Need ideas for a limiter circuit

[Gaz]

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*?

Yes, I reread that post many times, and I do get the concept now about 5 volts away from the rails. I will have to measure the rail voltage now that I understand that.

My main confusion now is what is the purpose of the LDR? I thought it needed to go in the preamp to lower the signal when the limiter kicks in.

[J M Fahey]

The LDR is floating and can go anywhere.

Typical is to put a resistor in series with amp input and the LDR from amp input hot to ground, so "between preamp OUT and power amp IN" but it can attenuate at an earlier stage.

In a nutshell: the *detector* part is Power amp business, and uses +/- rails and speaker output.

The attenuator part may be between preamp and power amp or at an earlier Preamp stage.

*IN THEORY" , the LDR might even be in parallel with Guitar/Bass passive volume control but for other reasons (not bringing the fox inside the hen house) I prefer not to bring powerful *speaker* signal towards early sensitive stages.

But ... but ... Iīm bringing the LDR, not speaker signal.
Whatever, LDR wires live in a loud signal environment (LED current IS part of speaker signal) and parasitic capacitance or inductance may some of bring it backwards, so ... I play it safe.

After all, many amps have radio interference problems ... and we are talking about a signal carried through insulating air and measured in micro Volts ... or less.

[Gaz]

Thanks for taking the time reiterate - this is all clarifying. My only issue now is trying to measure the positive and negative rails on the class D module since I have no schematic. Maximum output voltage is listed on the datasheet as 76Vp. Does that mean that is the rail voltage as well? Here is also a shot of the block diagram if that's helpful to find a measuring place.



Once I find that out, I'm wondering if it's advisable to tap off the module for the detector, or create the detector voltage from elsewhere so I don't inadvertently damage the module...

[eschertron]

Thanks for taking the time reiterate - this is all clarifying. My only issue now is trying to measure the positive and negative rails on the class D module since I have no schematic. Maximum output voltage is listed on the datasheet as 76Vp. Does that mean that is the rail voltage as well? Here is also a shot of the block diagram if that's helpful to find a measuring place.

[ATTACH=CONFIG]52110[/ATTACH]

Once I find that out, I'm wondering if it's advisable to tap off the module for the detector, or create the detector voltage from elsewhere so I don't inadvertently damage the module...

I'm following this thread with intent... I'm going to build something like this for a class D power module as well. My take is that the power rails are whatever is being fed to the module. In my case, it's a 24vdc single-ended supply so it'll end up looking like +/-12vdc to some arbitrary signal ground. The takeaway is that the rail voltage reference is the very rails themselves, not something derived from the rails. I think

[J M Fahey]

Not too good news.

Pity we got to know only on post#63 which Class D amplifier are we talking about.

Not only it should have been mentioned from the beginning, but also a direct link to its datasheet should have been shown:

https://icepower.dk/download/1652/

no mention of schematic or internal circuitry but from specs, connectors and interconnection with slave units (which is most unusual), it can be reasonably guessed that:

1) no +/-V rails whatsoever so Ampeg/Crate limiter is unusable here.

2) 700W RMS my *ss
It can meet that spec only for 5 minutes, only starting from cold (i.e. : "5 minutes once a day")
Of course, since FTC rules no longer apply, under pressure (and probably $$$$) from consumer products vendor associations, you can publish any rating as long as they hold for a few seconds, so ...

3) actual RMS power ... which it can sustain without shutting down ... is 270W RMS ,into either 4 or 8 ohm

4) you do have a direct DC out from power supply, meant to power auxiliary units (as in a triamplified PA cabinet) ; itīs +43V DC, at an unspecified current consumption.

5) almost forgot: even if we had +/-V split supplies, we do not have a single speaker out to limit but two, since output is bridged , two hot, no ground, so itīs another nail in the Opto limiter coffin.
Oh well.

6) so I guess youīll have to apply Plan B: build a conventional limiter, as fancy or simple as you like, use it between preamp and power amp, drive power amp to clipping (check on a scope screen) and then adjust limiter threshold or level out until clipped signal goes down to normal, and further drive increase does no make output clip; readjust as needed and leave it there.

Oh well.

EDIT:

I'm following this thread with intent... I'm going to build something like this for a class D power module as well. My take is that the power rails are whatever is being fed to the module. In my case, it's a 24vdc single-ended supply so it'll end up looking like +/-12vdc to some arbitrary signal ground. The takeaway is that the rail voltage reference is the very rails themselves, not something derived from the rails. I think

Sorry, same thing as with Gazīs amplifier, this very clever limiter does not apply in single source bridged output amplifiers because itīs connected rail to rail and detects where a single speaker out "almost" reaches one or the other.

[eschertron]

Sorry, same thing as with Gazīs amplifier, this very clever limiter does not apply in single source bridged output amplifiers because itīs connected rail to rail and detects where a single speaker out "almost" reaches one or the other.

gnats!

I appreciate how a wider voltage spread helps minimize the effect of the diode voltage drops. If I have a signal that's 3vpp, not much room to control.

I'm likely to play with a J111 as a control element at the lower signal voltages I expect to see.

[J M Fahey]

Excellent idea, had forgotten J111

Just ordered 1000 of them at Mouser in TO92 case, which has gone out of manufacturing, I guess they are practically "auctioning" the last ones.

As "obsolete" as they are, they are less "obsolete" than CDS cells , at least they are not forbidden.

Not good enough as amplifiers, but definitely usable as audio switches, compressors, and phasers.

[Gaz]

Ah, Juan, sorry I am such a solid state neophyte, wasting your time with all the particulars of this circuit. Hopefully it will help someone building a proper "lead sled"

You're right that the Class D 700W is bullshit. I actually got my hands on my bandmate's Mesa D800 Subway that uses the same Icepower module, and it does only 600Wrms into 4 Ohms when the limiter light comes on. All this amp has (from some minimal circuit tracing) is a passive zener limiter. It actually clips before the limiter light comes on, which is under 600W. This thing does not like to be run with a continuous sine wave as you mentioned, and while I haven't gotten into thermal shut down I can feel it heating up, even with a fan, extra heat-sinking, and the chassis open. Mesa says to only run with the clip light going approx 25% of the time, and their published spec of 800W is with 10% THD. I don't really understand THD measurements, but on the scope coming from tube amps that's what I would personally call noteworthy clipping.

I've had pretty good luck trying a back to back 2.7V zener diodes at the output of the preamp, but I'd like a little more control, or precision as it goes into clipping a little earlier than I'd like (albeit it is a smooth transition).

[Gaz]

gnats!

I appreciate how a wider voltage spread helps minimize the effect of the diode voltage drops. If I have a signal that's 3vpp, not much room to control.

I'm likely to play with a J111 as a control element at the lower signal voltages I expect to see.

What circuits have you been looking at? It goes without saying, but I'm looking for more ideas now too

[J M Fahey]

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.

[eschertron]

What circuits have you been looking at? It goes without saying, but I'm looking for more ideas now too

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.

[J M Fahey]

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

[Gaz]

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/