How to measure output voltage of a pedal?

Frequently asked questions regarding powering your pedal.
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diagrammatiks
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Post by diagrammatiks »

well if you take out the diodes you'll change how the pedal sounds but it will increase the output voltage swing significantly.

as too what the maximun input voltage of a transistor amp is...

that's hard to say.

it should theoretically be a little bit less then half the maximun supply voltage.

however it will clip way before that so most of that signal will be squashed at the input anyway.

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Post by FiveseveN »

With leds the extra energy goes into light, and not shortcircuit therefore right?
Nope. LEDs are not current-limited. They will destroy themselves if you just apply voltage to their terminals, they don't "know" how much current to pull, you have to set it by using a resistor.
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DrNomis
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Post by DrNomis »

Leds will typically draw up to about 20mA of current before they burn out and, or explode, the current limiting resistor prevents that from happening.... :hmmm:
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Post by FiveseveN »

Yeah, that's what I meant. I guess one could use them as (bad) low-power fuses :lol:
Anyway, it's one more reason to use such a resistor.
1N4148s and the like have 200 mA continuous / 450 mA peak forward current rating, which means an opamp would fry before it can damage the diodes.
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Post by mictester »

Lonkero wrote:
When using leds are diodes, shouldn'vt I be able to leave the 10k resistor out, since it's doesnt sound like a short circuit when the leds start conducting since they lighten up? (vs. other diodes)

With leds the extra energy goes into light, and not short circuit therefore right?
No - you still need a current-limiting resistor (just like connecting an LED across a battery). If you're going to use LEDs you can use a 1k resistor from the output of the op-amp.

If you do use LEDs, there will be much more level available (about 1.8 V p-p), so you'll have plenty of signal to put into a passive tone control. This could be a useful addition to a simple pedal like this - making a DOD 251 perhaps? :)
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Post by Lonkero »

Thanks for the info guys. I have one MXR plus modded and boxed clone, and reason I was asking about this resistor so much is because I'm using only one led there as clipper, and there's no resistor on output of op-amp ( i left it out when i noticed on bredboard the sound is better without it)

I haven't noticed any problems what so ever yet after a couple months of playing (even the led is still working), but it's better that I install a 1k resistor there.

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