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Thread: Need ideas for a limiter circuit

  1. #71
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Gaz View Post
    What circuits have you been looking at? It goes without saying, but I'm looking for more ideas now too
    Quote Originally Posted by J M Fahey View Post
    Oh, donīt worry, discussions are aways worth it because lots of new ideas appear .

    Pity this simple clever limiter which covers 99% of amps, from TDA2030 to 1000W monsters met the 1% which does NOT use dual rails.
    The one criterion I used for the class D amp that I selected, was that it was cheap. Literally, about USD $2, free shipping. If I end up finding something else that I like that also works better with the limiter design mentioned, I'd go for that.

    As far as using a J111 as a variable resistor, I'm back at looking for designs and discussions across the interwebs. If I'm using a 24vdc single-ended power supply, my signal path will be with +/-12vdc rails, otherwise indistinguishable from most stomp-box designs. There's plenty of compressor-type pedals that might help the design process. And I'm not above having a diode string to ensure a max level. As I see it, there are two design goals:

    1) prevent a signal level that causes the class D amp's output to collapse. Maybe some other modules are better in this regard, but the one I'm testing displays nasty behavior here. A simple diode clipper can do this.
    2) create a bit of compression just before clipping that models a traditional PP tube amp. This is where compression/sustain gives a little 'sweetness' to the notes. I expect a limiter circuit to be highly effective for this.

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  2. #72
    Old Timer J M Fahey's Avatar
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    As of plain clipping avoiding rail smashing, single diode drop is often not enough, so I often use a couple antiparallel red Leds, 1.9V peak each way so nice 1.5V RMS , enough for any regular power amp, and then add a trimmer after that, as a fixed Master Volume.

    I start with "Master" set to "1" , rise signal until Leds moderately clip (flat topped sinewave) , then slowly rise master trimmer until I see Class D amp clip on its own, which is clearly visible, sinewave becomes "squarier" and tops become flatter and wider, then back up a little until Class D amp is not clipping on its own.
    Then leave it there.

    In a way, same as old VOX Thomas limiter, only they varied clipping threshold by adjusting diode bias, I do the opposite by relying on fixed clipping, no bias involved, and varying clipped signal out level.

    End result ia about the same.

    You can use strings of plain diodes instead, but 2 Leds are equivalent to 6 diodes, for about same result,so I pick the simplest version.

    Itīs also easier to add to a pre existing circuit, no PCB needed

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  3. #73
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    Quote Originally Posted by J M Fahey View Post
    As of plain clipping avoiding rail smashing, single diode drop is often not enough, so I often use a couple antiparallel red Leds, 1.9V peak each way so nice 1.5V RMS , enough for any regular power amp, and then add a trimmer after that, as a fixed Master Volume.

    I start with "Master" set to "1" , rise signal until Leds moderately clip (flat topped sinewave) , then slowly rise master trimmer until I see Class D amp clip on its own, which is clearly visible, sinewave becomes "squarier" and tops become flatter and wider, then back up a little until Class D amp is not clipping on its own.
    Then leave it there.

    In a way, same as old VOX Thomas limiter, only they varied clipping threshold by adjusting diode bias, I do the opposite by relying on fixed clipping, no bias involved, and varying clipped signal out level.

    End result is about the same.

    You can use strings of plain diodes instead, but 2 Leds are equivalent to 6 diodes, for about same result,so I pick the simplest version.

    Itīs also easier to add to a pre existing circuit, no PCB needed
    I used your red LED idea (except add another in series to each side), and it clips ~3.6Vp. This is just about perfect, and I am using the trimmer after the clipper like you mentioned to take it down a little if needed. Like you said, this is simpler than the Vox scheme.

    I also found this circuit, and it works well! It has pretty much the same performance at the LEDs however, and even when I force them both to clip hard they pretty much sound the same. In practice that volume level (800Wrms or more) subtly is lost.

    https://www.allaboutcircuits.com/tec...ified-signals/

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