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Thread: Feedback on a magnetic pickup

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    Default Feedback on a magnetic pickup

    Iím struggling with feedback on my mandos. They both have K&K twins. I often see cheap second hand instruments with magnetic pickups. Will these be less prone to feedback? I like the idea of a beater that I can overdrive and crank. Thereís a Mayer down the road for £50 right now.
    https://www.gumtree.com/p/mandolins/...ic-/1441938510

  2. #2

    Default Re: Feedback on a magnetic pickup

    I think magnetic pickups will be a lot less likely to have feedback, but they will sound very different, more like an electric guitar.

    If you really want to kill all feedback, a solid body electric would be the best though.

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    Default Re: Feedback on a magnetic pickup

    Volume along with a hollowbody instrument is going to feedback no matter what pickup you use. The hollow body is what's causing it not the pickup as the instrument itself is resonating off of its own sound. The best remedy is to cut your live volume, but if that isn't an option there are a few others.

    A solid body electric would definitely cut the feedback but sound significantly different. See if you can angle your body differently to the speaker or get further away from it to see if it helps with the feedback. You could also shove a small towel into the cavity of the instrument that will dampen some of the sound as well.

    Keeping feedback in check is a game of constant minor adjustments.

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    Default Re: Feedback on a magnetic pickup

    If you put a soundboard pickup on an acoustic mandolin , you have made a spruce diaphragm microphone...

    Try, via stage craft, to keep it from hearing its own monitors ...

    or reflection of the mains coming bounced back off the walls & ceiling ...

    magnetic pickup sees the steel string vibrating in a magnetic field if sitting on a acoustic top it's less subject,

    But better in a solid body ..
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    Default Re: Feedback on a magnetic pickup

    There is, of course, a halfway house between a soundboard pickup and a magnetic one. Have you considered a Fishman bridge? They don’t sound totally like an acoustic mandolin but likely more-so than a magnetic. Shadow also make (made?) one. There was one on a ‘14 Gibson I bought years ago - http://www.mandolinarchive.com/gibson/serial/23871 - but I took it off without trying it. (Before anyone asks, the signature had been removed before I bought it!)

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    Default Re: Feedback on a magnetic pickup

    Quote Originally Posted by onswah View Post
    Volume along with a hollowbody instrument is going to feedback no matter what pickup you use. The hollow body is what's causing it not the pickup as the instrument itself is resonating off of its own sound. The best remedy is to cut your live volume, but if that isn't an option there are a few others.

    A solid body electric would definitely cut the feedback but sound significantly different. See if you can angle your body differently to the speaker or get further away from it to see if it helps with the feedback. You could also shove a small towel into the cavity of the instrument that will dampen some of the sound as well.

    Keeping feedback in check is a game of constant minor adjustments.
    INTERESTING! I must assume this to apply to guitars as well, does it not?
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    Default Re: Feedback on a magnetic pickup

    “Spruce diaphragm microphone” is exactly apt! So, between the characteristic sound of a magnetic pickup, and one related to mechanical movement at a bridge, there may be a middle ground if the bridge itself has the pickup (piezos, say) and is then isolated from the soundboard with damping, say, rubber. For music applications that tend to require extreme loundness, it might work. Or take a can of expanding foam to the F holes.

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    Default Re: Feedback on a magnetic pickup

    Magnetic pickups work by the string 'cutting' the magnetic field and inducing a voltage. This only occurs when the string is in motion i.e. vibrating. Therefore the resonating spruce does not produce sound in the pickup, only where the strings themselves are excited, either by the external volume directly or perhaps as a secondary vibration from the spruce. This makes them fundamentally different to the K&K which is deliberately picking up as much vibration from the soundboard as possible.
    I realised in my electrical apprenticeship that guitar enthusiasts have discovered completely separate laws of physics to the rest of the world; 'output impedance' being a particular favourite.
    Thanks for all the replies. Do you have any direct experience with magnetic pickups you can relay? (pun intended)

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    Default Re: Feedback on a magnetic pickup

    While a fully acoustic instrument can still feedback with magnetic pickups (e.g. Beatles' "I Feel Fine") it is less likely as the feedback route is indirect, unlike with a contact pickup on the top. But adapting one is a challenge, and will not sound "acoustic".

    Undersaddle bridge pickups are much closer to acoustic tone and provide very loud playing above feedback. Drew Emmitt of Leftover Salmon uses one on his Nugget, as does Don Stiernberg. Theirs are the discontinued L.R. Baggs. I have Headway undersaddle pickups on my Buchanans, which is satisfying when playing fairly loud contra dance gigs. I met a couple of British pickers that love their Headway undersaddle on a Sobell octave mandolin and bouzouki.

    Those require careful installation but are thereafter very reliable. A bit of tinkering to achieve best string balance is helpful.
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    Default Re: Feedback on a magnetic pickup

    Quote Originally Posted by rumpystumpy View Post
    I’m struggling with feedback on my mandos. They both have K&K twins. I often see cheap second hand instruments with magnetic pickups. Will these be less prone to feedback? I like the idea of a beater that I can overdrive and crank. There’s a Mayer down the road for £50 right now.
    https://www.gumtree.com/p/mandolins/...ic-/1441938510
    Both piezo and magnetic have their advantages and disadvantages. Tonally neither is correct; I find a blend of the 2 to be the best short of micing.

    K&K are piezo-based and will feedback at moderate to loud volume. This is because they rely on the body to resonate, which is exactly what causes feedback.

    Magnetic pickups help with the feedback problem but can introduce 60 cycle hum (single-coils) or darken the tone (humbuckers).

    My mando has 2 piezos attached under the soundboard and a Lace sensor (magnetic) pickup on the floating fingerrest. Each has their own volume control; the 3 way switches piezo/both/mag. The best sound is a mix of the 2 but the ability to switch one or the other helps control live situations.

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    Default Re: Feedback on a magnetic pickup

    Fun fact - the definition of a "microphone" is a diaphragm, the movement of which is sensed by a transducer.
    In this case, the pickup (whether piezo or magnetic) is the transducer, and the mandolin functions analogously to a diaphragm.

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    Default Re: Feedback on a magnetic pickup

    Quote Originally Posted by Pittsburgh Bill View Post
    INTERESTING! I must assume this to apply to guitars as well, does it not?
    I haven't found that hollowbody instruments "feed back no matter what." They're a lot more finicky than solid-body instruments, but they don't usually feed back.

    If you avoid having a speaker pointed at the soundhole or soundholes and keep your volume at a reasonable level, that's usually all it takes. In dire circumstances, like a very loud band or an inexperienced sound person, some players stuff rags or socks in their soundholes. B.B. King solved it by getting his own signature Gibson with no f-holes.

    My mando never feeds back. The repeat offender is my squareneck resonator guitar. They face the ceiling, so every soundwave in the room seems to drop in at the bridge to say hi. I can beat it, but it takes some patient EQing and some pestering the sound person. Fortunately, they don't like feedback, either.
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  17. #13

    Default Re: Feedback on a magnetic pickup

    Quote Originally Posted by Marty Jacobson View Post
    Fun fact - the definition of a "microphone" is a diaphragm, the movement of which is sensed by a transducer.
    In this case, the pickup (whether piezo or magnetic) is the transducer, and the mandolin functions analogously to a diaphragm.
    Just a technical note for clarification - piezos and magnetic pickups work differently from most microphones in my experience.

    A microphone has a diaphragm with a voice-coil of windings around the center magnet. As air and sound vibrations hit the diaphragm they cause it to move the windings which causes a magnetic flux in the magnet that is transmitted as an electrical signal to the amplifier.

    A magnetic pickup works in a different yet similar manner. In this case it is the movement of the metal within the strings that causes the magnetic flux resulting in an output signal. Generally speaking there is no diaphragm needed although some pickups are made in a way that is 'microphonic' which bleeds into the signal.

    A piezo is quite different; the piezo material emits an electronic charge when it moves, which is independent of the material making it move, unlike microphones and magnetic pickups. Piezo 'quack' is how a piezo overdrives and distorts when hit too hard.
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    Default Re: Feedback on a magnetic pickup

    I have often played a completely hollow electric guitar (Gibson ES 125) on stage and rarely had feedback issues. I had to keep my distance from the amp, use little gain, and keep my hands on the strings to stop them when not playing.

    D.H.

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    Default Re: Feedback on a magnetic pickup

    A guitarist friend suggested that I try stuffing my mandolin with some damping material which is a trick he used for a hollow body electric guitar. I cut strips of foam type carpet padding about 1" wide by 6" long and stuffed them into my mandolin through the f holes until it was full (a somewhat tedious task) but it worked really well as in zero feedback. The piezo pickups sounded clear and produced a nice natural acoustic type of tone. When not amplified the mandolin is rather quiet as expected. Try it, it works and if you play amplified on a gig you can push the volume without fear of feedback.

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    Default Re: Feedback on a magnetic pickup

    Magnetic pickups that are not potted well are prone to feedback. When it is secured to a top, and the top is vibrating, it can be prone to feedback at higher volume. Piezo pickups are also prone to feedback at high volume as they are attached to a vibrating top. Reducing the vibration of the top will help immensely with the feedback.
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    Default Re: Feedback on a magnetic pickup

    Just to close this loop.
    I didn’t buy a beater with a magnetic pickup, but I did take the Fishman rare earth blend (mag and mic sound hole pickup) out of a guitar and get it into my octave mando. Sounds fantastic and only feeds back if your right next to the amp.
    Nightmare to install as the soundhole is too small so the clamps have to be dismantled and re-assembled in-situ, but worth it.

    Thanks all.

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