Looking for help with soft diode clipping circuit.

Started by armstrom, October 26, 2008, 10:50:18 PM

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I'm planning on building a hybrid amp consisting of a tube preamp stage and a solid state power amp stage. I'll start with two gain stages from a 12AX7 tube and may try other tubes in the 12A_7 family. Ultimately, I will use a simple voltage divider to get the output voltage swing "into the neighborhood" of safe signal levels for my power amp. However, I would also like to add a soft diode clipping stage to ensure that I don't get any voltage spikes above the safe maximum peak-to-peak voltage for my power amp. I've found some clipping circuits but they all seem to be designed to clip very early in order to produce distortion/overdrive sounds (tube-sound pedals, for example). I want to keep the signal clean as much as possible and then only clip it to avoid nasty clipping in the SS power amp.

I would want to clip the voltage to a maximum of 2.5V Peak-to-Peak. Could someone suggest a circuit segment that would acomplish this with nice soft clipping characteristics? Since I'm doing this more for circuit protection would it be worth using germanium diodes? Also, I'm looking for any information regarding the calculations to determine the clipping characteristic (when clipping starts, maximum output voltage, etc...) of circuits like this. That way I can experiment with different values rather than being locked in to a particular circuit design.



look up headphone limiter on Google...the traditional ham radio way of clipping high peaks used 2 diodes and a pot in a setup familiar to us (clipping parallels), but had a back voltage to 'ease' how they conducted.  That may give you some softer limiting than you'd ordinarily get...
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R.G. had posted a circuit fragment from old VOX solid state guitars amps as  mystery circuit 1 or 2 some years ago IIRC.  Some cool stuff in the old Vox solid state amps.