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.

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

13 comments:

  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.

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  2. Hi Paul!
    Didn't see that.
    I've used Brian's schematic from his website:
    http://www.madbeanpedals.com/EP/index.html
    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!:)

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  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.

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  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.

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  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.

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

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    Replies
    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.

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  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.

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    Replies
    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!!!

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    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!

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    3. Cool! Thanks for the suggestions Neil!

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  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.

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  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...

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