Date: Thu, 14 Sep 1995 12:18:49 -0400 From: "Timothy J. Stanley" To: YUEP69A@prodigy.com (Sean Ryan) Subject: Prescription Elec. Experience Pedal help! Hi Sean, | I don't know if anyone else out there owns or uses | this incredible pedal that features 3 different effects | in one: fuzz, octave, and swell, but I have a problem w/ | mine. It is picking up tons of am radio signals. Is this | just natural for the transistors or have I lost a ground? | I love the pedal but, I can't use it at gigs anymore b/c | of this. Any help would be great! Thank you! Sounds like a cool pedal... Yeah, I have had this sort of problem before. Any kind of hyper-high-gain circuit (aka fuzz) is capable of this sort of thing. My Big Muff Pi from when I was a kid could easily do this, especially when I used it with my brother's Bad Stone phase shifter. BTW - does this problem happen everywhere, or only at certain gig locations that are especially near an AM station? All of this is IMHO, of course... The problem is essentially that something in your circuit is acting as an antenna (obviously), and other aspects are actually capable of a degree of tuning/filtering any carrier frequencies, and presenting some amount of that signal to the input of your high-gain pedal. What can you do? 1) Try very high quality cords between guitar and pedal. I bet it is this long cord and its capacitance that is causing you the antenna trouble. I am *sure* from experience that cable quality and length affects this sort of problem. Try a shorter cord, as an experiment, to identify if this is the source of the problem. Always use your best cord between guitar and pedals, rather than between pedals and amp. The signal coming out of the pedals is much more substantial than the signal coming out the the guitar, and as such, the cord quality is a little less critical. 2) Consider shielding your guitar internals. Any shielded plane in line with your pickups will shunt much (not all) of this sort of electrical energy to ground. Yup, depending you angle w.r.t. the angle of the energy source, etc. The degree to which the wires in your pickup coils act as an antenna is thus (hopefully) reduced. Remember, that the inductance of your pickup coils interact with the capacitance of your cable and the resistance of your volume/tone potentiometers - that is a classic RLC simple frequency tuner! Textbook. So, if you can keep the darn AM signal out of the signal path (which starts at those inductive coils), things ought to improve. You certainly can not change the fact that the pickups are inductors, and the cable is a capacitor. But, you can change how much external electrical energy they each pick up. Finally, you would get the other noise reduction advantages of shielding. I am *not* sure that shielding will fix this, but I honestly think it would :-) per the above argument. As you are a regular on these newsgroups, you know that my www page has faqs and digests on shielding, so please surf on over. I think that maybe perhaps I recall that I used to get this problem with a humbucking SG with the big shielding chrome covers on the pickup. Which weakens my argument, doesn't it? But, it could have been my first $6.00 used guitar, too. Maybe it was my brother's Tele...I just can't remember my youth quite that well... As an experiment, turn the volume pot on your guitar all the way down. Does the AM signal go away? If so, one of two things are true: a) the AM signal is getting picked up in the pickup/antenna. Therefore, shielding will help. b) by removing the inductive pickup from the circuit, your system no longer tunes in the AM. Which teaches us nothing. But! if the AM signal remains, that points to something other than a shielding problem, eh? 3) Check that your pedal's input and output jacks are solidly connected to its metal (I hope) chassis, and that the internal ground connection is still solid. But, I doubt that this is the problem. If the chassis is not metal, consider housing it inside yet another metal box or something that will shield its internals from the AM signals. Or, just try that as a quick experiment to figure out where the AM signal is coming from. Also, if possible, bend the input and output wires inside the chassis to be as far apart as possible. If you are an electronics hacker, consider replacing the input and output leads with shielded cable, connecting the shield at *only* the jack end. This is a good practice for homemade effects, and it definitely would not hurt with this effect. Let us know how you solve this... --------------------------------------------------------------------------- Tim Stanley Graduate Student Research Assistant University of Michigan Advanced Computer Architecture Laboratory tjs@eecs.umich.edu http://www.eecs.umich.edu/~tjs/