Diode/LED clipping?

[einziger76]

I am in the middle of a MXR Distortion Plus build.  I have the circuit on a bread board.  I do not have any diodes or LED's in the circuit but it is working.  If I put either diodes or LED's in the circuit it doesnt make a difference in tone.  It sounds the same with them off of the circuit.   I can only come to the conclusion that I am not putting them on the circut correctly.  Can someone explain how these need to be intergrated into the circuit? 

[Pharod]

Hello einziger!

Ive built the Distort+ right from the schematic and it worked. So, the schem is alright. As with any diode clippers, you have to connect them antiparallel to the signal line. Thats means; take two diodes and turn one of them around. With normal diodes, you have the marking ring on the opposite side of each other. For LEDs the big and the small plate inside the led are opposite. Place the diodes/LEDs next to each other. For the distort+, they are the last part before the output jack (or to the stomp switch). You have to connect one wire per diode to the signal line, and the other wire of each to ground.

Here is a schematic of an amp mod with switchable diode clippers. http://www.tube-town.net/diy/tt-ga5/03-vj-schaltplan-mod-1-clipping.jpg

[einziger76]

I have gotten the diodes to change the tone of the distortion but if i take them out the pedal acts as a boost and if i put the LED's in the signal stays the same.  The diodes were working, not the LED's now.  I want to use LED's as I play mostly 80's-90's metal and will be using it for an over drive.  My LED's could possibly be bad.  I Will try to find a way to check them with out burning them up.  Thanks.

[darron]

if you take the diodes out the volume will dramatically increase as there's not much limiting the output voltage anymore.

there's no necessary rule that diodes must be used for metal. actually, diodes will give you a lot less clipping distortion, so less of a 'gain' sound and in metal you often want high gain/distortion.





LEDs are just another kind of diode of course, and they have a higher forward voltage. so that means the output will be much louder than a regular diode. there shouldn't be anything wrong with your LEDs from using them in the circuit but you can always just test them with your multimeter on the diode setting?

[Mark Hammer]

Changing over from silicon diodes to LEDs will automatically raise the output level because the ceiling on where clipping occurs has been raised.  It will also likely require more gain from the chip to produce the desired intensity of clipping.  A stock Dist+ has a max gain of 213x, which is nough to push the signal up to the clipping threshold for much of the note's lifespan, but may only produce momentary crunch at the very start of the note using LEDs.  I might recommend replacing the 4k7 resistor in series with the Distortion/Gain pot with a 2k7 resistor, which will bump the max gain up to 371x.  I will also suggest increasing the value of the .047uf cap to .22uf.  This will letmore bass in and provide even more oomph.  It will still roll off the bass below about 260hz at max gain, so I wouldn't consider it a bass boost, but it will be easier to achieve the signal levels required to bring the LEDs into clipping.

Note that the 741 is perfectly capable of clippingon its own, without diodes of any kind.

[einziger76]

Thanks again for the help Mark.  I think the LED's I have are burnt.  The pedal is working fine now with the diodes in it.   I will try to get more LED's.  I had 2.4v 5mm green LED's from radio shack.  Should these be ok if I buy a new pair to ensure they work? When I put the LED's I have now in the board it does what darron was saying and makes it sound like my amp is on ten when its on one.  Also thanks for the tips!  I will be trying the things you recommended to get a little more gain and oomph from it.   Thanks for all the help from everyone!   This morning I was wanting to beat my head on my work bench and with the help of everyone I built my first pedal which I will enjoy no matter if it ever gets to the full sound I want.  Still a blast to see something come together and work when you DIY!!!!!!

[Le québécois]

it sound like my amp is on ten when its on one. 

As you say, this is exactly what you expect. Your LED are not burn. No need to buy others.

Easy way to test your LED :  just take a 22, 10, 4.7,  .... or 1 k resistor not less to be cautious. Wrap one resistor leg to one LED leg. just connect (physically touch) the + end of a 9v battery with the other end of the resistor and touch the - end of the battery with the remaining LED leg. You should see light. If not, reverse your contact. When you see light, note which LED leg is connected to the + side of the battery (normally the longer leg). You can use this information to make sure they are connected correctly on your D+ circuit. That is: a positive leg to ground for one LED and a negative leg to ground for the other LED.

[ashcat_lt]

Most of the time when we talk about using LEDs for clipping we're talking about 1.4V red LEDs.  It's pretty much a given, and I'm sure that everyone who's replied so far has done so under the assumption that these were the ones you are using.  The green ones you've got will allow quite a bit more voltage out, and won't end up clipping nearly as hard.  Switch to red.