Saturday, 27 February 2016

Madbean Holy Roller / Baja Optical Limiter

This is Brian's version of Bajaman Optical Limiter.
Here you can find the original thread.

You could swap D3 with 100R resistor and 100uF cap with 220uF if too noisy.


  1. Hey Alex - very cool choice, thanks for this. the LA2A is a famous compressor, so I thought I might try this -

    I had some trouble with white noise on my Diamante compressor, so I am curious about the notes at FSB where the OP says "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 pots )."

    I don't see a 100-ohm R on the input on the design above, or a 220 uf on output. I am just curious if you chose to leave off - or what? I did not read the whole thread over there.

    Any input on this would be appreciated. I can always just build it and see what happens, and add those things if I get white noise - just curious if you know more.

  2. Hi Paul!
    Didn't see that.
    I've used Brian's schematic from his website:
    If you want to try it, you could simply swap D3 with a 100R resistor and the 100uF cap with a 220uF cap.
    Let me know if it works!:)

  3. Thank you for the help! Since I put the limiter up front (currently using a Diamante from this site) the noise can build up, so I will most likely give it a try soon.

  4. Well, you can mark this verified, and it sounds great. I did not have an MPF102 so I used a 2N5484, I used a 3mm green LED and dropped the resister drawing current off to to ground down to just 470R. Otherwise all the same.

    This circuit is spanky - almost like it already has the post EQ. I know that Skunk Baxter used to like to record direct into a console through an LA2A limiter (because I recorded him once) - this has that spanky "Last Dance - Donna Summer" sound.

  5. Oh, and I also did one recommended mod in the first post where D3 becomes a 100R. I did not do the 220 uf (who wants one of those monsters on their board? and I have no noise problems at all.

    When you compate this to the Diamond they are both great, but that one squishes & adds sustain (longer attack & release) while this one just pops - making everything more alive. I guess you have to love limiters to appreciate this like I do.

  6. Cool! I'm glad it works. Thanks Paul!

    1. I just build a second one of these and I am having trouble getting the LED to work. It flashes on when I insert the DC power, so I know it is good - but it does not flash when I am playing. I can also tell the LDR is good just by covering it while playing. I get some limiting, but I know the LED worked in my first build. Any suggestions appreciated.

  7. A wonderful and highly recommended build from a wonderful layout!
    Way more usable than a compressor... (at least for me)
    I did a few changes though:
    - Changed the D3 diode to a 100R (as recommended) and left the 100 uf as original.
    - Since I did not have the correct opamps, I tried TL072 and it worked as a charm.
    - Instead of rolling my own Optocoupler, I slapped a NSL-32 in there, and it works as expected.

    1. One more thing:
      I mounted a 3mm Red LED in parallel with the optocoupler, and I now have a limiting indicator LED. Very neat!!!

    2. Wow! This Limiter is absolutely amazing.
      It really sounds like a LA2A!
      I swapped the top IC with a OP275 (not recommended for the other one, keep as is!), and the result was mindblowing: more clarity and more "spanky".
      This one is a keeper on the board!
      You'll never use a compressor ever again after trying this one!

    3. Cool! Thanks for the suggestions Neil!

  8. Okay - I am frustrated. I just built two more of these and neither one works correctly. In short - the LED is not lighting up at all. I know it is connected correctly, because it flashes when I power on - and the LDR is working because it gets louder when I put my finger over it. I have audio (and in fact it is even limited, so there must be other limiting circuitry in here besides the LED.)

    Unfortunately I gave away the working one.

    I have the exact same problem on both new builds. Now, I now I ended up with a different OpAmp than the 353P - because I still have all 12 I bought. And from my comments I can see I used a different transistor (even though I have MPF102 in stock now).

    Now, I tried putting in TL072s and I accidently bent pin 8 of the lower one - and the LED strayed full on (weird). But otherwise it does not light up with signal at all.

    I hope someone has some ideas.

  9. After building two of these, I have noticed that there is a quite nasty switch-noise when engaging the pedal.
    So I would recommend that you add a 1M to ground at the input as pop-prevention.

    I see that there is a 1M to ground at the gate of the JFET, but this is probably just for setting the input impedance, as it is not preventing pops when engaging...

  10. Hi there. This pedal sounds great! Because of this I want to build some of these and use them as a Insert Effect for my the mixer.
    I noticed, that with a hotter line level the pedal starts to distor. Anyone an idee to ajust it for line level?
    I tried a trimmer but to level the signal down. But it´s not that easy. It´s not enough and at the end there is not enough headroom to bring it back to line level.

  11. Hi there. I built this last week and I have a sort of fuzzy noise on top end, especially when used with a bass guitar. Any idea?
    BTW, is there a way to modify its "treshold"? I find it quite subtle, even with the "limit" pot all the way up: the optocoupler's LED is not as modulated by audio as I expected...

    1. That happens to me with Chinese knockoff op amps. Cheap chips, if you will!