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DIODES

Discussion in 'Effects [BG]' started by nil, Feb 13, 2002.


  1. Does anyone know of diodes that can be driven (ie work) from instrument-level voltage? Or do they all require a power source higher?
    I simply want to hook one up on a project to stop the output feeding back into the input.
     
  2. You are correct that a diode will only conduct in one direction. However it only goes for DC voltages (not music signals) and when conducting, a diode will drop the voltage level about .6 volts, which is more than the signal level of a bass guitar or effects box.

    So, it can't be done with a diode. Give me more specs of what you're trying to do and I'll see if I can think of an easy solution.

    Regards,
    Joris.
     
  3. OK, it's pretty simple.

    ProCo Rat 2 distortion pedal - great overdrive, shame about the loss of bottom end.

    So, i've installed an extra switch that simply joins the tip of the input and output sockets (the ring is already connected thru the -ve AC adaptor jack terminal).

    Which means, that even without power, with the switch on the direct bass signal will flow through. The idea is that with distortion on, the direct signal will always be there, making up the low end loss (also meaning that the effect signal can be blended in more precisely).

    The problem? With the switch on, and the pedal on, the output feeds back into the input and creates a nasty feedback tone.

    I was hoping that I could use a diode of sufficiently low DC voltage and current rating to prevent the feedback. So far Schottky diodes seem to be have the lowest rating (.4v/1mA), so I was kinda hoping/assuming that the instrument-level signal would be enough to drive it.

    Have I got it completely wrong? I'm no electronics expert, just using common sense and a bit of a clue i've gained over the years! ;) Any advice would be golden! :D
     
  4. Cool page...thanks for that. I think i'll try replacing the .022uf cap in the Rat's input section with a .2uf this weekend for a laugh...

    At least so far in my experiments i've been able to add diode clipping that sounds like a cat being throttled... :eek:
     
  5. In effects pedals, the amount of low end/high end is determined by resistor and capacitor networks known as filters. If arranged in a certain way they can be either High pass filters or Low pass filters. The low pass, obviously passes the low frequencies in that secion of the circuit and block the higher frequencies.

    The Xc (reactance of the capacitor) really determines what frequencies are passed. Try substituting larger caps in for some of the smaller value ones. Say 1uF in place of a .1uf.

    As for diode thing, if you get Germanium diodes they can run off a DC voltage between 0.3 and 0.4 volts. If Diodes are used for AC purposes, and the signal out of your bass is glorified AC (its a sinewave), then the diode will act as a rectifier chopping off the bottom of the sinewave, and your output would be distorted.

    If you want to stop that kind of feedback, try maybe placing a high value capacitor (in the order of Picofarads) from the base of the first transistor in the circuit to the collector of the transistor. Also see if u can put one in the line say from the collector of the last transistor to the base of the first transistor.

    What you would be creating is a negative feedback loop. It samples the ouput and feeds it back to the input via the capacitor. By doing so u eliminate any oscillations that can become present and are common in audio circuits. It balances the ouput and reduces the amplification to a suitable level if the input becomes excessive.

    Have fun and experiement!

    :D:D

    Merls
     
  6. I hate working out PCB circuits to diagrams...takes bloody ages, esp. when you're not sure whether the diagram is correct! :)

    Anyways, I replaced the 0.022uf input cap with a 0.1uf, and there is a little more bass through the unit now...might try a larger value, but I kinda like the low-mid snarl that i'm getting now.

    Additionally, I fitted a 0.22uf cap between the input tip and the output pot (with a switch to activate it). Direct through my mixer in my studio it has the nice effect of removing high-end sizzle, giving a more "swampish" fat buzz. Nice. But, played thru my rig it's not really noticable (again, just need to experiment with different value caps), apart from the fact that it kills feedback very nicely.

    Anyone else who's fiddled with a Rat (or variation on the circuit) with success, lemme know!