never have enough 'fresh' answers on old questions...

Where's that Tubezipper schematic? PT has escape all tracing here...

bajaman · Post by **bajaman** » 28 Nov 2007, 01:59

anyway - mine only has two knobs

bananaman

paulc · Post by **paulc** » 28 Nov 2007, 03:27

Bajaman,

that looks pretty cool. I've got something close to that myself, and I've got some tricks for it that you might want to try if you're into it.

The main one is I use two LDRs instead of one. The LED's are wired back to back like a diode clipping pair, and the resistive elements are paralled in the same place you have them. I add some series resistance to one of the resistive elements, and a parallel resistor across both. This lets me putz around with the "taper". You can spend a lot of time there getting things perfect to your ears.

I've found that running the driver at a higher voltage can help. Once you hit the rails you wont be able to get the LEDs brighter. This can limit the range, so by increasing the headroom you can hit the LED's harder making the resistors drop more. If you can get them to drop low enough (having the two in parallel helps right off the bat) then you can make the effect stronger without having a large series resistor on top reducing level at quiescence. Less recovery gain will be needed keeping the noise down.

Also adding a tone control type circuit around the driver can make the comp react to different freqs. This can be cool to keep bass from being boomy, or if you crank the highs you can get it to act like a deEsser comp'ing the highs more to keep things from getting shrill. But before it get's said I know that's an old trick... still cool though.

I love these type of opto comps much better than the VCA's. 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 and what not. It's sort of like making discreet ops. you can get in there and go to town instead of being stuck with what's in the "package".

One of the things I regret most is in a house move I tossed some old Urei optos I'd been saving for a LA2A build. I was bummed when I unpacked and couldn't find 'em.

Later, PaulC
HEritage amps/ Tim & timmy pedals

seniorLoco · Post by **seniorLoco** » 28 Nov 2007, 03:52

I've found that running the driver at a higher voltage can help

Pfffttt ...done before !

Roland Sustainer AS-1 ....

(gut shots coming soon)

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 !?

BAJA LA2A Style Optical Limiter [documentation]