guitar overdrive

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Bernius1
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guitar overdrive

Post by Bernius1 » Wed Jan 05, 2005 6:47 am

Most guitar overdrives use diodes in the feedback loop of an op-amp, to clip the tops off the signal. I saw one using two Ge transistors looped, each of which provided gain to overdrive the other. I saw a similar circuit in an elecdesign.com 'design brief' using two xistors in a differential setup ( in feedback), acting as a tanX slope controller, yielding a knee cleaner than Bessel or Chebyshev. Nice. So, from memory ( mistake ?) I drew up this schematic:
Image
Look at the transistors. Something looks wrong to me, but it's not making sense. Comments appreciated. The LM311 ( I have some ) inserted is allowance for a digital delay :
Image
That part I'm not worried about, I'll just steal his idea, & work it in.
It's for a Hohner 15B amp (old), that I'm 'hot-rodding'.
Thanks - in- advance.
..George
Can't we end all posts with a comical quip?

Enzo
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Re: guitar overdrive

Post by Enzo » Wed Jan 05, 2005 3:53 pm

Diodes just clip. These things look like the caps introduce a spikiness to it.

Ron H
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Re: guitar overdrive

Post by Ron H » Wed Jan 05, 2005 7:59 pm

The transistors with caps in series with the bases won't work. Even if you get rid of the caps, it won't work. You need some resistors in that circuit. I don't know enough about the distortion you want, to make recommendations.
Your gain control circuit also needs a resistor from U1A inverting input to GND.
The Sigma-Delta modulator has an error in it that was pointed out by someone shortly after it was published. The top end of the resistor needs to connect to the output of the D flipflop, not to the output of the comparator.

Enzo
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Re: guitar overdrive

Post by Enzo » Wed Jan 05, 2005 8:24 pm

I admit being unclear on this, but my first impression here was that as the audio output at U1B yanked C or E of the transistors up and down, they would at any instant be at some potential away from the instantaneous charge voltage in the caps and thus their bases so there would be moments of conduction. The caps would lag the waveform trying to keep up.<p>So a negative pulse on the output would haul the emitter of Q2 down with it. The base of Q2 would be at zero volts DC until it charged through the transistor. But during that moment, the current could flow through the transistor back to the front of the 1458. The other xstr works on the pisitive side of thre waveform.<p>The result then would be a more severe distortion than simple clipping.<p>It would have to be a substantial signal to overcome the BE potential of the xstr.<p>Please disabuse me of this notion if I am mistaken. Thus far I have just been rationalizing the thing. SO I would love to know the real story. It does look like a few parts are missing though.

dyarker
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Re: guitar overdrive

Post by dyarker » Wed Jan 05, 2005 9:46 pm

Q2, the PNP looks okay. When point "A" goes positive, the BE junction of Q2 is forward biased.<p>I think Q1 needs the collector and emitter connections swapped so it's emitter is connected to "A" and collector to op amp input. Then the BE junction will be forward biased when "A" goes negative, and the collector will be positive relative to the emitter.<p>Also, maybe, connect the bases together, then one capacitor to common. that should prevent DC built up that would stop the transistors from turning on after the first few cycles "music?".
Dale Y

Bernius1
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Re: guitar overdrive

Post by Bernius1 » Thu Jan 06, 2005 2:40 am

Thanks to y'all so far, & keep 'em coming.
Dale, I think you're right. I'm used to Com-E gain feeding the base of the next stage. But if I reverse C & E on Q1, they'll both be common base. But there's no 'upper rail' to provide voltage gain, so unless it's just pulling U1B's inverting input to GND faster, I can't see a gain. What I want is a harmonic type of distortion that clips the slope as it deviates away from GND. With regular distortion, as signal increases ( moreso in bass ) , the clipping removes thr higher frequency nuances, leaving excessive 'growl'. What I'm looking to accomplish is; As the output goes up or down AWAY from GND, the cap will charge long enough to provide feedback, yielding a reversed-slope spike, regardless of input signal strength (0.7V no longer a threshold). I'll have to breadboard to find which Cap's preserve the 'fidelity of the higher frequencies in the presence of the distortion'. ( How's THAT for a quote !!) But I don't want to flatten the slope, just insert an inverted spike. So, at a max 10kHz, I need a .1mSec 'on' time for the Xsistors.
Can't we end all posts with a comical quip?

dyarker
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Re: guitar overdrive

Post by dyarker » Thu Jan 06, 2005 4:48 am

Lower than oh, about 0.3V the op amp gain is the open loop gain. By the time output get to 0.7V gain is down to about 1.<p>To get more than 0.7V out will need a resistor somewhere in feedback path.
Dale Y

softek
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Re: guitar overdrive

Post by softek » Sat Jan 22, 2005 5:39 am

The way its laid out I think it was supposed to be a common emitter circuit. Very rare in design.

Bernius1
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Re: guitar overdrive

Post by Bernius1 » Mon Jan 24, 2005 2:41 am

Just to update; I shouldn't try a ckt from memory, unless it's 440/3-phase !!! I found a similar x-istor feedback setup, & it's a log-x amp. The x-istor is set in common base mode, input at the emitter. Signal strength at -4dB is .775V RMS, so the peaks will clip. I remember the comment that the x-istors would overdrive each other. And, of course, a log-x amp does modify slew rate !!! When I get it working I'll post the schem.
Can't we end all posts with a comical quip?

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