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Less Treble When Increasing Distortion


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11/11/2002 12:14 AM
Vic
Less Treble When Increasing Distortion
I have several distortion pedals that I enjoy, but have a problem with them when I turn the Distortion control past ~2:00. When turning the control past this point, I find the tone becomes trebly and harsh, with a significant loss of bottom end.  
 
Is there a way to modify the pedal in order to negate this annoying artifact? Perhaps a capacitor change may be in order?  
 
Thanks in advance,  
 
Vic
 
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11/11/2002 9:42 PM
frank_clarke

In a tube screamer, increase the 51pF cap. That cuts more treble at high gain levels. Amps can have bright caps on volume controls which have the same effect, you could do that with a Big Muff.
 
11/12/2002 10:37 PM
frank_clarke

Mind you, with the Big Muff (add cap from wiper to non-ground), you are increasing treble at low gain, doing nothing at high gain. But that might be OK.  
And what Mark said, of course :).
 
11/12/2002 5:28 PM
Mark Hammer A micro-tutorial to let you fine tune things..
Many distortion pedal designs use an op-amp as their primary gain stage.  
 
Gain in an op-amp can be altered by changing the value of the feedback resistor between output and the appropriate input (either - or +), the input resistor going to "-" (in the case of inverting op-amp configurations), or a resistor from "-" to ground (in the case of non-inverting op-amps).  
 
Typically, high gain stages have a feedback capacitor in the feedback loop to eliminate unwanted oscillation at high frequencies and also to keep hiss and other high-freq noise from getting out of hand (we'll call this C-h).  
 
Most non-inverting op-amps also have a capacitor between the resistor from the "-" pin and ground to block DC and set the lower frequency rolloff (we'll call this C-l).  
 
Both C-h and C-l set high and low rolloff points depending on their value and the value of the accompanying resistor. The 3db rolloff point can be identified with the formula freq=1/(2*pi*R*C). In each case, as the resistor value decreases, the rolloff frequency goes up.  
 
When the gain is set with a feedback resistor, as you increase the value of this resistor by turning the pot up, in combination with the feedback cap it simultaneously trims back on high end. That's kind of handy for high gain situations. If the cap is a fairly small value, it merely keeps parasitics and stuff in control. If the cap is big enough, it will also roll off some of the audible high end as well. In the case of distortions like the DOD 250 or MXR Dist+, that can be useful because the clipping diodes that follow it are going to add harmonics anyways.  
 
When the gain is set with the resistor to ground, turning up the gain involves *reducing* the value of this resistor, which will move the low-freq rolloff point up. For instance, if the cap is .047uf and the gain pot is set to 10k, the low end rolloff is 338hz. Set that same pot to 100k (reducing the gain by a factor of 10) and the low-end rolloff moves to 34hz. Big difference in bottom end!  
 
All signs point to - as you suspected - a capacitor change to alleviate this problem. If you can swing it, it might be worth installing a toggle to simple add in a capacitor in parallel so that it is still possible to get the "bumble-bee" fuzz sound when you want it, in addition to a more full-bodied one.
 

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