BAJA LA2A Style Optical Limiter

bajaman · Post by **bajaman** » 25 Apr 2008, 20:49

As I mentioned in the Opto limiter rev 2 thread, a suitable LDR can be obtained from http://www.futurlec.com - PHOTOCELL 1 is how it is listed - 5k light resistance, 500k dark resistance.
cheers
bajaman

kierc · Post by **kierc** » 01 Dec 2009, 18:25

Sorry to bump this up - just finished putting mine together but pretty sure I wired it wrong because it isn't working!

I used "Photocell 1" from Futurlec... not sure if it's the same one Bajaman had, but it's pretty small (smaller than a 5mm Yellow LED anyway)

Well, I couldn't figure out how to wire it so I just went with the way pictured below, and I'm guessing it's wrong!!

Symptoms :

No signal passing in either switch position, panel LED doesn't light up - not sure about internal one at the moment...

If anyone could help with the wiring that'd be great!!

Jarno · Post by **Jarno** » 02 Dec 2009, 08:39

Have you seen these sites?
Beavis, stompboxwiring
Gauss Markov, wiring 1590b

kierc · Post by **kierc** » 02 Dec 2009, 12:41

Hi Jarno,

Thanks for the links - I've seen them before and have wired up pedals using the Beavis way a couple of times, but this pedal has different connections which confuse me, such as having 2 Outputs from the board and 2 pads for a Switch....

Somebody please enlighten me!!

Jarno · Post by **Jarno** » 02 Dec 2009, 13:25

Not sure what the two pads for the switch are for, but you don't really need to use the two outputs. If you want it to be true bypass, don't use the buffered out. If you do want to use the buffered out, you only need a DPDT footswitch and just switch the output from buffered to effect out (and a LED on the remaining pole).

Edit:
Could the two switch pads be to both switch the indicator led AND the effect on/off led?

kierc · Post by **kierc** » 02 Dec 2009, 15:59

Hmm... That's what I thought I was doing when I wired it like my sketch

I thought with my sketch that to the right it would cut off the Indicator LED and pass the Buffered Output to the output jack, and to the left it would turn the LED on and pass the effected signal out...

I've placed the Layout over the PCB to get a clearer look on everything :

Looks like the Switch breaks the circuit between the 2 LEDs which are in series(?), so I think it would cut both the indicator and effect LEDs off...

I've been out of the effect world for a while now, and I just can't see how to get this running through my tired eyes

Is the orientation of the 3PDT switch in my sketch the right way? with the pins horizontal and the holes vertical?

kierc · Post by **kierc** » 02 Dec 2009, 17:31

Just been playing with the wiring a bit...

I de-soldered everything from the 3PDT, and soldered the two "Switch" wires together to close the connection and try and get some light from the LEDs...

Seems that no power is getting to them

I wired the battery 9v + directly to the board rather than from the DC jack just in case, but still no lights

was hoping it was just the wiring or something simple!

Anyways... could it be the LF353N's I got from Futurlec? the 2SK117? (this was labelled just "K117" on the devise?)

I'll try and get some voltages and post back

kierc · Post by **kierc** » 02 Dec 2009, 18:21

Got some voltages :

Top LF353N:
1. 4.54
2. 4.49
3. 4.07
4. 0
5. 4.07
6. 4.46
7. 4.47
8. 8.88

Bottom LF353N:
1. 4.43
2. 4.42
3. 4.32
4. 0
5. 4.43
6. 4.43
7. 5.79
8. 8.79

2SK117 ( haven't got the pinout handy so I'll say left, middle, right as per PCB layout )
Left. 8.72
Middle. 2.17
Right. 4.69

Took these with a fresh 9v battery that read 9.46v at the board input...

Don't know if anything looks off or what, but I'm completely lost as it is!

Jarno · Post by **Jarno** » 02 Dec 2009, 19:45

I think those voltages are ok, but it would be really helpful if Baja would help out, as I haven't built this thing (yet). I am working on the compulator, which is surprisingly similar (although that's a compressor and this is a limiter ????).

kierc · Post by **kierc** » 03 Dec 2009, 21:43

Bit of a good update I guess!!

Got it working to an extent - seems my 3PDT is fubarred (it's been in a couple of pedals now I think...) I'll find something else at some point...

Have hard wired the "Switch" connections closed so the LED's are always working, and also hard wired the effected output to the output jack, so that its always working

Now, the only thing is that when it "limits" the signal is distorted - is this the photocell? I used Photocell 1 from Futurlec paired with a standard 5mm Yellow LED...

Cheers for your help so far Jarno - hope your Compulator build goes nice and smoothly

I'm tempted to build both and try to hear the different results...

Jarno · Post by **Jarno** » 03 Dec 2009, 22:35

My first reaction was that it shouldn't be the LDR, but looking back in this thread, apparently they can cause mayhem.
Maybe add a cap on the input side of the LED driver opamp, to smoothen out some of the ripple?

Have you measured the "light" and "dark" resistance of the LDR? Although, apparently, if it is causing problems it's because of it reacting too fast, I don't think your going to catch that behaviour with your DMM.

kierc · Post by **kierc** » 04 Dec 2009, 22:29

Could anybody chime in on this clipping/distortion/whatever it is?

I've changed the switch to a nice Alpha DPDT I stole from my Echo Base build

(was noisy and haven't got round to fixing that one yet!)

Through the buffered output the signal also clips to an extent but not as drastic as when limited... I thought the buffered signal wouldn't be passing through the LED/LDR combo to share the problem? If so - could it be something else causing it?

@Jarno, could you explain adding the cap a bit better for me? I'm not much of a modder/tinkerer with circuits, usually just build them stock!

I would try another LDR, but I'd have to order specially which is expensive - and the Vactrol VTL5C10 used in the Compulator is like £5?!?

I might have an LDR left over from my Neovibe build - doubt it would be useable if I even have one though!

Anybody have a spare to sell?

bajaman · Post by **bajaman** » 04 Dec 2009, 22:39

smallbear has VTL devices if I remember correctly.
bajaman

kierc · Post by **kierc** » 05 Dec 2009, 21:27

Cheers for the input Steve, but (to me anyway) that's still a hell of a lot of money for such a small part

From my symptoms (in my last post), do you think the LDR is the culprit? as you yourself said the Futurlec part is suitable, or were you just going from the light/dark resistance on paper? as they state it has a fast response time...

I don't suppose you would have a suitable LDR spare that you could sell me do you?

swt · Post by **swt** » 08 Dec 2009, 13:33

hi guys. you can go with a standar ldr, the problem with the strange distortion is the second opamp rail clipping. in the compulator, i solved this going higher with the trimpot, the 1k1 suggested just made the distortion. In this case, the 560 ohm resistor didn't work for me, can't remember the value i chose, but go higher and the distortion will disappear. But, it does change my guitar tone compared to the bypass signal, it makes it a little thinner, don't know why...

kierc · Post by **kierc** » 15 Dec 2009, 13:35

Got my VTL5C10 from Small Bear today (fast! was shipping on Saturday I think?!), got it all wired in and I changed the LED to an Orange one as I think I saw someone suggest it... (couldn't be bothered to swap some out to try, just went with it!!)

s'all working fine now - will have to try it better later as I could only play quiet earlier, but it had little to no distortion, and did make it sound a bit fatter!

Will have to try a Compulator next time!

roseblood11 · Post by **roseblood11** » 20 Dec 2009, 13:12

Hi,

Bajaman, why did you chose a 1n4148 as polarity protection diode? Wouldn´t the voltage drop be lower with a Schottky (1n5819 or similar)?

pablopearljam · Post by **pablopearljam** » 23 Feb 2010, 14:17

Hi guys,

I have been trying to build the Demeter Compulator, but it is impossible to get the VTL5C10 here in Argentina. I have been trying to find an alternative, and then I found this project.

I have a few questions:
1. Has anyone tried this with a bass? Do you think it can sound similar/better to/than the compulator?
2. Does anyone have the layout and PCB for this circuit?
3. Are there any audio samples of this little beast working?

I hope this is not much trouble for you to reply to

Thanks in advance!
Pablo

elziel · Post by **elziel** » 20 Dec 2010, 09:16

Hi!

Im wondering Is it possible to add Va meter showing gain reduction?.

matt239 · Post by **matt239** » 26 Jul 2013, 19:13

This seems very cool. There is some stuff I don't get about it:

What makes it a limiter vs. compressor? Feed-forward doesn't automatically make it a limiter.. Is something happening to raise the threshold here so it only acts on peaks?
How does the detector/envelope section work; how do the two 1N4148 diodes function? (I saw what PaulC said about DC offset, but it's a bit over my head HOW it actually works..)

Could more control be added: threshold? ratio? attack? release?

BAJA LA2A Style Optical Limiter [documentation]