Need ideas for a limiter circuit

[Gaz]

...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!

[J M Fahey]

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

[Gaz]

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.

[J M Fahey]

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.

[Gaz]

Thanks, I got a couple ordered and will give this circuit a shot and report back!

[mhuss]

Sadly, Vactrols are also going the way of the Dodo (CdS!!! ). I started using MOSFET output optoisolators (e.g., NEC PS-7141 series) instead for switching. As Juan pointed out, if you just need a switch, linearity, etc., doesn't really matter.

[Gaz]

Sadly, Vactrols are also going the way of the Dodo (CdS!!! ). I started using MOSFET output optoisolators (e.g., NEC PS-7141 series) instead for switching. As Juan pointed out, if you just need a switch, linearity, etc., doesn't really matter.

Thanks for the advice, Mark. What application are you using the mosfet optos for?

[olddawg]

I used to use 4 diodes to limit the input to approximately line level. But they will hard clip anything over the propagation voltage. Another old trick is to take a signal off of the main amp input, isolate it with a cap, rectify it with a diode, filter it with a cap, and then use that DC voltage as a preamp output control voltage using a couple of transistors or an opamp. The problem is it isn’t instantaneous. When it turns on there may be something audible. You have to play around with cap and resistor values, fast diodes, etc. Time constants matter. There used to be an application specific IC used in a lot of high powered stereos that basically had a power supply pins, a ground, an in, an out, and a control voltage input. Sometimes it was used as a volume control too. The chip number escapes me.

[mhuss]

So far, just channel switching. The optos are available with 400v ratings. I attached a simple example.

[Gaz]

Cool. Thanks for sharing. Gonna put that in my bag of tricks for sure.

[Helmholtz]

I used to use 4 diodes to limit the input to approximately line level. But they will hard clip anything over the propagation voltage.

Any diode clipping can be made softer by wiring a resistor (e.g. 100 Ohm depending on source impedance) in series with each diode. (Also works with overdrive circuits.) But limiting will be less effectve as the resistors still allow for some level increase.

The problem is it isn’t instantaneous.

For clean limiting/compression, the circuit response must not be really instantaneous. This is why LDRs (photocells) work fine, as they react slowly enough having a HP corner frequency of 10Hz or lower.
Instantaneous limiting (like with diodes) inevitably produces distortion.

Time constants matter.

That's the main point, especially regarding attack. I think something in the 100ms range should do.
Too fast response (< 10ms) causes low frequency distortion, too slow response causes noticeable level overshoot. And instantaneous response produces distortion at all signal frequencies.

[Gaz]

Ok, this is the very simple Ampeg/Crate Opto Limiter.

[ATTACH=CONFIG]51898[/ATTACH]

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.

Ok, so I got the parts in, and got this functioning... kind of. I am using a NSL32SR2 opto, and subbed 1N4007's for the 1N914's (I don't believe it would matter in this circuit).

Here's a schem of how I have it setup. I can get the opto to turn on with signal, but need help setting where that happens. I am using the LDR in the bottom half of a divider (as you suggested) feeding the limiter. My goal is have this kick in when signal reaches 3Vp. Does that mean I drop the the +/- rails to be 3V? Still a little confused, but think I've almost got it

[J M Fahey]

Why 3Vp?

I thought we were talking about limiting a big nasty Class D module, hundreds of watts RMS, so speaker out NEVER reaches power rails but stops 1V shy of them, never clipping.

This power amp limiter does not know or care about what the preamp does.

[Gaz]

Why 3Vp?

I thought we were talking about limiting a big nasty Class D module, hundreds of watts RMS, so speaker out NEVER reaches power rails but stops 1V shy of them, never clipping.

This power amp limiter does not know or care about what the preamp does.

Ok, I see, so I need to be taking the input from the speaker output? Someone else asked about that earlier as well, so I was confused. It reaches about 60Vrms (~84 peak), so I take get the +/- rails from the preamp B+, and drop them down to 80vdc or so? Then I use LDR the same way as the bottom of a voltage divider at the preamp output?

[J M Fahey]

Ok, I see, so I need to be taking the input from the speaker output? Someone else asked about that earlier as well, so I was confused. It reaches about 60Vrms (~84 peak), so I take get the +/- rails from the preamp B+, and drop them down to 80vdc or so? Then I use LDR the same way as the bottom of a voltage divider at the preamp output?

1) Forget the preamp, period.

2) re read and apply the first 3 asterisked answers in post #40 , itīs all there.

If you do not understand some bit of it, ask.

Forget RMS volts, all we care is rail volts and setting voltage references 5V away from them.

Original design applies to +/-51V ... what are *yours*?