Flexi Clip
Background
This is very simple and yet useful project to do. The idea is to replace clipping diodes in any pedal with flexible circuit that will allow user to experiment with many clipping diodes. The most common ways of getting overdrive effect with clipping diodes is putting them back-to-back in the negative feedback loop of an Op-Amp (like on Tubescreamer and similar circuits) or shunt to ground (like on Rat distortion, MXR Dist +, etc).
Diodes will conduct only when signal is above given threshold. When wired to ground or in the feedback loop of the Op-Amp they will clip the signal louder than the diode threshold is. That’s how we get to distortion. Different diode types have different threshold levels, but also same diodes can vary pretty much so it’s not a bad idea to measure all diodes before putting them into circuit. Diodes with lower threshold level (GE) will clip before ones with higher level (SI) giving more distortion but also less volume.
Typical diode configurations
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This figure shows original waveform of guitar signal. |
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Adding one diode will clip one side of waveform giving asymmetrical clipping. |
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Changing diode orientation will clip the other side of waveform. |
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Adding second diode in back-to-back configuration will clip both sides of waveform giving symmetrical clipping. |
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And finally, putting two diodes on one side or putting different diodes makes asymmetrical clipping where both sides of the waveform are clipped, but one is clipped more than another. Threshold level of diode array is sum of threshold levels of each diode in the array. |
Layout
Diode overview
This circuits lets you try many symmetrical and asymmetrical clipping types combining few most used clipping diodes: Silicon, Germanium, Rectifier, LED and FET (used as diode: Drain and Gate for cathode and Source for anode). Below is chart of forward voltages I measured on all parts used here.
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Measured forward voltage
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SI 1N4148
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~790mV
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GE 1N60
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~265mV
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LED 3mm red
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~1600mV
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Rectifier 1n4001
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~740mV
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Schottky 1N5819
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~205mV
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FET BS170*
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~810mV
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Note that FET clipper threshold is sum of thresholds for Schottky diode and FET alone.
Germanium diodes
Most circuits use either SI 1N914 (same as 1N418), GE 1N34A (or 1N60) or combination of those two so it should be good starting point for experimenting. Germanium diodes will clip the signal softer than Silicon and LEDs have harder fuzzier clipping. Below is board layout for this circuit. Diodes in rows a-e clip one side of the waveform and the rest of diodes clip other side. Switch poles 1-5 control one side of waveform and poles 6-10 control the other. Only two poles should be ON at the moment: one in each section. Switching pole ON will connect respective diode to circuit. As you can see only one diode can be assigned for each side of the waveform, but you have plenty of options with this setup.
Bill of Materials
- 2x 1N914 (or 1N4148) SI diodes
- 2x 1N34A (or 1N60 or similar) GE diodes
- 2x 1N400x rectifier diodes
- 2x LED 3mm red
- 2x 1N34A (or 1N60 or similar) GE or Schottky 1N5819 diodes
- 2x IRF520 FETs or 2N7000 or BS170
- DIP switch with 10 poles
- Perfboard 10×10 holes
- 2x Transistor sockets, not required but recommended for testing different FETs
* bolded parts are the ones used in prototype.
Installation
Installation to existing OD/Dist circuit is very simple:
- Remove existing clipping diodes.
- If there is second clipping diode per waveform side in original circuit place jumper instead of it.
- Connect solder pads for cathodes (or anode, doesn’t make difference) to input and output of Flexi Clip.
Installation diagram
When installation is complete plug guitar in and try every combination to find the one you like the most. Clipping setup is not a thing one would need to change every day so there’s no need for external controls; mini DIP switch is small enough to fit in existing effect enclosure.
By the way, I’ve heard that Black Ice and similar passive on-board devices are nothing but two diodes in back-to-back configuration so do not be fooled. You could build exactly the same for much, much less money…or you could build Flexi Clip instead 
Useful links
Article on overdrive and distortion circuits
Geofex article on distortion and clipping
DIYStompboxes.com thread about FETs as clipping diodes
I’ve noticed that your “flexi clip” only clip one side of the signal at a time you switch it on, how about if I want to test a back-to-back configuration or asymmetrical two back-to-back with one?..
dude all you do is slide more than 1 switch across across at a time!! like ‘e’ & ‘f’ for symmetrical, or ‘b’ & ‘g’ for assymetrical. there’s a ton of combinations available from this little unit! the designer of this is a bloody genius! 10 x thumbs up!!!
Why stack the FET with the Shottky?
it gives a voltage of 810 mV + 205 mV = 1015 mV or 1.015V
just a way of obtaining a different voltage that’s all it is mate!
every different voltage you can get will give a slightly different sound!