Scramjet led question

[miter53]

The Scramjet build doc says that the clipping diodes Fv should be less than 3v, but not too much less. I have a bunch of red and yellow leds that I measure Fv at 1.8-1.9. Should I get some blue or green ones that ought to be a little higher?

[RobA]

The logic for going with LED's that are close to but less than 3V Fv is that that's about where the MOSFET clipping section sits. The LED clippers pretty much modify the base clipping that is set by the MOSFET. In my testing, if I used a pair of LED's that were near the MOSFET clipping voltage, it would sound different than if I just used the LED's without the MOSFET clippers in place. So, it seems the knee of the MOSFET clipping extends down a bit into the range of an LED with a high enough Fv. I think you would lose that part of it if you use LED's with too low an Fv.

The other reason for using an LED with an Fv close to the Fv of the MOSFET's is the single LED clipper. I really don't like the sound if the clipping voltages of the two sides of the signal are really different. Other people probably have a different opinion on that though.

I used the Tayda standard red LED's in all three positions on one build and I like the way it sounds. I did another one using a mix of white and water clear blue in the paired diodes and a white in the single. I really liked the way that one came out. It's a bit softer than the red version. But, I don't think that I can give you a definitive answer. I think it really comes down to what you like. You could socket them and try different options.

I'm really interested in hearing what you think of it and what parts you've ended up using too.