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Thread: Need ideas for a limiter circuit

  1. #36
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Gaz View Post
    I can't just passively attenuate down to that level with the gain control and master volume cranked, or I will never be able to get a clean sound at full power.
    It's not really volume that makes the class D power amp clip miserably, but max signal. What ever output headroom V3 has, it must be less than anything that clips the power amp. Even momentary peaks on an otherwise 'clean' signal. Regardless of how much quieter the volume appears.

    With tube power amps, the momentary signal peaks don't immediately offend. In fact, there's some mojo to the compression/limiting that the power tubes do to the signal before it gets noticeably clipped. That luxury is not available with class D power amps. So the limiter circuit must provide a replacement effect to that expected soft compression.

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    Quote Originally Posted by eschertron View Post
    It's not really volume that makes the class D power amp clip miserably, but max signal. What ever output headroom V3 has, it must be less than anything that clips the power amp. Even momentary peaks on an otherwise 'clean' signal. Regardless of how much quieter the volume appears.

    With tube power amps, the momentary signal peaks don't immediately offend. In fact, there's some mojo to the compression/limiting that the power tubes do to the signal before it gets noticeably clipped. That luxury is not available with class D power amps. So the limiter circuit must provide a replacement effect to that expected soft compression.
    Yes, that's a really good way of putting it. I am already missing auto-limiting-tube-mojo. In the previous post I am referring to the master volume control, that when turned up, creates more signal voltage

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  3. #38
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Gaz View Post
    Yes, that's a really good way of putting it. I am already missing auto-limiting-tube-mojo. In the previous post I am referring to the master volume control, that when turned up, creates more signal voltage
    Right. So you can turn the signal amplitude down for balancing heavily-distorted sounds versus clean sounds, but you can't turn it up to balance clean sounds versus heavily-distorted sounds. Make sense? The overall effective volume of the amp is less, if you take the clean sounds as the benchmark. This is one argument for why a SS amp needs to be 2x to 3x more powerful than an equally 'loud' tube amp.

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    Quote Originally Posted by eschertron View Post
    Right. So you can turn the signal amplitude down for balancing heavily-distorted sounds versus clean sounds, but you can't turn it up to balance clean sounds versus heavily-distorted sounds. Make sense? The overall effective volume of the amp is less, if you take the clean sounds as the benchmark. This is one argument for why a SS amp needs to be 2x to 3x more powerful than an equally 'loud' tube amp.
    Yes, I certainly understand the dilemma. I am looking for possible solutions

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  5. #40
    Old Timer J M Fahey's Avatar
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    Ok, this is the very simple Ampeg/Crate Opto Limiter.

    Click image for larger version. 

Name:	OptoLimiter.jpg 
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    How it works:

    * There is a string of resistors from +V to -V : R45 - R48 - Ground - R47 - R52
    Values are calculated so R45 and R52 drop about 5V , so R48 and R47 must be scaled up/down depending on actual +/-V present, values shown match 50V rails but it can work with as low as +/-12V ones if necessary.

    * so we have +V-5V at the junction R45/48 and its complement at the -V side.

    * we have a diode net (D11-13-14-17) feeding the Opto LED, normally reverse biased, and it will pass current only if Speaker peak voltage "almost" reaches +V or -V rails.

    * you use that lowered opto resistance as you wish.
    Here they lower NFB resistance so gain drops; standard use is to put, say, 4k7 or 10k in series with amp input and LDR to ground.

    * the beauty is that besides simplicity itīs *external* to power amp and does not care a bit about its internal circuit, I bet it will also work well with amps such as yours.

    The speaker peaks never reach rails, so they never clip, and itīs auto adjustable: if rails drop or even mains voltage does, this peak detector self adjusts and tracks that.

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    Thanks so much posting the schem and explaining that circuit to me! Super appreciate that. I am eager to try it out. Just have to get that opto...

    One (perhaps dumb) question about the circuit: I plan on using the LDR in the lower leg of a divider like you mentioned. Does the LDR need to be ahead of the limiter input (SPK) like in the schematic? My guess is that it does, but my brain is going in loops trying to figure it out.

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    Last edited by Gaz; 01-12-2019 at 11:30 AM.

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    Old Timer J M Fahey's Avatar
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    Worst case, and if you can still buy a few CDS cells (already forbidden in Europe) you can cook your own Opto.

    As of the limiter, itīs fully external to the amp, it simply compares amp peak output against rails and when too close, LDR attenuates, just like that.
    Donīt worry about internal signal path, NFB loops, nothing, just a resistive attenuator at the input.

    In fact, I made a small stamp sized PCB and add this to "other" amplifiers which need it, in general small PA, Keyboard or Bass ones, but specially Piano amps where you have very high peaks and the least distortion sounds horrible .... mine *all* include some kind of limiter of course, although this one is "too clean", I prefer Fets which add more "flavour".

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    Last edited by J M Fahey; 01-12-2019 at 04:53 PM.
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    ..although this one is "too clean", I prefer Fets which add more "flavour".
    If a limiter reacts too fast, it produces (low frequency) distortion. Response time should be at least a few cycles of the lowest signal frequency for clean operation.

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    Yes, if you want clean operation

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  10. #45
    Don't forget the joker g1's Avatar
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    So, it does not matter if you are sampling at the speaker output, or any other part of the circuit, you just use whatever rails for that part of the circuit?
    In this case, you do not want to hear the harsh sound of the power amp clipping, so it would be best to sample in the power amp section, like in the example (post #40)?

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    Quote Originally Posted by J M Fahey View Post
    ...I prefer Fets which add more "flavour".
    I'd love to see an example... especially because I can't even find where to buy a vactrol. They seems to be obsolete!

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  12. #47
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    TO92 Fets are also going the way of the Dodo

    You can still get CDS cells, non ROHS compliant of course, at none less than Adafruit: https://www.adafruit.com/product/161

    Or ready made vactrols: https://modularaddict.com/parts/synt...s/max-vactrols

    or: http://store.synthrotek.com/Vactrol

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    Quote Originally Posted by J M Fahey View Post
    TO92 Fets are also going the way of the Dodo

    You can still get CDS cells, non ROHS compliant of course, at none less than Adafruit: https://www.adafruit.com/product/161

    Or ready made vactrols: https://modularaddict.com/parts/synt...s/max-vactrols

    or: http://store.synthrotek.com/Vactrol
    Thanks! Those links are helpful. Are there anything in particular I should be choosing for the specs, or will all these sold from specialty synth sites be in the ballpark for this purpose? I realize I will have to adjust the top leg of the divider depending on the On resistance of the LDR in the optocoupler.

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  14. #49
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    You will only adjust resistor values to get 5V reverence below or above rails, nothing else.

    Opto specs are important if you use them for something needing precision, such as tracking volume controls, Studio Compressors, tone controls, Auto Wah, Phasers or Jimi Hendrix type "chorus" , but here they are used individually and almost on an on-off fashion, so my experience is they are not critical at all.

    Just for the record, pass, say, 5 or 10mA through the driving diode and measure resistance across LDR.

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