Pleasant distortion without clipping

[puretube]

I was imagining some diode or biased transistor that keeps the door closed if there's enough voltage, but then opens up more and more to a "lows-to-ground sink" as voltage starts dropping.

as opposed to the "series-diodes simple noisegate-thingy"?

using this snippet as a sensor,
to find out where/when the desired low-voltage -region is situated...

[tca]

^ Or something like this?!?

[Gus]

R.G. 
I seem to remember you posted a circuit fragment as a kind of challenge to this forum some years ago.
It was a limiter circuit from a Vox solid state amp IIRC.
I can seem to find it.
It was clever simple circuit.

[R.G.]

Yeah, it was the limiter out of the Thomas Vox "big head" amplifers, notably the Royal Guardsman and Beatle. It had a pair of diodes to do the clipping, each with something like 100R in series with it, but biased with a variable DC current through a couple of other resistors. By adjusting the DC in the string, you moved the limiting point to let the signal be bigger or smaller.

Thomas used it to keep from blowing the output transistors, I think. But it was useful as it added a soft clipping to a solid state power amp that kept it from doing the razor-edged clipping that most SS amps of the era did.

I've done something similar with both diode-connected MOSFETs and a resistor-diode ladder setup to give a broadly curved transfer function to a power amp.

It's hard to get this right in a 9V power supply just because the voltages and signal levels are so low. 'S why I'm enjoying reading the other travelers on the trail. 

[tca]

R.G. 
I seem to remember you posted a circuit fragment as a kind of challenge to this forum some years ago.
It was a limiter circuit from a Vox solid state amp IIRC.
I can seem to find it.
It was clever simple circuit.

This one?



Ref.: http://www.premierguitar.com/Magazine/Issue/2007/Sep/Adjustable_Clipping_in_VOX_Amps.aspx

[R O Tiree]

How many people looked at that pic and thought, "J201? Where is it?" and then face-palm, "Junction 201"

[WaveshapeIllusions]

I've heard a bit about the Vox limiter. First time I've seen it though. That's a really clever setup. It looks like the negative peaks are limited at the same level though? The bias voltage only goes to the one diode... Wait, nevermind, I see it.

Since this is moving to distortion methods in general, I have another thing to add. What if we took that limit level pot and replaced it with... let's say an LDR and then run an LFO through the corresponding LED? For a somole version, just two diodes clipping to a reference voltage that is modulated by an LFO. I'm sure that has been done before? I imagine that it might sound a bit like PWM?

[Johan]

It's hard to get this right in a 9V power supply just because the voltages and signal levels are so low. 'S why I'm enjoying reading the other travelers on the trail.  

What if you replaced r257 with a large value cap, so that node becomes signal ground but can still be biased with enough dc to keep the diodes almost open?
J
edit, nevermind. I didn't pay close enough attention to the direction of the diodes...

[screamersusa]

Has anyone adjusted this for 12v? Id like to play with it in front of a j201.
I can go 15v if necessary. What transistors would work?




http://www.premierguitar.com/education/images/pic_200709_techviews_1.gif

[Johan]

perhaps this could be of interest?
http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=50220&g2_serialNumber=1

its something Im working on and have not built yet, just simmed. its in Another thread that gets next to no attention due to not precenting a finished design ( poweramp for guitar concept)

note that allthou the combined gain is 40db(voltagegain 100) the output doesnt flat top untill the signal Before the limiter is allready clipped
blue trace is input, green is just after the first opamp and red is output.

my intent is to replace the second opamp with a Power ic such as lm1875 or similar ( a bridged lm1875 amp actually ~50watt at +-15volt)

any thoughts

Johan