[2538] Changing Diode Vf

Date: March 15th, 2019 | Comments : [1] | Categories: DIY.

The forward voltage of a diode is not fixed, and this is easily seen on the datasheet for the 1N4148, for example. At 200ma, the 1N4148 diode Vf is over 1v but if the current is only 1ma then the Vf is less than 0.625v. As the current goes lower, so does the diode’s Vf.

This circuit will vary the peak clipping of the diodes, but only over a modest range. Also, the opamp driving it must be capable of sourcing enough current to make a difference, and you have to be careful that the power rating of the 47 ohm resistor is not exceeded.

The circuit is of limited practical use, but there it is for your experimentation anyway!


[2526] Fet Bypass Distortion

Date: March 6th, 2019 | Comments : [1] | Categories: DIY.
fet bypass distortion

While getting acquainted with my new audio distortion analyzer, I decided to do a quick test to compare a true bypass pedal with another that used jfet bypass switching.

Since there was a new Boss SD-1 in a box next to my workbench, that is the pedal that I chose for the fet bypass test. The true bypass test was actually performed twice using a prototype of the Mosfet Boost module pedal and a similar mosfet boost pedal that I made using through-hole parts, but there was no difference in the results when the pedals were bypassed.

The bottom line is that jfet switching with buffers, as used in the Boss pedals, results in 135 times more distortion! It also has a signal loss of 0.81dBV while true bypass has no loss.

True bypass did not introduce any distortion and the THD shown in the listing is the residual distortion from the sine wave generator. It was the same with the true bypass switch as with a straight loopback without the pedal.

I will post a more complete article when I have had time to run further tests and optimize the settings on the test equipment.

Update: Further tests on bypass switching were conducted and the article is now in the Lab Notebook. Enjoy!



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