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How To Build Passive Filter
How To Build Passive Filter
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Old 19th June 2013, 03:15 PM   #1
f69tele is offline f69tele
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Join Date: Sep 2011
Default How To Build Passive Filter

Can this be done?

I apologize in advance for being a "green horn" but I am trying to learn.

So, I have a pickup on a steel guitar that always needs to have certain mid frequencies cut, say in the 1K range. Is it possible to solder in some components between the pickup and output jack to cut specific frequencies by, say, -3db? What would I need to do it?

Also, what would be needed (different project) to build a passive high-shelving boost from 2.5K up @ +5db? I either want to incorporate this inside the instrument cord, or in a small un-powered box. Am I dreaming? or is it possible?

Thanks for any help on this, and again I apologize if these questions seem pathetic and silly - I just don't know.
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Old 19th June 2013, 04:39 PM   #2
DF96 is offline DF96  England
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Google 'twin-T notch filter' for the first one. In the ideal case this gives infinite attenuation at the notch, but you can unbalance it for a smaller notch. It is quite broad, though. If you want a narrower filter then look at LC notch filters.

A boost is harder to do passively. You can attenuate everything else instead.
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Old 20th June 2013, 12:04 AM   #3
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
DO you always use the same amp? Or do you just plug into whatever shows up on stage?

Two pieces of gear would handle this in general, a parametric EQ or a graphic EQ. You could build a tiny parametric and put into the guitar, but these would more likely go into the FX loop of your amp, or if it has them, between the preamp out jack and the power amp in jack. These can be had as rack gear or as pedals. either way they would sit with your amp and not clutter the stage under your guitar.

They are not passive of course, but a passive filter is going to knock your guitar output level down.

A graphic lets you fine tune the overall frequency response, so you could use it to not only notch out your problem band, but also do your high shelving boost at the same time.

The parametric is adjustable to a particular frequency, then also for boost or cut and Q or bandwidth, ie a narrow notch or a wide gentle one.

Does your guitar need this no matter what amp and speaker? Or mainly just your usual rig? Might consider a different speaker, speakers can be peaky.

Why do you need them to be passive and right after the guitar? If you use active circuits, they can run in the higher level parts of the signal path and will be less likely to add noise than if between guitar and amp.
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Old 21st June 2013, 04:41 AM   #4
f69tele is offline f69tele
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Join Date: Sep 2011
Originally Posted by Enzo View Post

They are not passive of course, but a passive filter is going to knock your guitar output level down.

'Nuff said right there. I guess the most logical thing would be to get a graphic pedal of some sort. I was just trying to eliminate the batter/power supply thing.

And yes, depending on the amp and what settings are available, it is not always possible to dial in a perfect "sweet" sound.

Thanks for the replies.
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Old 21st June 2013, 05:07 AM   #5
picowallspeaker is offline picowallspeaker  Italy
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As the matter of facts, all the amps need a specified bandwidth to work safely, but that because of the power devices that pump current. At low level -signal or pickup - current is negligible, and the RLC net needed to do such filter puts some poles in the curve or equation that defines it; well, that' s the very technical part which I leave to more skilled people
So an amplifier lifts the level by a defined ratio ( which is expressed in deci Bels) so gain is a matter of bandwidth, too.
By putting a passive filter before it, you can narrow the BW at the expense of loosing some dB or you can make it a notch type where the center of the 'bell' has a high Q .
Making sort of a notch filter that has a 'telephone band' behavior ...
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