AMZ FX ANALOG ALCHEMY GEOFX MEF ROG DIY WIKI

fuzzys.. what's going on here

Started by Eddododo, April 07, 2014, 11:24:03 PM

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Eddododo

April 07, 2014, 11:24:03 PM
So... I had a project on a breadboard I forgot about.  I don't remember some of the decisions but it was definitely a tweak'n'check
What's interesting is the jfet. Its horribly misbiased you get almost no signal from it, but by disconnecting the jfet drain from v+ or by switching the bias gate resistor to v+ and connecting it to the output along with the mosfet you get these crazy gated sounds.  Also when using the crossover distortion at the output (I actually have them on a switch from shunt to crossover clipping not the pot I drew) when taking the output from the crossover clipping and using a V+ biased jfet you get these crazy jawari ish starving gate craziness.


The diodes are GE pair and mosfet diode pair



So what's with the jfet interaction?

Keppy

#3
April 08, 2014, 01:28:04 AM
In the case of connecting the JFET gate, to V+, you are making the gate positive with respect to the channel. When the gate of an N-channel JFET is positive with respect to the channel, it becomes a silicon diode. The JFET diode is conducting all the time, since the DC level is so high the signal is unable to turn it off. Basically, you've bypasses the MOSFET, so you have your input signal (through the JFET) fighting things out with the inverted amplified signal (through the MOSFET). Add in the variable clipping arrangement and stuff gets weird. If the JFET is acting as a diode:

1) Your input impedance might be tiny. Any frequency that gets through the coupling cap to the JFET is one diode drop from the 1k/.047u pair. The exact impedance depends a lot on JFET bias resistor.
2) There's a ton of negative feedback because the drain of the MOSFET (which is inverted) has an AC path back to the gate through the 2.2k resistor (or crossover diodes), JFET diode, and four caps. If the feedback is clipped, the clipping reduces the negative feedback, which increases signal, causing more clipping. This thing should clip HARD.
"Electrons go where I tell them to go." - wavley

Eddododo

#4
April 08, 2014, 08:30:30 AM
It clips like nobody's business that's for sure. ..  the variety of options just by switches is incredible, rather pleased with my happy accident

Eddododo

#5
April 08, 2014, 10:45:22 AM
Clip 4 shows the crossover dIstortion with the jfet in the circuit
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