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Thread: Need ideas for a limiter circuit

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    Need ideas for a limiter circuit

    I'm mating a tube preamp to a Class D module (Icepower), and would like to include a limiter circuit (solid state is aok) to keep the power amp from clipping. I'm a bit lost on where to look, but my preliminary research make me think that I need something that is "soft-limiting/clipping." Any basic circuits I can learn from (although I realize most of this circuits are relatively complex). Thanks!

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Gaz View Post
    I'm mating a tube preamp to a Class D module (Icepower), and would like to include a limiter circuit (solid state is aok) to keep the power amp from clipping. I'm a bit lost on where to look, but my preliminary research make me think that I need something that is "soft-limiting/clipping." Any basic circuits I can learn from (although I realize most of this circuits are relatively complex). Thanks!
    3 possible solutions:

    1) if you have no problem with power amp eventually clipping but "want it to clip like a Class AB amplifier and not a Class D one" just add a couple (or a string) of plain diodes across its input, so "they" clip first.
    Diode clipping has quite a rounded and smooth edge .

    2) if you do not want the power amplifier to ever clip, no matter what, add a non clipping limiter between preamp and power amp.
    Scope output so you have as much as possible without squaring peaks.
    I suggest the simple and elegant Crate/Ampeg opto limiter, which stops waveform peaks a couple Volts before reaching positive or negative peaks.

    3) set preamp to maximum clipped output, balls to the wall, add a screwdriver adjustable "master" at power amp input, scope output and sllloooowwwwllllyyyy rise Master, starting from 0, until you see the wonky tube preamp waveform starts to squarewave on peaks (indicating now the Class D amp has started clipping by itself), then back down a little.
    Now nothing the Preamp does will clip the Power amp.
    FWIW that is the method I use.

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    Quote Originally Posted by Gaz View Post
    I'm mating a tube preamp to a Class D module (Icepower), and would like to include a limiter circuit (solid state is aok) to keep the power amp from clipping. I'm a bit lost on where to look, but my preliminary research make me think that I need something that is "soft-limiting/clipping." Any basic circuits I can learn from (although I realize most of this circuits are relatively complex). Thanks!
    The output stage of your tube preamp clips, although not cleanly. You could put an attenuator between it and the amp so that the preamp cannot quite drive the amp into clipping.

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    By the way, I think most of those Icepower modules have a built in soft limiter; who knows how good it is.

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    I've looked into doing something like this for a hybrid amp but I'm yet to put it to the test:
    http://sound.whsites.net/articles/soft-clip.htm

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    It's good to see this technique being accepted in the larger community. Pre-clipping the preamp before the power amp gets the signal is a neat trick. It can effectively eliminate the razor-edged clipping of high-feedback power amps - the power amps never clip, and always stay linear.

    I'm sure it's older than this, but the pre-powe-amp limiter was used to good effect in the Thomas Vox amps of the early-mid 1960s. The "bias" procedure on these amps was actually a pot that adjusted the clipping threshold on the preamp limiter. The procedure was somewhat as JM menioned - turn the limiter to minimum limiting, run the signal into the power amp until the power amp was just hinting at clipping, then turn the mixer/limiter control to make the output noticeably more clipped. This ensured that the limter, not the power amp, was clipping.

    As to what clipper to use, that gets tied up in signal level and adjustability. The Thomas Vox limiter had a wide clipping range, from about +/-0.6V to about +/-1.8V. It took two transistors and two diodes plus a handful of R/C to make that work. The Thomas Vox clipping/limiting circuit arranged for a variable amount of DC to run through resistors attached to the clipping diodes, which were ordinary silicon devices. Increasing DC current pulled the clipping points of each diode apart, letting the clip level be adjusted. It's a touchy circuit but sounds good.

    There are a variety of clipper/limiters that can be used. About the softest simple limiter is a pair of small signal MOSFET with gates tied to drains. This produces a clipped waveform that never entirely flat tops. It's actually a softer than many tube circuits' clipping. The only real drawback is the clipping voltage is around +/-3V or more depending on the threshold voltage of the MOSFETs. This is a pretty big voltage compared to the input voltage of a solid state power amp (including the IcePower), so you pretty much have to juggle the preamp signal up to clip, then volume control it down to run the power amp inpu at the right level. And it's not all that adjustable as to voltage clipping levels.

    @Gaz: I'd scope out the amount of signal needed to clip the IcePower module, then design a clipper that clipped at just less than that, and check this out by switching the clipper in/out while watching the output waveform. The IcePower module will have a relatively fixed input clipping point, so once you get the input clipping point set, you won't need to adjust it. Fixed is always better if you don't really need adjustable.

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    Quote Originally Posted by J M Fahey View Post
    3 possible solutions:

    1) if you have no problem with power amp eventually clipping but "want it to clip like a Class AB amplifier and not a Class D one" just add a couple (or a string) of plain diodes across its input, so "they" clip first.
    Diode clipping has quite a rounded and smooth edge .

    2) if you do not want the power amplifier to ever clip, no matter what, add a non clipping limiter between preamp and power amp.
    Scope output so you have as much as possible without squaring peaks.
    I suggest the simple and elegant Crate/Ampeg opto limiter, which stops waveform peaks a couple Volts before reaching positive or negative peaks.

    3) set preamp to maximum clipped output, balls to the wall, add a screwdriver adjustable "master" at power amp input, scope output and sllloooowwwwllllyyyy rise Master, starting from 0, until you see the wonky tube preamp waveform starts to squarewave on peaks (indicating now the Class D amp has started clipping by itself), then back down a little.
    Now nothing the Preamp does will clip the Power amp.
    FWIW that is the method I use.
    Thanks, Juan!


    #1, Do you think an arrangement like @zozobra posted would be worthwhile? http://sound.whsites.net/articles/soft-clip.htm

    #2 - Which Ampeg/Crate design would be a good one to study? (i.e. the simplest one for me to distill the limiter circuit from).

    #3 Having no knowledge of limiting circuits, my first inclination was to do #3, and to be honest I'm not sure why not all preamps aren't just attenuated to that point before power amp clipping Is it really to just make the amp feel more tube-like?

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    Quote Originally Posted by Mike Sulzer View Post
    The output stage of your tube preamp clips, although not cleanly. You could put an attenuator between it and the amp so that the preamp cannot quite drive the amp into clipping.
    Thanks, I may just do this, and if I understand correctly, you mean the same as JM did in his #3 suggestion.

    Quote Originally Posted by Mike Sulzer View Post
    By the way, I think most of those Icepower modules have a built in soft limiter; who knows how good it is.
    I'm not sure. I emailed them, and they did not mention it, just that input sensitivity was 2.3Vrms, and no need to go past 2.5Vrms (limit before damage is 12Vp).

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    Quote Originally Posted by R.G. View Post
    It's good to see this technique being accepted in the larger community. Pre-clipping the preamp before the power amp gets the signal is a neat trick. It can effectively eliminate the razor-edged clipping of high-feedback power amps - the power amps never clip, and always stay linear.

    I'm sure it's older than this, but the pre-powe-amp limiter was used to good effect in the Thomas Vox amps of the early-mid 1960s. The "bias" procedure on these amps was actually a pot that adjusted the clipping threshold on the preamp limiter. The procedure was somewhat as JM menioned - turn the limiter to minimum limiting, run the signal into the power amp until the power amp was just hinting at clipping, then turn the mixer/limiter control to make the output noticeably more clipped. This ensured that the limter, not the power amp, was clipping.

    As to what clipper to use, that gets tied up in signal level and adjustability. The Thomas Vox limiter had a wide clipping range, from about +/-0.6V to about +/-1.8V. It took two transistors and two diodes plus a handful of R/C to make that work. The Thomas Vox clipping/limiting circuit arranged for a variable amount of DC to run through resistors attached to the clipping diodes, which were ordinary silicon devices. Increasing DC current pulled the clipping points of each diode apart, letting the clip level be adjusted. It's a touchy circuit but sounds good.

    There are a variety of clipper/limiters that can be used. About the softest simple limiter is a pair of small signal MOSFET with gates tied to drains. This produces a clipped waveform that never entirely flat tops. It's actually a softer than many tube circuits' clipping. The only real drawback is the clipping voltage is around +/-3V or more depending on the threshold voltage of the MOSFETs. This is a pretty big voltage compared to the input voltage of a solid state power amp (including the IcePower), so you pretty much have to juggle the preamp signal up to clip, then volume control it down to run the power amp inpu at the right level. And it's not all that adjustable as to voltage clipping levels.

    @Gaz: I'd scope out the amount of signal needed to clip the IcePower module, then design a clipper that clipped at just less than that, and check this out by switching the clipper in/out while watching the output waveform. The IcePower module will have a relatively fixed input clipping point, so once you get the input clipping point set, you won't need to adjust it. Fixed is always better if you don't really need adjustable.
    Thanks, R.G. I've actually read your article on the Vox limiter (https://www.premierguitar.com/articl...-in-vox-amps-1)

    I did email Icepower a while back about input sensitivity, and this was their answer:

    "Yes the differential input voltage needed for 720W in 4 ohm is 2.3Vrms. If you input is 2.46Vrms differential you will get 836W at 4 ohm with 5% THD+N, so more than 2.5Vrms input does not make much sense. Keeping the max input level at ~2.5Vrms (3.6Vpeak) also gives better/lower idle noise than with larger gain in your preamp."

    The actual input max to the module is 12Vp.


    I will have to audition some clippers, but if like the effect, what would be reasonable threshold to set? 2.5Vrms according to their comments? I'm thinking it may be cool to add a clip light for myself, so I can have a visual for when the threshold is being crossed.

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    Whenever I see "clipping" I always shift over to peak/instantaneous voltages, not rms. 2.3Vrms is 3.25V peak, 2.5V rms is 3.535V peak. So you're talking about clippers that limit voltage to between 3.25V and 3.54V.

    I'm interpreting the comments from IcePower as saying in effect "... the input is not distorted (much, anyway) at input voltage peaks of 3.25V, but another quarter of a volt starts pushing the amp into the beginnings of clipping", which is how I interpret any amplifier maker saying that it's XXXX W at Y% distortion.

    In my mind, the difference in real sound level between 720W and 836W is not enough to pursue, even given that you do retain some hearing after using those kinds of levels So for me, I'd try to set the limiter to go max outputs of 3.3 or 3.4V peak and call it a day.

    As an architectural side note, the power amp is running linearly at all points under these 3.3-3.4V peaks, so you can in fact put a "master volume" after your clipper, and give yourself the ability to turn down the blasted thing and retain exactly the same kind of overload/overdrive from the power amp at lower power levels. I would be horribly tempted to design the clipper to run the IcePower at its full 720W or so with the "master volume" full up, and let it be turned down for lower effective powers.

    But back to the main thread. So you want to design a clipper that never lets its output signal go over +3.3V (for instance...) nor under -3.3V. We as a group believe that softer clipping is better on this point, so the trick becomes designing a clipper that starts softly clipping somewhat below that point, maybe 2.6, 2.8, 3.0V or so, and softly increases clipping until it won't do any more output above 3.3. One way is to hand-pick small signal MOSFETs as I mentioned to easter-egg the right clipping points. Two will be needed, and they probably won't match, but a little asymmetry in this clipping just might sound good. Matter of taste.

    MOSFET clippers are so soft that it might not be clipp-y enough. In that case, either a string of diode pairs, diodes tied to clipping voltage references (which is what the Thomas Vox thing really did, they just varied the clipping references), or some such. This will take some auditioning.

    I'd like to say I have a canned design all ready for you, but I don't. My last foray into this was for a +/- 1.4V clipper at the input of a more standard SS amp.

    Another issue lurking here is what AC voltages your preamp puts out. If the tone you like from your preamp that you just want to be made [MUCH] louder, you'll need to need to figure out the happiest marriage of padding down or amping up the sweet spot for tone from the preamp output and then massaging that down to the right place for how much you want to overdrive the clipper circuit, if any. This is going to be hugely subjective, I think. And should be.

    It's an interesting design question. I'd be happy to add any design help I can.

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    R.G., thanks so much for your thoughts, and I'll take any help I can get!

    Quote Originally Posted by R.G.
    Whenever I see "clipping" I always shift over to peak/instantaneous voltages, not rms. 2.3Vrms is 3.25V peak, 2.5V rms is 3.535V peak. So you're talking about clippers that limit voltage to between 3.25V and 3.54V.
    Note taken - how I will sPEAK about voltages from now on regarding clipping issue

    As an architectural side note, the power amp is running linearly at all points under these 3.3-3.4V peaks, so you can in fact put a "master volume" after your clipper, and give yourself the ability to turn down the blasted thing and retain exactly the same kind of overload/overdrive from the power amp at lower power levels. I would be horribly tempted to design the clipper to run the IcePower at its full 720W or so with the "master volume" full up, and let it be turned down for lower effective powers.
    That's interesting, and I always imagined the master volume (there's actually two switchable as it's two channels) before the clipper, so any clipping from the 'soft-limiter' would be obscured by the sheer volume, and perhaps even feel more like a tube power amp starting to clip. I will surely have to try both.

    But back to the main thread. So you want to design a clipper that never lets its output signal go over +3.3V (for instance...) nor under -3.3V. We as a group believe that softer clipping is better on this point, so the trick becomes designing a clipper that starts softly clipping somewhat below that point, maybe 2.6, 2.8, 3.0V or so, and softly increases clipping until it won't do any more output above 3.3. One way is to hand-pick small signal MOSFETs as I mentioned to easter-egg the right clipping points. Two will be needed, and they probably won't match, but a little asymmetry in this clipping just might sound good. Matter of taste.

    MOSFET clippers are so soft that it might not be clipp-y enough. In that case, either a string of diode pairs, diodes tied to clipping voltage references (which is what the Thomas Vox thing really did, they just varied the clipping references), or some such. This will take some auditioning.
    Ok, thanks, I can envision now how the clipping will function in relation to the input sensitivity of the module. What are some small signal MOSFETs to look at? I am FET illiterate (besides the high voltage ones I've used for a tube-voltage FX loop). I'm also going to have a go at the VOX style. The Icepower module has a handy .5A 15V supply on it I could use for the reference.[/QUOTE]


    Another issue lurking here is what AC voltages your preamp puts out. If the tone you like from your preamp that you just want to be made [MUCH] louder, you'll need to need to figure out the happiest marriage of padding down or amping up the sweet spot for tone from the preamp output and then massaging that down to the right place for how much you want to overdrive the clipper circuit, if any. This is going to be hugely subjective, I think. And should be.
    Yes, I'm pleased with the preamp tone, and am starting to realize there's no real reason I can't just passively attenuate the preamp signal so that it never exceeds 3.4Vp, but now am set on trying the clipper I measured my preamp, and it puts out about 7Vp before clipping, and 20Vp maximum on the Overdrive channel. The clean channel is a bit less at max.


    Also, I was able to find one schematic that is a hybrid preamp, and uses an Ice module - The Fender Bassman TV series. I'm having a bit of trouble understanding exactly how they are doing it. Are D1 and D2 zeners being used as a limiter (their voltage rating seems to high), and then the following opamps being used as buffers (the Icepower modules have a 7K input impedance)?

    Fender Bassman TV Series Schematics.pdf

    Thanks again, R.G.!

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    Old Timer J M Fahey's Avatar
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    Are we talking Guitar amplifier or Bass amplifier here?

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

    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 itīs always clean, does not colour sound and might as well have been made with Fets or even Op Amps.

    Tube amps (and preamps) "do their thing" when clipping or at least being driven real hard into non linear areas which does not happen here.

    ICE power module is an excellent Hi Fi/PA type power amp, VERY SS, sharp edge squarewave clipping, high damping, zero compression ... the absolute opposite of any Tube Bassman power amp.

    Diodes D1/D2 are just a fail safe protection, keep signal below 5V peak or 3.5V RMS ... what Dr Icepower recommends

    Same as D4/5/6/7, they protect U1a and U2a .
    In theory there is 0Vac there, those are virtual ground points ... emphasis on *virtual*.

    If circuit malfunctions or simply loses 1 or 2 15V rails you do NOT have the self cancelling action any more and Tube signal out , if untamed, can and will destroy delicate Op Amp inputs,so these diodes keep peaks below 700mV peak.

    Now for a Guitar preamp:

    Tubes are not magic or fairy dust or Talismans, acting by mere presence (contrary to 99.99% Internet disinformation being spread out there), they do some very definite things when driven hard, which we happen to like (me included )

    IF you get a nice sounding tube waveform, reamplify it faithfully and drive guitar speakers, your ears will have a hard time finding it is not "100% tube".

    Just imagine a Champ driven full blast, going through a Jensen speaker, being picked up by an SM57 and being reamplified by tens of kilowatts into a Stadium audience, through a *clean* power amp bank (to boot, triamplified, limited, etc.) into a mountain of high performance PA speakers, including horn drivers, bullet tweeters, etc. ... sound will still be very recognizably TUBE type.

    Now if you feed a preamp straight into a loud clean SS power amp you start by losing 50% of the Tube sound, since there is neither a clipping (or crunching) Tube power amp and damping is "all wrong".

    And if you nicely symmetrically re-clip that wonky Tube waveform you took so much trouble to get, with any kind of SS clipper, pick any of them: Diode/Zener/diode connected FET/LED/biased Diode (VOX)/you-name-it ... then whatīs the point of using Tubes in the first place?

    Beyond cosmetics or Mojo that is.

    Thatīs 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.

    Which I can predict (not surprisingly ) will let clipped Tube waveform peaks NOT surpass 3.5V peaks or whatever ICE Power specifies

    In a nutshell: the adjustment suggested is *visual* , by clipping preamp out, monitoring speaker out, and setting non-user-accesible Master Volume trimmer to just clipping.

    Which will also be stupid loud and destroy any sensible Guitar cabinet you can build .. unless you find a 24x12" cabinet acceptable, that is.

    Being realistic, exact same adjustment procedure applies to a Tube preamp driving a 15 to 25W chipamp usable in a bedroom, or an LM3886/TDA7294 used in a Club.

    FWIW my own "100W Marshall/Mesa head killer" uses a 300W RMS Class AB amplifier padded down to reproduce faithfully *any* waveform and level those heads can send to a speaker.

    And speaker does not know the difference, all it sees is a certain voltage and current waveform driving its voice coil, has no clue where that came from and reproduces it as air pressure variations.

    Ear canīt tell the difference either.

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    "...Tube preamp, since used that way itīs always clean, does not colour sound and might as well have been made with Fets or even Op Amps."

    What am I missing? It looks to me as though a humbucker could easily overdrive the second tube with the gain up. Even a strat ought to do it. But also tubes do not have to be dried hard into clipping to color the sound somewhat.

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    That's exactly right, J.M. - if you can hear tube tone on a PA or a recording, that means that you can reproduce tube tone live through a non-tube power amp on stage. There is one caveat on that, and I'll go there in a moment.

    I once had a neighbor who wanted to perfect his lawn. (there is a point here, and I'll get to it in a second )
    We live in a very rocky area, and so when he found a small rock sticking up through his grass, he got his shovel and started to dig it up. This is very common here. However, after about an hour of digging and prying, attempting to lift the small rock, he had enlarged the hole to about 1.5m wide. At this point, he realized that the "rock" was not a chip. It was, instead the PLANET protruding through his grass, and he began filling the hole back up. We sometimes find what looks like a small problem that gets bigger the more we dig into it.

    Guitar amp clipping is one of those. There are a lot of special cases, caveats, and other things to think about.

    What helps me is to try to split problems into smaller pieces and examine the smaller pieces. Here, as J.M. clearly explains, you can hear tube sound from any amplifier that clearly reproduces the originally-produced tube sound. The only question is where do you stop -producing- and start -re-producing?

    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.

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    Quote Originally Posted by Gaz View Post
    Thanks, I may just do this, and if I understand correctly, you mean the same as JM did in his #3 suggestion.



    I'm not sure. I emailed them, and they did not mention it, just that input sensitivity was 2.3Vrms, and no need to go past 2.5Vrms (limit before damage is 12Vp).

    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.

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    Quote Originally Posted by R.G. View Post
    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.

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    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.

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    Have you looked into speaker distortion mechanisms as well?
    Can you provide relevant data for quantitative evaluation?

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    Quote Originally Posted by J M Fahey View Post
    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 itīs 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.

    Thatīs 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!

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    Quote Originally Posted by Mike Sulzer View Post
    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!

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    Quote Originally Posted by R.G. View Post
    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?

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Gaz View Post
    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?
    Itīs 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 Worldīs 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 itīs 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 Iīll suggest how to adapt it to ICE Power.

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    Quote Originally Posted by Gaz View Post
    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.

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    Quote Originally Posted by Helmholtz View Post
    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?

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    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 itīs fully cooked and seasoned, but if you pull some essential elements from the recipe, taste wonīt 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 70īs: Kiss and Black Sabbath.

    Heīs 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 Pandorasīs 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!!!!!

    Whatīs 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 itīs 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:

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    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.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
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    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

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    Quote Originally Posted by loudthud View Post
    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

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    Quote Originally Posted by nosaj View Post
    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.

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    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!

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    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.
    That is certainly true and shows the importance of adequate filtering after the clipping stage. This is best accomplished by a guitar speaker .

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    Quote Originally Posted by Mike Sulzer View Post
    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.
    This idea is very old and has some validity when listening to recorded music (especially acoustic music) in a quiet room. Second harmonic levels of 1% or so make recorded music sound more alive. The odd harmonics if present sound harsh or discordant. When you are playing loud guitar next to a drummer, it doesn't apply. As you can see from my post above, harmonics out to #10 and beyond are quite common, but they get filtered by guitar speakers. I might be proven wrong but I don't think a clipped guitar amp will produce even harmonics without plenty of odd harmonics being present.

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    Quote Originally Posted by nosaj View Post
    Even Harmonics = enjoyable
    Odd Harmonics = irritating.

    Odd Man Out

    nosaj
    You must find it irritating how most classic guitar amps distort

    Measurements of some classic guitar amp circuits:
    5E3
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    5E5A
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    GIBSON GA-something
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    Single ended amp
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    In push pull amps you mostly get odd order harmonics and even in single ended amps odd order harmonics dominate once they're cranked.

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    Quote Originally Posted by J M Fahey View Post

    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.
    I thought you said earlier, you just passively attenuate?

    3) set preamp to maximum clipped output, balls to the wall, add a screwdriver adjustable "master" at power amp input, scope output and sllloooowwwwllllyyyy rise Master, starting from 0, until you see the wonky tube preamp waveform starts to squarewave on peaks (indicating now the Class D amp has started clipping by itself), then back down a little.
    Now nothing the Preamp does will clip the Power amp.
    FWIW that is the method I use.
    Maybe that's just for clean preamps with no master volume? I need quite a bit of wiggle room so that I can vary the preamp gain vs. master volume and still get full output power (e.g. master full up and gain low for clean, and preamp up master low for preamp break up).


    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 itÂīs 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 IÂīll suggest how to adapt it to ICE Power.
    Is this the one you mean (from Ampeg SVT Pro 6)?

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    I would really appreciate how to adapt for the module. I'd like it to kick in when the voltage reaches 2.5Vrms. Thanks!

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    Thanks for your thoughts. I actually experimented with some of the simple diode limiters discussed here and the Vox style, but had trouble dialing them in. I'm not sure if I adapted it correctly. I do have a better idea for what my needs are. I basically just need a limiter that will kick in at 3.6V peak. I realized 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. If my preamp was totally clean I could get away with that, but I want to be able to use the preamp clean and still have enough level to drive the module to full power, and also use it with the preamp gain full up for dirty sounds. Here's a partial schematic, so you can see how sloppily this exists in my head. I didn't include all the preamp, but just from the tonestack onward and a buffer at the end. Plan would be to use R1/R2 to attenuate down to a reasonable level so I can still have the preamp gain set low and output 3.6Vrms to drive the module to full power.

    Click image for larger version. 

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