Solid State Amplifiers That Have Soft-Clipping

[joecool85]

Just curious - since a SS amp is great at just doing what it's told at anywhere less than clipping, why wouldn't you do an entire soft clipping circuit ahead of the power amp and then just feed that to the accurate power amp?

The soft clipper only has to soft clip the input at a voltage less than would drive the power amp to clipping. As long as that happens, the power amp is completely unaware what's happening to it.

As long as a SS amp never clips, it never sounds ugly, just accurate. It's possible to follow a soft clipped signal accurately.

Thanks for the words of wisdom R.G., it couldn't have been said better.

A good ss power amp circuit will not color the sound, will not alter it in any way other than making it louder.  So what you do is make your preamp do the clipping, soft clipping or whatever you want.  Or, run a pedal into it.  This is part of the reason why tube/ss amp combos are so popular.  If you can get a tube preamp to sound really amazing, then hook it to a good ss power amp, it sounds great.  Keep in mind thats not the best example however, because a real tube amp has a tube preamp as well as tube power amp and tube power amps DO color the sound.  So to get a tube/ss hybrid to sound the same as a tube amp, you need to get the preamp to sound the same as a tube preamp and a tube power amp!

Regardless, listen to what R.G. said and you'll be good to go.

[teemuk]

Limiting the preamp signal only is practically the easiest way but it also has some problems:

What will happen if the voltage gain of the power amplifier is not consistent throughout the audio band - this is true with some specific feedback configurations. It is quite hard to build a limiter that has a compensation for the odd frequency response in the detector; it is much easier to build a limiter that has a detector monitoring the difference between output and rail voltage. For example, limiter kicks in when output has 1V difference to Vrail. This way the operation is not tied to constant signal amplitude threshold and can even tolerate supply sag etc.

Most limiters are also quite slow, which means that they cannot react to initial transient fast enough. In such cases it is a merit if the power amp can also soft clip as clipping circuits are not troubled by the delay issue.

[joecool85]

LM3886 chips are anything but soft clipping, and I've never had an issue with mine.  I drive it hard and loud too.

[R.G.]

What will happen if the voltage gain of the power amplifier is not consistent throughout the audio band - this is true with some specific feedback configurations. It is quite hard to build a limiter that has a compensation for the odd frequency response in the detector; it is much easier to build a limiter that has a detector monitoring the difference between output and rail voltage. For example, limiter kicks in when output has 1V difference to Vrail. This way the operation is not tied to constant signal amplitude threshold and can even tolerate supply sag etc.

As I noted, SS power amps are notorious for being accurate. With the LM3886 particularly, the voltage gain of the power amplifier is dead flat across the audio range unless you work at making it not flat. Non-flatness of the power amp voltage gain is simply not an issue with modern power amps. They're easier to make flat than not-flat.

As to limiters with odd frequency responses - well, don't do that. It is far easier to control limiter response at a line-level stage than by tinkering with a power amp. You have the same tools available in terms of circuit responses and components, and the advantage that a mistake will not vaporize your power stage. You have the same problems and solutions available, and lower downside risk of failure.

And for limiters kicking in with 1V to the rail - don't do that. Soft limiting doesn't happen right up at the power rail. That's hard clipping. Soft limiting starts kicking in maybe 20db down from max power and gradually increases as power approaches max. A good soft limiter starts clipping ever-so-gradually way down at lower powers. You may want a last-resort limiter to keep the power amp from ever clipping that hard limits at a fixed voltage down from the rail, but it is a last resort. It's not very musical.

[teemuk]

This is not an argue R.G. I stated my opinions accompanied with what I consider as pretty valid arguments. Why persist that there is something wrong with them? Both methods - limiting based on PA output signal and preamp signal - can work fine. Both methods have been used in numerous applications. If you combine these methods you can get even better results.

Guitar power amplifiers do not have to have a dead flat frequency response but can be made to react to speaker impedance similarly as tube amplifiers do. It is very common that an amp like this exhibits higher voltage gain at speaker's resonant frequency and on higher frequencies. You can even make LM3886 exhibit the same character if you want to: It's all about the feedback configuration. Some amplifier models even have active tone control circuits in the feedback loop of the power amplifier. A reason why to make PA circuit's frequency response flat is as valid as the reason why not to. It's all about what the designer wishes to do. If the frequency response of PA is not flat then it's tough luck for limiters that detect only the preamplifier's signal level.

Anyway, a limiter that has a detector hooked to PA output and that attenuates the signal fed to power amp: Is that a preamp-based limiter or power amp-based... Well, we have a semantics issue here and it's quite hard to determine where to draw a line in this. It is easier to make a division between limiters that control the power amplifier's gain or power amplifier's input signal amplitude.

Vaporizing the power amp is a bit far-fetched but I guess it's valid to assume something like that would happen if someone doesn't quite know what he/she is doing. For example, I would not recommend tampering with resistance values in power amp's feedback loop (gain control) as it may severily disturb the DC offset and this something I already mentioned somewhere in this thread. If one does not know how to deal with the PA section safely then one should not tamper with it. Simple.

In my opinion, having the detector at the amplifier output is more accurate way of guessing when PA is approaching clipping levels than determing a certain preamp signal voltage level threshold where the limiter kicks in - at least in cases when power amplifier's gain is dictated by impedance (valid assumption) or when supply sag has to be considered (another valid assumption). It's the very same idea as in clipping detectors: Sampling PA's output gives more accurate indication of what's happening than sampling preamp's output. That's why you put clipping detectors to PA output not to preamp. The 1V-Vrail was just an example and naturally real values would depend on the circuit.

I do know that two limiter types are practically needed: One rectifying the sampled signal and controlling the limiting level gradually according to (usually filtered) DC potential of the rectified signal and one (usually) comparator-based that detects potential differences and acts faster but on a constant limiting level. Both serve their purpose: One cannot trust the first type to operate fast enough to protect the amplifier (or more importantly - the speakers) from the initial clipping transient. It is also quite difficult to make this limiter type to "hard limit" at a certain signal level so if the input signal amplitudes are high overdriving the power amplifier is always a possibilty with them. The latter type has at least a moderate chance for preventing both - although it's "not very musical".

If power amp has dead flat frequency response and doesn't exhibit a notable sag then there is really no need to make things any harder than needed. In practice, many amplifiers work fine even without limiters. ...But they also would work better with them - especially with all kinds of them. Keeping mind open for alternative methods is always beneficial.

[R.G.]

This is not an argue R.G. I stated my opinions accompanied with what I consider as pretty valid arguments. Why persist that there is something wrong with them?

I agree - this is not an argument. I have different opinions, and I consider the exploration of them to more than one ply of depth to be a discussion. No argument was intended.

Guitar power amplifiers do not have to have a dead flat frequency response but can be made to react to speaker impedance similarly as tube amplifiers do.

That's true in general; however, this forum was started with high consideration for the LM3886 amps, and that continues to be a major consideration. All of the "chip amp" style amps do have ruler-flat response unless you do something to change that, as they are all high-feedback, low output impedance amps unless you go to some lengths to make that not true.

You can even make LM3886 exhibit the same character if you want to: It's all about the feedback configuration.

This being one way to make that not true.

Some amplifier models even have active tone control circuits in the feedback loop of the power amplifier.

And this being another. Note that this method still leaves a low impedance output with a tight "grip" on the speaker.

A reason why to make PA circuit's frequency response flat is as valid as the reason why not to. It's all about what the designer wishes to do. If the frequency response of PA is not flat then it's tough luck for limiters that detect only the preamplifier's signal level.

That's another version of what I said. We agree there.

Anyway, a limiter that has a detector hooked to PA output and that attenuates the signal fed to power amp: Is that a preamp-based limiter or power amp-based... Well, we have a semantics issue here and it's quite hard to determine where to draw a line in this. It is easier to make a division between limiters that control the power amplifier's gain or power amplifier's input signal amplitude.

I may not have been clear.

You can do the limiting entirely feed-forward by limiting at the preamp output or feed-back by watching the output of the power amp. There are multiple methods for each one.

Power amps are inherently high power opamps these days (neglecting tube amps, since this is a solid state guitar amp forum), and since they are, unless you are forcing them into some nonlinearity, the output follows the input as dictated by the feedback network. Whether you use a low power opamp or a high power opamp to do your limiting makes little difference if you make the assumption that you are not going to let the amp doing the limiting go into clipping - which is the goal you're after in the first place.

That being the case, it is a lower-risk technique to limit before you go into a high power stage, and to do the limiting in a way that whatever you feed the power amp will never force the power amp into clipping under any condition. The down side of doing your limiting in a low power stage ahead of the power amp itself is that (1) you have to use an additional US$0.15 opamp to do it instead of the power amp you would otherwise use (2) you have to do your power limiting at lower voltage, which is more of a design problem than it sounds like, and (3) you may give up a few watts of potential power out in the real power amp by designing not to drive it to the very edge of clipping.

It could be a problem to some people, but I don't consider 1 to be a problem, especially since it removes the risk of getting something wrong in the power amp stage and blowing it. Power amp chips are not as stable as general opamps, and high power oscillation will kill them. The LM3886, for instance, is not stable for gains under five. The TL072 is unity gain stable. You can also put in an RF decoupler and keep any mistakes from the limiting stage from getting into the power amp. 

2 is an issue when you are depending on using semiconductor junctions as references for where you do your limiting. The smaller voltages in an opamp limiter as opposed to a power amp limiter make this a bit trickier and require more ability on the part of the designer.

3 is something you've already signed up for by doing soft limiting. The logarithmic response of human hearing make this almost inaudible anyway.

Vaporizing the power amp is a bit far-fetched but I guess it's valid to assume something like that would happen if someone doesn't quite know what he/she is doing.

Yep. I can promise you that it can happen. I've done it. I guess I'm one of those people who didn't know what they were doing at one time in the past. Not all of us can always know what they're doing.

In my opinion, having the detector at the amplifier output is more accurate way of guessing when PA is approaching clipping levels than determing a certain preamp signal voltage level threshold where the limiter kicks in - at least in cases when power amplifier's gain is dictated by impedance (valid assumption) or when supply sag has to be considered (another valid assumption).

It is in my opinion too. But my opinion also is that there are other design reasons for making not having to make this determination accurately. If you don't need to make a razor-sharp determination of where something clips, so much the better.

If power amp has dead flat frequency response and doesn't exhibit a notable sag then there is really no need to make things any harder than needed. In practice, many amplifiers work fine even without limiters. ...But they also would work better with them - especially with all kinds of them.

I believe that. Of course, I'm after soft-clipping for musical sound, not for protection of the power amp or speakers. IMHO, SS amps should never be let clip. Period. It's too nasty sounding. It's the sound I'm after. I'd like to control how the clipping happens so it's musical. I think this is easier to do in a low power stage than a high power stage. It's not the only way, just the one I like.

Keeping mind open for alternative methods is always beneficial.

I could not agree with you more. I think that keeping an open mind is essential, maybe crucial, to improving one's design abilities.