Robert_the_damned Posted April 1, 2006 Report Share Posted April 1, 2006 Hi guys, just wondering if someone would like to suggest a resistor and capacitor for a bandpass filter for a buzzy effect I have? I've already stuck in a temp band pass using a 270 ohm resistor and an unknown value cap and I'm loosing serious amounts of trebble in the on and off possition (its not true bypass obviously!). Any help would be appreciated! Robert Quote Link to comment Share on other sites More sharing options...
ansil Posted April 1, 2006 Report Share Posted April 1, 2006 Hi guys, just wondering if someone would like to suggest a resistor and capacitor for a bandpass filter for a buzzy effect I have? I've already stuck in a temp band pass using a 270 ohm resistor and an unknown value cap and I'm loosing serious amounts of trebble in the on and off possition (its not true bypass obviously!). Any help would be appreciated! Robert hmmm schematic? where are you putting the filter. most people put a .001uf to ground at input to knock it down a bit. http://www.muzique.com/lab/hum.htm Hum and Noise in Effects (and how to squelch it) Questions about hum and noise in FX designs come up repeatedly in the AMZ e-mail. These problems are of three basic types: powerline hum, high frequency hiss and radio frequency interference. The problem cannot be fully evaluated until the circuit is mounted in its enclosure and sealed up. PC boards and wiring that are strewn across a workbench are especially susceptible to induced noises and will not be indicative of the performance of the finished unit. You cannot adequately evaluate the extent of your hum or noise problem while the circuit is on a breadboard or open perfboard. Hum is leakage from the AC power source that has made its way into the circuit and subsequently amplified. It may be 60 cycles per second, which is the fundamental frequency of the power grid, or it may be a multiple of the base frequency. A 120 Hertz frequency is produced by a full-wave rectified power supply and 180 Hz., 240 Hz. and higher harmonics are also produced but at a lower level. The multiples of the base frequency are difficult to remove and the best plan is to eliminate the source of the interference. A common way that hum enters a circuit is through the power supply, or DC input to the circuit box. Poorly regulated AC adapters are often to blame because of inadequate filtering. Putting some series resistance before the pc board is a way of reducing powerline induced hum. A value of 100 ohms for the resistor is usually adequate though it may be increased if required. If this is not sufficient, or if there is no filter capacitor on the pc board (as with the Fuzzface), adding some additional capacitance from the voltage supply to ground on the pc board can prove helpful (this is the 220uF capacitor in the drawing above). Why is my 9v adapter putting out 12v+? This is usually detected when one is measuring the voltage of the adapter without a load on it. An unregulated AC to DC adapter will have a voltage and a current rating. When the circuit being powered is drawing the rated current from the adapter, the voltage will sag from the 12v or 13v unloaded down to the rated 9v. If the current requirements are less than the full rating of the adapter, the voltage will be proportionally higher since the load is lighter and there is less sag. The solution to this potential problem is to use a voltage regulated adapter which will supply a steady voltage at any current draw from nothing up to its maximum rating. They are more complex than the unregulated versions and more expensive. Radio frequency signals can also be picked up by the power supply lines and instead of the resistor shown in the previous example, a pair of ferrite beads used on the wire connecting the power input to the pcb. The beads will increase the impedance of the wire to radio frquency signals while offering little resistance to the dc power. The Lab Notebook article about the Radio Shack choke demonstrates an easy and inexpensive inductor for powerline filtering that is more effective than ferrite beads. Hum may also enter a circuit via routes other than the power input. Light dimmer switches, fluorescent lamp fixtures and computer monitors radiate substantial amounts of hum and noise that may be picked up and amplified by the circuit. Move your pedals as far away from these sources as is practical, but even that may not be enough. While the hum from these sources may be induced into the power supply line, the filtering of the power circuit previously shown will eliminate that source of the noise. An alternate point of entry for the hum is the input connection from the jack or footswitch to the circuit board and any hum or interference that finds its way into the input will be further amplified by the circuit. The wire connecting the input to the pcb should be shielded cable with the shield grounded at the jack end only. RG-174 wire is small diameter and flexible enough for this purpose. Additionally, the box that holds the circuit should be metal. The circuit ground and the power supply grounds should be connected to a common point on the box. A convenient place that is most often used is a ground lug on the input or output jacks. This effectively places the box at ground potential and it acts as a shield against induced hum and radio frequencies (RF). The cord connecting the guitar to the effects box should be top quality since it can act as an antenna which is how some effects boxes can receive and amplify powerful nearby radio stations. A flexible cable with a good shield connection is required. Braided wire is best but some types of foil shields are acceptable. In some locations, radio signals are so close and powerful that they still manage to find their way into an effects circuit, especially if it is a high gain design such as a fuzz. The way to eliminate this radio pickup is to add a couple of ferrite beads to the signal input wire between the jack and the pc board and a small capacitor to ground on the signal input to the circuit. A 100 to 1k resistor may be substituted for the ferrites. This usually has no audible effect on the tone except with certain low impedance circuits such as the Fuzzface and even then it will be hard to detect. Select the smallest value resistance that eliminates the interference. The capacitance value may be increased for additional filtering. Start with the values shown above and increase the capacitance (or resistance) until the problem is eliminated. By applying the simple techniques shown above, most of the noise and hum problems in your effects devices can be eliminated. GROUNDING: There have been entire books written on the subject of grounding and it is a hotly debated subject among homebrew amp enthusiasts. Fortunately, it is somewhat simpler for fx builders. The box should ideally be made of a conductive metal though I have made and used numerous pedals that had plastic casings. The input and output jacks will be automatically be grounded to the metal case when mounted in the holes in the box (though there are isolating jacks with plastic bodies that do not do this). A well designed pc board usually has a single ground point that is connected by a wire to one of the jacks. I usually use the output since the input jack may already have battery wires going to it. A single wire to one jack is all that is needed. If the box is plastic, a wire should be added from the ground lug of the output jack to the ground lug of the input jack as well. Off-board controls are usually not too critical about grounding and I usually jump their ground points together and take a single wire from there to the output jack ground lug. That should do it! -------------------------------------------------------------------------------- ©2002 Jack Orman All Rights Reserved This page last modified on Monday, 18-Aug-2003 18:40:02 PDT 04093 hits since AMZ Main Page Quote Link to comment Share on other sites More sharing options...
Robert_the_damned Posted April 1, 2006 Author Report Share Posted April 1, 2006 (edited) Thanks for that Ansil! This is for a bought effect (a Behringer UO100 Ultra Octaver) that's noisy...I like the sound it has but its unbareably noisy un-modded. Its defenatly not a power problem because it does it on batteries and on mains. the filter I have in there at the moment has removed all the buzz but also my highs. Thats the filter as it stands...the 33nF capacitor was on there in the first place and I was unsure if I needed to remove it or not so I left it be and just added the 270 ohm Resistor and the other capacitor. I've done some more searching and found the formular f= 1/(2*pi*R*C) for a filter (low or high). Can I just use this to calculate the correct capacitance for the right frequency cut? (at 20,000Hz right?) will this give the desired results or isn't it applicable in this case? Robert Edited April 1, 2006 by Robert_the_damned Quote Link to comment Share on other sites More sharing options...
gripper Posted April 1, 2006 Report Share Posted April 1, 2006 I just was looking for a similar noise with my active crossover. Lifted grounds, star-grounded, moved filter and bypass caps, etc. 120 cycle hum persisted but just on the high-pass. Capacitive filtering just killed the highs that I love so much. Ended up with a 1:1 isolation transformer on that output only. Now it is dead quiet but I have to find a good place to mount it. I know I didn't find the problem but I got rid of it and that's all I care about. By the way, the transformer is from (blush) Radio Shack. Made for car audio, I think. Bi-amping ROCKS! Quote Link to comment Share on other sites More sharing options...
Robert_the_damned Posted April 1, 2006 Author Report Share Posted April 1, 2006 mhmm seems like a bit of an extream measure for an effect that didn't really cost me a lot. I'd rather try some differant cap values first. Anyone got any suggested values? I've had a look at the cap I put in and I think its a 0.1uF cap (a 104?) so maybe if I change the value that'd fix my loss of highs...I was thinking a 0.03uF? seeing as when I pluged the numbers into the equation I mentioned I came out with 0.0297blahblahblah so 0.03 seems sensible Robert Quote Link to comment Share on other sites More sharing options...
Robert_the_damned Posted April 1, 2006 Author Report Share Posted April 1, 2006 I've done yet more fiddling with this confounded pedal.....I've tried a 22nF in place of the capacitor 'C'...its still not buzzing....its also still not letting my highs through....I've tested all my cables and its not my amp settings (tried playing without the pedal in my signal chain) its bugging the hell out of me now why what I'm doing isn't doing what I expected to.....I'm sure someones going to come along and tell me what I've done wrong.....hopefully! Robert Quote Link to comment Share on other sites More sharing options...
lovekraft Posted April 3, 2006 Report Share Posted April 3, 2006 I think you're trying to solve the wrong problem - hum/buzz is not RF interference, and won't be solved by low-passing the input. Try going back and checking all your grounds, shielding, and power supply - that's where the problem is likely to be. Now, if you're getting radio stations punching through in the middle of your solos, then a cap on the input is the way to go. Quote Link to comment Share on other sites More sharing options...
Robert_the_damned Posted April 3, 2006 Author Report Share Posted April 3, 2006 I think its got to be some sort of bacground RF though...its defentaly not powersupply (it buzzes equally with batteries and mains addaptor). The filter I've added does stop the noise though but I think I need to use a lower cap value to stop my highs all going to ground. Quote Link to comment Share on other sites More sharing options...
ansil Posted April 3, 2006 Report Share Posted April 3, 2006 is this an octave down unit. ? 2. Are you sure its a 22n cause yeah you did the math right. in the forumula http://www.muzique.com/schem/filter.htm btw are you sure its a 270 ohm.. i would double check everything with a meter. also i tend to put the filter before the coupling cap. if you dont' have a capacitance meter then i would suggest another cap, perhaps a 1k and a .022uf will give you around 7238hz which i wouldnt' really go over since the highest fretted note on a guitar is only about 1.35khz. add one octave overtones and you get 2.7khz and one more octave you are at around 5khz. at 20khz you would be 4 octaves over the highest fretted note whiuch would let in noise. if it is rolling off your high i would bust out the meter and double check it. Quote Link to comment Share on other sites More sharing options...
Robert_the_damned Posted April 4, 2006 Author Report Share Posted April 4, 2006 : ) thanks Ansil yeah it is an octave down I'm going to try putting in the 1K and the 0.022uf and I'll reverse the order of the coupling cap and the filter that's the kind of advice I've been waiting for Thanks again Robert Quote Link to comment Share on other sites More sharing options...
Mike Sulzer Posted April 4, 2006 Report Share Posted April 4, 2006 I agree with Lovecraft; I do not think this is rf. When does it buzz? Does it buzz with the input shorted? if you plug a cable in with the other end of this cable shorted does it buzz? Quote Link to comment Share on other sites More sharing options...
Robert_the_damned Posted April 5, 2006 Author Report Share Posted April 5, 2006 yes...and I've put a 25V 2000uF electrolytic cap accross the DC imput so I'm pretty sure its not power hum...I'm going to make the alterations that Ansil suggested and see if that works. If not I think I'll just have to leave it be. Quote Link to comment Share on other sites More sharing options...
Mike Sulzer Posted April 5, 2006 Report Share Posted April 5, 2006 yes...and I've put a 25V 2000uF electrolytic cap accross the DC imput so I'm pretty sure its not power hum...I'm going to make the alterations that Ansil suggested and see if that works. If not I think I'll just have to leave it be. OK, if it hums with the input shorted then you are not picking up rf at the input. If it hums with a battery as a power supply, then it is not the power supply. It sounds like a problem involving a ground loop with the the amp you are feeding this to. Quote Link to comment Share on other sites More sharing options...
Robert_the_damned Posted April 5, 2006 Author Report Share Posted April 5, 2006 : / hmm sounds intresting...possible too...I'll try a cable that's grounded at one end if I get it working again! darn things crapped out completely now. could my problem have been a bad op-amp that's now died? I'm going to try new opamps...they're SMD which means I'll have fun soldering them! Robert Quote Link to comment Share on other sites More sharing options...
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