Need ideas for a limiter circuit

[Mike Sulzer]

The only question is where do you stop -producing- and start -re-producing?

Well, you do not stop producing at the preamp output, but have a good low power output stage as well. Then you follow that with the reproduction: an attenuator/speaker impedance emulator, and only then the class D SS amplifier. Then you have all the flexibility of different kinds of tube overdrive, from 0 watts up to whatever the SS amp does.

[R.G.]

I got tired of typing early, I see. Thanks for reminding me.

That all should have had a paragraph that the last link in the chain is the link from the power amp output to the speakers, and the kinds of speakers themselves - or emulations thereof. Included in there is the semi-mystical interaction of the OT, amplifier output impedance, speaker characteristics, and any nonlinearities in the speakers themselves.

With Class AB amps, it's possible to pull an opamp trick or two to make the amplifier behave as if it has a higher output impedance, loosening the amp's death grip on the speaker cones and letting them flop a bit independently. All of the SS amps I've seen recently include something like this dodge to let the speaker sound honk through.

I know you've been looking at speaker impedance simulation recently. Have you looked into speaker distortion mechanisms as well? That bit of polishing ought to make for a real step forward in speaker emulation.

[Helmholtz]

Have you looked into speaker distortion mechanisms as well?

Can you provide relevant data for quantitative evaluation?

[Gaz]

Are we talking Guitar amplifier or Bass amplifier here?

If Bass, you need tons of clean pkower, period, and preamp is used basically clean.

It's for bass, but one of the channels is very high gain (4 gain stages). Of course you know some people like distorted bass!

FWIW the Fender "Bassman TV series" you linked to is a fraud, pure and simple, the only "DNA" they show is *cosmetics*.
Oh, and "do nothing Tube preamp, since used that way it´s always clean, does not colour sound and might as well have been made with Fets or even Op Amps.

Yes, I agree, and of course this is a gimmicky use of a dual triode - I was just interested in the details of how they mated the tube pre to Class D module since I could find no other examples.

That´s why for me the only "honest" way to do it (which is also the simplest one) is to clip preamp **even if it will later be used "clean"**, what we are doing is finding its limits, then feed that into any loud clean Power amp you like, and carefully adjust it (a screwdriver adjustable Master Volume is as good as it gets) until it *just* clips and leave it there.

The more I consider my particular situation (where ALL the tone and clipping is coming from the tube preamp), then I think your approach is the simplest, and I may just include some protective diodes for good measure, especially since @Mike Sulzer pointed out the module has soft-limiting built in, which I did not know!

[Gaz]

Go here: https://icepower.dk/products/amplifi...mparison-chart

Scroll down and click on "see comparison chart" . Look down for the soft limiter entry.

Yes, I did not mean to double post Juan's idea. I had not seen his post yet.

Mike, thanks! I did not see this in the datasheet, and so the fact there is already some soft limiting built in is news to me!

[Gaz]

There are some other sub-cases. Gaz, if your preamp produces 7Vpk on the normal preamp output, and 21Vpk on the overdrive, you'd see two different kinds of clipping - (probably) the preamp tube soft distortion on the normal channel, and (probably) the power amp input clipping on the distortion channel. The bigger signal (probably) causes a different kind of distortion when driving the power amp input, and (probably) different yet when the PI is a LTP versus a concertina. The details start multiplying.

I have been using this preamp into a clean 200W tube power amp from another amp (a clone of a Hiwatt), and have not been driving the PI into clipping. In fact, I don't like the sound of the power amp breaking up, which is why I started looking into the Icepower modules in the first place. From this conversation I have a much better understanding of how limiting actually functions, however it still perplexes me a little that so many SS amps have such complex limiter circuits built in. I realize these must be more for tonal reasons, and not to merely protect the poweramp? Why not just passively attenuate before the power amp to a safe level in all cases?

[J M Fahey]

it still perplexes me a little that so many SS amps have such complex limiter circuits built in. I realize these must be more for tonal reasons, and not to merely protect the poweramp? Why not just passively attenuate before the power amp to a safe level in all cases?

It´s not for protection but because:

a) SS power amps clip *ugly*

b) you go from sweet but dull 0.1% distortion to buzzy flat top squarewave, add "farty" if you were not careful cutting unwanted deep Bass within 1 dB or two , say "trying" to go from ultra clean 95W RMS to clipped 120 to 150W RMS on a 100W amplifier.

Same on a tube amp will go smoothly from polite sound to fat growly, which is not bad at all.

c) so you are at ease playing loud with a tube amp , while with an SS one you either play far from maximum power, or stand buzzy ugly sound (some do) or get the World´s Best Limiter to let you reach 90 W rms but never ever surpass 95W RMS (both on a 100W amp).

FWIW *ALL* of my Bass amps have a "self adjusting" distortion detecting limiter, similar to pioneering Peavey DDT who in dues time copied the IOC present in early Crown amps.

Some customers ask me whether I use Limiters or not, since they see no switch or Led showing its action, my answer is that they are hardwired and I do NOT want customers to be able to turn them OFF, period.
In my book, NO Bass SS amps can be used without a limiter, period.

d) another technique is the Crate/Ampeg optical one, it does not detect distortion BUT measures output peak voltage and does not let it get closer than 5V to either rail.

The beauty is that it can be added to *any* amplifier, even if it´s a "black box" without access to its innards (ahem!!!! ICE Power module cough! cough! )

Post the simplest Crate amplifier using one (they even use it in a humble 10W Bass amp, go figure) and I´ll suggest how to adapt it to ICE Power.

[R.G.]

I realize these must be more for tonal reasons, and not to merely protect the poweramp? Why not just passively attenuate before the power amp to a safe level in all cases?

No, they're almost never there to protect the power amp. Not any more, anyway. The Thomas Vox "Watchdog Limiter" was (IMHO) put there to protect the power amp because at the time, early 1960s, they didn't know any other way, or could not afford it. There are a few who use input signal for power limiting, but it's not common.

SS amp techniques instead branched off into protecting the output devices, and right at the output devices themselves. With modern multislope V-I limiting, you can make an SS amplifier's output stage so robust that you can leave it shorted, opened, anything in between, even highly reactive loads, without any real risk of damaging the output transistors. It's humbling to realize that your power amp is humming at power line frequency and the PT getting hot but no signal being heard in the speakers, and finding that you accidentally plugged in the speaker test leads to the power amp to make a dead short. I makes you respect the amp to find that the power amp simply resumes playing as though nothing has happened when the short is removed.

But that diversion of design thought into instantaneous robustness didn't include the kind of limiting you're thinking about. A few hifi amps did adopt soft limiting of power amps, but for different reasons. "Average" music, if that exists, runs with about at 20db peak to average power ratio. That is, the nominal listening level has huge power peaks riding on it. These peaks need large instantaneous power from the amp. Although the human ear is tolerant of short squarks of peak distortion - otherwise, we'd all HATE recorded music - hifi golden ears with oscilloscopes are not tolerant of anything, so the recommended amplifier power for hifi crept up from 10W or so to fractional-kilowatts over the years. This is not for running PAs, it's for listening to, say, light chamber music you can hold conversations over. The idea is that if the amp is big enough, it will reproduce even the peaks cleanly all the time. There is a surprisingly large amount of hifi literature on this topic.

What we're talking about here became a minor divergent path in hifi amplifiers. Some otherwise pristinely clean hifi power amps include something like a very, very soft clipper to make the transition from uber-clean average levels to softly, softly clipped peaks to let the soft clipping hide the vastly uglier razor-like clipping lurking just above the soft clipping, in theory anyway. But the whole idea here is to make clipped peaks be soft-clipped or soft-limited peaks and prevent them being little blats of clipping. Er, which the normal human ear tolerates pretty well, anyway, but then, this is not a logical process, is it?

So no, limiting is pretty much not done for protection any more. It's for avoiding distortion, which I suppose could be thought of as tonal benefits.

[R.G.]

Can you provide relevant data for quantitative evaluation?

Absolutely not.

But I knew if I didn't say that, it would come up in the next few posts.

Right?

[J M Fahey]

Just to throw a little more gasoline into fire: a very clean SS power amp and very flat speaker will faithfully reproduce miked up Tube sound, once it´s fully cooked and seasoned, but if you pull some essential elements from the recipe, taste won´t be the one you exopect, by far.

Removing the Tube amp clipping is a big loss; removing its interaction with speakers even worse.

Here Johan Segeborn clones 2 very famous but very different Guitar sounds from the 70´s: Kiss and Black Sabbath.

He´s been experimenting a lot, and to his surprise found that although they used different Guitars: SG and Les Paul, comparing just guitar to guitar actually showed little difference.
Then he compared heads: Marshall Plexi Vs. Laney Supergroup: basically same thing, since they are very similar internally.

Then compared their favorite cabinets, and then Pandoras´s box opened.
Some used Celestions in both and sound was quite similar (duh!) , then he tested those using Goodmans speakers.

Difference was SO amazing that he redid the test , playing classic riffs from both, using same guitar (since he had proven them very similar in another video) , same head (same thing), but different cabinets .... WOW!!!!!

What´s my point? : that those very different cabinets will NOT sound that way if driven from a clean high damping Class D power amp.

Does it mean that then it´s a waste of time trying to replace Tube amps for Class D ones?
Not at all, my point is that there still a lot of work ahead to add back those "errors" which are actually so tasty to us.

The real test starts at 2:50 :

Notice also that Power Tube clipping is bright, sharp, punchy, aggressive, not the "rounded top" fairy tale which is usually shown when trying to explain differences.

Think this:



rather than this:

[loudthud]

I posted this back in 2017 but it never got much notice. Only 49 views. It shows the difference between symmetrical clipping like a limiter discussed here might produce and clipping from a solid state amp with grid current emulation. Notice the absence of even harmonics in the symmetrical clipping case. This sounds cold or hollow.

I disagree with some of what's been said in this thread, but we'll have to just agree to disagree.

[J M Fahey]

Cool

Real scoping beats any "theory-without-practice" opinion any day of the week.

That said and for a fuller picture (literally ) , can you please also post each waveform besides its spectral "signature" as you showed above?
Thanks.

PS: and if available, also the schematics of "what" produced each

[nosaj]

I posted this back in 2017 but it never got much notice. Only 49 views. It shows the difference between symmetrical clipping like a limiter discussed here might produce and clipping from a solid state amp with grid current emulation. Notice the absence of even harmonics in the symmetrical clipping case. This sounds cold or hollow.

I disagree with some of what's been said in this thread, but we'll have to just agree to disagree.

Even Harmonics = enjoyable
Odd Harmonics = irritating.

Odd Man Out

nosaj

[Mike Sulzer]

Even Harmonics = enjoyable
Odd Harmonics = irritating.

Odd Man Out

nosaj

That sounds nice, but what makes more of a difference in practice is keeping the percentage of really high order harmonics low, and emphasizing the lower ones.

[R.G.]

I would add to the fire that intermodulation distortion (sum-and-difference distortion) is even more irritating than harmonic distortion. Every distortion process produces intermod and harmonic on mixed signals, but as a generality, the softer the clipping the less intermod as a percent of the total distortion.

Aaaaaannd we're off into the Original Weeds!