Need ideas for a limiter circuit

[Helmholtz]

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 .

[loudthud]

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.

[Lauri]

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

5E5A

GIBSON GA-something

Single ended amp


In push pull amps you mostly get odd order harmonics and even in single ended amps odd order harmonics dominate once they're cranked.

[Gaz]

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)?



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

[Gaz]

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.

[eschertron]

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.

[Gaz]

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

[eschertron]

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.

[Gaz]

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

[J M Fahey]

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



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.

[Gaz]

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.

[J M Fahey]

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".

[Helmholtz]

..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.

[J M Fahey]

Yes, if you want clean operation

[g1]

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)?