BAJA Optical Limiter revisited - rev2 [documentation]

[bajaman]

Hi folks
Here is my latest offering to the community - a revised version of my LA2A style optical limiter.
this time I have built it on one board (board is flipped too).
One thing i noticed when I powered this from a small 500ma 9v switch mode power pack, was a very loud white noise. At first I thought I had a noisy op amp but this was not the case. The solution was to replace the polarity protection diode with a 100 ohm resistor and decouple the output side with a 220uf electrolytic to ground (this is seen in one of the photos elegantly tacked on to the board under one of the control potis ).
After fitting this decoupling of the dc supply, the limiter was totally noiseless - no hiss whatsoever.
Notice the fuck up with the switch wiring - the indicator limiter panel light was working in bypass instead of effect .
The cute red capacitors and alpha pots came from futurlec, sockets from Aron at that other stompbox forum, switch from pedal parts plus (thanks soulsonic ). The LEDs used are both standard yellow 5 mm types. the LDR has a resistance of about 90 ohms when exposed to the light - sorry I cannot give you the exact number for this part but i found by purchasing from a number of suppliers that there are two common types in this size (5mm circular with top and bottom sliced off). You can test with a multimeter across the terminals - expose it to sunlight and observe the reading - if it is 350 ohms or higher, then you have the wrong type for this project - it should read around 90 ohms.

anyway enough background info - here are some pictures for your perusal and PCB, layout drawings etc.
I built this one in a see through red ABS Hammond case - just for a change - by the way it does not hum either ( I did run a ground wire connecting the control pots to the input socket ground)
Enjoy
Cheers
bajaman

[swt]

Hey Steve! i have build the first version of the la2 but have some problems. If i set the comp pot to ground, the pedal works fine, but when i max it, or raise it, it sounds like a distorted signal riding on top of the clean signal, just like a misbiased op amp. Changed opamps, nothing happens. used an external led test the ldr, and it sounds ok, in fact it's limiting. The led works as suposed also, but the misbiased sound is synchronized with the distorted sound. Clues?. Thanks a lot!.

[swt]

Sorry Steve! didn't know the rule for new topics. Just got the answers on the other thread. Will report any advances. Thanks for the terrific design.

[bajaman]

Hey no problem swt
I do hope you get the distortion problem sorted - it is a great limiter when working properly.
Let us all know if you are still having problems with it.
cheers
bajaman

[minor7th]

Pardon me for drooling, and forgetting my usual bows and scrapes towards you, and you knowing how much I love this ciruit already, but there is one thing that stands out.

The enclosure... The enclosure... Where did you get the enclosure... I want the enclosure...

[bajaman]

Hi minor7th and others
It is a Hammond 1591B type case - I bought it from South Island Component Centre right here in ChCh New Zealand.
cheers
bajaman

PS: the brass threaded inserts for the case are in larger than normal plastic pillars - this is why the DC socket is not flush with the outside opf the case as it would be if I had used a metal 1590B or NSC1290 case. This is why there is no battery mounted in the see through case - it would not fit unfortunately
cheers
bajaman

PPS: just uploaded two more better focused pictures of the case.

[floris]

Thanks for sharing how you fixed the white noise in the pedal from the power supply. I'll try that on my noisy PT80 see if it helps.

[bajaman]

Yes - that is what this forum is all about - sharing information that we can ALL benefit from
Unlike some others like Analretentiveman who tells you he has fixed the power supply hiss problem but will not share any details
cheers
bajaman

[bajaman]

By the way - futurlec have a suitable LDR for this project - they are listed as PHOTOCELL1 (light resistance 5k and dark resistance 500k) - US$0.25c each!!!
cheers
bajaman

[swt]

ok. Changed the led for a 5mm and the ldr for a 10mm and guess what...it works. guess i learn something new each day....now...i'll go and enjoy a usefull limiter. It does enhance the tone in a good way. Thanks Steve!

[kagaxdx]

The solution was to replace the polarity protection diode with a 100 ohm resistor and decouple the output side with a 220uf electrolytic to ground

Just corious.. couldn't it be fixed just adding the 220uF decoupling after the diode instead of having to remove the diode? Would the diode works as a resistor (for the sake of filtering)?
Thanks for the project Baja!

[bajaman]

I think I tried that but found the resistor capacitor combination less noisy. Glad you like the project - one of my personal favourite pedals too
bajaman

[puppiesonacid]

Very nice build baja man!!!


thanks,


puppiesonacid

[modman]

I built this yesterday and it worked perfectly from the start -- I should have built a compressor long time ago

Did face some building problems:
- In/out jacks: they are not standard (?) pcb mount jack I have lying around, spacing between the legs are smaller. Is that the 'socket from Aron'?
- LDR: I relied on the first specifications: 90 ohms dark resistance, but the photocell specified in a later post. But seems fine still
- Can't figure out how to mount the pcb in a case with the jacks sticking out -- I'm on the verge on desoldering them and mounting the jacks first. There must surely be some trick but I admit I'm at a loss here. Jacks are 2mm and 3mm beyond the edge of the case and the large footswitch is not helping matters. It might be that I had to mount the pcb higher towards the surface of the case because of the larger jacks, and this is causing problems...

Trying to avoid sealing this build Expandora style
Really eager to box this one up for a recording on Sunday.

anyways, thanks for another terrific project Baja!

[bajaman]

built this yesterday and it worked perfectly from the start -- I should have built a compressor long time ago

Hey - glad you like it - it is one of my favourite pedals and so versatile in use - BUT - it is NOT a compressor - it is a limiter.
I do not wish to split hairs but limiters function completely differently to compressors - no pumping involved with limiters and below the preset limiting threshold - NO compression whatsoever!!

I have a dislike for compressors but limiters - well I love them - especially a good optical one like this.

- In/out jacks: they are not standard (?) pcb mount jack I have lying around, spacing between the legs are smaller. Is that the 'socket from Aron'?

Yes these are the PCB jacks from Aron

- Can't figure out how to mount the pcb in a case with the jacks sticking out

EASY - I drilled the holes in the case and then I hacksawed the case - so I ended up with "wheel arches".
Once the nuts are screwed on and the lid fitted you would be hard pressed to see that they are arched shaped slots.

Jacks are 2mm and 3mm beyond the edge of the case

Sorry - this is always the problem with board mounted components - The jacks I got from Aron are slightly different leg spacings to the Cliff and Rean types. a further complication with Aron's jacks is that the socket hole is offset to one side as well, but hopefully you will get yours to fit - once you start using it you will wonder how you ever got a good sound without it!!
It works for me at the end of my effects chain - just before the input of my tube amplifiers.
Have you built the silent selector yet???
Hmmmm - I have been a bit slack recently - my mother just died last week and it has put me a bit behind with my work schedule. I will miss her even though her last few years were spent fighting off the effects of type II diabetes, Parkinson's disease and a series of mild strokes that eventually made it impossible for her to swallow food, drink or medicine.
She was 86 when she died - it was on my late father's birthday - i could see him having a party and calling out to her to hurry up and join him for the celebrations.

Take care all my friends here on freestompboxes.
cheers
bajaman

[MPUMarko]

Hi bajaman,
thanks for the optical limiter. I'm at the process of making one at the moment. I'm a total newbie on stompboxes and electronics and there's a couple of components I'd like to ask you about:
-on the schematics there's a 10n capasitor on the input in front of 2SK117. On the component layout there's however no 10n capasitor.
-on the layout there's a 470ohm resistor close to the power input socket. On the schematics there's no 470ohm resistor.
-on the schematics there's only one 100n capasitor. On the component layout there's three of them.

I hope you find time to answer,
thanks in advance.
Greetings,
Marko

[bajaman]

Hello Marko
Use the component layout and PCB for construction - the schematic is essentially the same (there is a 100n instead of the 10n for the input coupling capacitor as on the schematic - no consequence).
i have built more than one limiter rev2 from the layout and pcb and they all work fine.
One day I shall get around to changing the schematic - unfortunately I lost some work on my computer a while back and this was one of those files. I only have what is on freestompboxes fortunately though.
cheers
bajaman

[floris]

I find the limiting part of the circuit a bit hard to follow especially the opamp stage that drives the limiter LED (bottom right in schematic).

The two 1N4148 diodes that are part of the feedback loop must set/influence the signal max before the limiter kicks in.
Somehow, until that level is reached, the opamp hardly drives the LED but beyond that, the opamp must drive the LED hard probably because the gain suddenly increases.

The opamp gain is set by the ratio of:
- 10k resistor between opamp output and the inverting input.
- Combined resistance of resistors (1k5 10k 4k7) and the two diodes between inverting input and Vbias/Gnd.

That last "combined" resistance is a a bit unclear to me especially how its dependency is on signal level.
Assuming that the two diodes behave either as a short (with voltage difference over them larger than 1.4V) or a high impedance (small voltage difference over them), I get:
=> Short: "combined" resistance as seen by the opamp's gain structure is 0 ohm. So the opamp gain becomes very high (10k / 0 = "infinite") as soon as the diodes start conducting which drives the LED hard on.
=> High impedance: "combined" resistance as seen by the opamp's gain structure is 1k5 // (10k + 4k7) = 1k4 ohm. So the opamp gain is set to 10k / 1k4 = 7.3 as soon as the diodes stop conducting which probably results into the LED not (hardly) being driven.
The construction with the 4k7 resistor to Gnd must set the bias for the LED (when no signal is present) so that it is turned on just a little "in rest".

Can someone please explain that part of the circuit and tell if I'm on the right track?

[RnFR]

hey BM!

first of all my condolences for your loss. i couldn't imagine how that must feel, but hopefully their birthday party was a good one.


i was wondering about whether this would be a good project to rack up and use in my live sound duties. maybe running at a higher voltage via a charge pump? burr brown op amps? would it be possible to do balanced in/outs? or would it even be worth it at all?

[MPUMarko]

I just got mine working except the indicator LED. I've checked the LED and it's ok and correct but it's not working in either position of the switch. I'm using this limiter for bass with very high output pickup and I think there's a bit too little usable range on the limit pot before it starts to really eat lows. Any tips?
Marko