The last few days I've been trying to figure out how the soft clipping cell on the end of this circuit works. It's used in the Ethos TWE-1 overdrive pedal and I think many other circuits. How does a sine wave pass through it? Surely diodes back to back in opposite polarities will block this? As even if the first lets the positive part of the waveform through, the second diode being in opposite polarity will block that. And then how could they let small signals through if they only turn on at 0.7V?
https://patents.google.com/patent/US20130136278A1/en
Soft Clipping Circuit - How?
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Littlewyan
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Soft Clipping Circuit - How?
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Littlewyan
- Posts: 1955
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Re: Soft Clipping Circuit - How?
Is it because of the DC voltage turning the diodes on? 4.5V present at Op Amp output and then a resistor network setting the middle part of the diode strings to 4.3 and 3.8V? Turning the diodes on allowing AC to pass?
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martin manning
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Littlewyan
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Re: Soft Clipping Circuit - How?
Yes that’s one of the pages I’ve been reading, along with this https://www.diyaudio.com/community/thre ... 678/page-2
Re: Soft Clipping Circuit - How?
I suppose it qualifies for patenting under the "novel combination of known devices" approach to patents. It's a combination of previously known waveshaping techniques.
The even-harmonic generator makes a different gain for positive-going and negative-going polarities of signal. the zero-crossing processor looks like a bipolar DC-restorer, common in early analog TV days for color-signal processing. The diode bridge is a combination of approaches from the Thomas Vox amplifiers.
Thomas used almost the exact same diode bridge as an amplitude modulator for their fancier tremolos. Diodes have a variable resistance region about 25mv to 50mv wide as they just start to conduct. Thomas cut the signal level down to about that variable resistance region and modulated the resistors on the high and low sides of the bridge. This made the diodes into a variable resistance which let more or less signal through depending on the amount of current flowing through the bridge.
Thomas also used a very similar concept to the zero-crossing and soft clipping sections for a "limiter". They put a diode clipping pair, each diode with a series resistor, to ground, fed this through a capacitor with signal, much like the ZC section; The diodes each had a parallel resistor and a side-feed through a biggish resistor with DC from the power supply. This current feed set a DC voltage on the diodes that amounted to setting the clipping threshold for the diodes. This affair could set soft limiting (i.e. clipping thresholds) to any level in a few volts peak.
The old circuits for wave shaping don't get much play these days.
The even-harmonic generator makes a different gain for positive-going and negative-going polarities of signal. the zero-crossing processor looks like a bipolar DC-restorer, common in early analog TV days for color-signal processing. The diode bridge is a combination of approaches from the Thomas Vox amplifiers.
Thomas used almost the exact same diode bridge as an amplitude modulator for their fancier tremolos. Diodes have a variable resistance region about 25mv to 50mv wide as they just start to conduct. Thomas cut the signal level down to about that variable resistance region and modulated the resistors on the high and low sides of the bridge. This made the diodes into a variable resistance which let more or less signal through depending on the amount of current flowing through the bridge.
Thomas also used a very similar concept to the zero-crossing and soft clipping sections for a "limiter". They put a diode clipping pair, each diode with a series resistor, to ground, fed this through a capacitor with signal, much like the ZC section; The diodes each had a parallel resistor and a side-feed through a biggish resistor with DC from the power supply. This current feed set a DC voltage on the diodes that amounted to setting the clipping threshold for the diodes. This affair could set soft limiting (i.e. clipping thresholds) to any level in a few volts peak.
The old circuits for wave shaping don't get much play these days.
"For every complex problem there is an answer that is clear, simple, and wrong"
H. L. Mencken
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Littlewyan
- Posts: 1955
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- Location: UK
Re: Soft Clipping Circuit - How?
Thanks for the explanation R.G. I also found a video of Pat Quilter explaining it too https://youtu.be/GAEdc1l-t6g?si=_6Ph1cQxKCP4XTCi
So if we take the zero crossing processor example the diodes are biased on at idle by the difference in voltage either side of them. Two diodes, one positive facing the other negative facing. Take the positive example, lets assume the voltage at the anode is 5.1V and at the cathode it is seeing 4.5V. When a 1V AC signal is passed in the positive side goes in absolutely fine, as it goes negative it effectively pulls down the anode voltage biasing the diode off and thus clipping the signal. Do I have that right?
So if we take the zero crossing processor example the diodes are biased on at idle by the difference in voltage either side of them. Two diodes, one positive facing the other negative facing. Take the positive example, lets assume the voltage at the anode is 5.1V and at the cathode it is seeing 4.5V. When a 1V AC signal is passed in the positive side goes in absolutely fine, as it goes negative it effectively pulls down the anode voltage biasing the diode off and thus clipping the signal. Do I have that right?