How do I Implement Soft Clipping in an Audio Circuit? Or should I just Attenuate the Signal?

Question

I am designing an audio circuit that takes in line-level audio, amplifies the audio using an amplifier (most likely this 3.1w amp - http://www.ti.com/product/tpa6211a1#parametric) and outputs the amplified signal to a 1w nominal, 2w maximum speaker.

To ensure that I do not overload the speaker I have been recommended to implement soft clipping. So am looking for details on how to implement soft clipping. I prefer the solution to be analog rather than digital, as my signal path, including the amplifier are all analog.

Someone also suggested the best way to deal with protecting the speaker is to attenuate the signal. If this is a better solution, where do you do you attenuate. Before the line-level signal enters the amplifier or just after the amplified signal leaves the amplifier? And how much should I attenuate the signal (or at least what ranges)?

Questions:

1. Soft Clipping or Attenuation?
2. If Soft Clipping, then what does an analogue circuit look like?
3. If Attenuation, where do I attenuate, before or after the amplifier? And by how much?

Answer 1

Soft limiting can be a really simple way of preserving the gain for smaller signals whilst limiting (with some measured distortion) the output volume for bigger signals. Feeding an input to a PA stage can be intercepted with back-to-back diodes so that for small signals the diodes are barely conducting whilst higher amplitudes get limited to about 1.4 Vp-p.

Another way is to use an AGC (automatic gain control) amplifier such as this one: -

Q1 is non-conducting when small signals are fed into the circuit. This means that Q2 (P channel JFET acting like a variable resistor) is biased into pinch-off by R8 and, doesn't attenuate the incoming signal (to ground) via R2. As the signal gets bigger Q1 starts to conduct and this drags the gate voltage on Q2 down towards ground turning on the JFET and attenuating the input signal. C4 acts to smooth Q1's effect and give more of a DC control signal to Q2 based on what the op-amp's output amplitude is.

Answer 2

Generally speaking, it's a good idea to place signal-processing circuitry before the final amplification stage (especially in the case of speakers, where you are using quite a lot of amplification). This allows you to work with lower-level signals. For soft-clipping this is especially nice, as it allows you to use diode or transistor turn-on voltages as your threshold. In your case, a soft-clipper can be implemented using a unity (or otherwise, if necessary) gain op-amp topology with parallel, inverted diodes in the feedback path, potentially with resistors to set the change in gain as the diodes enter their operational biases. For more information, look into guitar distortion circuits on your search engine of choice. Many of these incorporate soft-clipping elements of this style (in addition to diodes on the amplifier output performing a harder clip). You will also want to tune the circuit by ear, as different diodes have slightly varying behaviors that can lead to dramatically different sounds under clip.