1N4002 opposed to 3mm LEDs ??

[blanik]

i was modding my BD-2 and trying stuff around when i tried a 3 mm diode at D3 (is something similar to the 1N914 on the original) and Keeley suggest a 1N4002 (or 4001, i'm not shure if there's a difference except for total voltage), i can't read the LED's value with my meter (works for reg diodes but LEDs can't be read?!) but i figured the LED would clip a lot higher than the 4002 but i tried both and there's no difference...

i also put 2 LEDs at D9 D10 and 2 4002 at D7 D8 it's still quite fuzzy at max but less harsh in the trebble dept...

R.

*edit* i would also like to know if a 3mm LED is more prone to burn than a 1N4002 ?  (i.e. if the sound difference between both is small, is it safer to just put a 4002 in there so it doesn't crap on me live)?

[blanik]

ok.. i will rephrase that 

i tried to search on the net and in here about the clpping treshold of 3mm LEDs... no can't find

i know Si clip around 530, Ge 200-300, but i can't measure LEDs with my meter (any side... dunno why)

because i put 3mm LEDs instead of 1N400X diodes and the sound doesn't seem to be affected...

and if the values are close to one another, is it safer to use 400X  (to prevent busting the LED)

hope i was clearer... 

R.

[brett]

Hi
the current flow during clipping is extremely low, so there's no chance that diodes will burn out.

There's no standard forward voltage for 3mm LEDs because the chemical composition of the diode could be any one of a dozen different types.  Ordinary red LEDs have a forward voltage of about 2200mV for 1mA.  Other LEDs range from 1500mV to 3500mV.  Your meter probably reads up to 999mV.  For the 1N400X family, the equivalent would be about 650mV.  As you've discovered, Ge diodes are about half this.

The effects of increasing Vf are (i) to increase the signal volume after clipping, and (ii) increase the ratio of signal volume to conduction "knee".  Assuming that there's no real value in extra volume (coz there's a volume control on the pedal, anyway), the value lies in the ratio of volume to knee.

The latter effect makes clipping "harder", which is why Ge is softer than Si, and Si is softer than LEDs. It seems that you can't hear the differences, which is not surprising because they are kinda subtle.  I find that the greatest improvement in tone comes from using dissimilar or different numbers of diodes in each direction.  For example, using 2 x 1N4148 antiparallel with 1 x 1N4148 offers extra harmonics and richness froma tubescreamer.

cheers

[blanik]

thank you for that perfect awnser... exactly what i was looking for!

R

[WGTP]

Measuring LED's on my DMM they usually come out  1.7-1.8v.

The BD-2 is a more complex and non-typical design than usual.  I had asked about D3 and it may not contibute to the distortion, but be a limiting element.  It uses discrete op amps which probably contribute distortion.  D7-D10 probably would have a more noticeable effect on distortion.   

[blanik]

i did notice a slight increase in volume when i changed D3 for a 3mm LED... but the overall clipping of the pedal doesn't seem to be radically changed by any diodes combination i tried so far... i guess the real clipping in this pedal happens somewhere else than the diodes (compared to other effects i tried like the TS or Rat where the diode change is obvious...)

R.