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Hi,
I have a customer with an unusual problem....

he has a Gibson j150, and the A string (when amplified) feeds back like crazy. All the other strings are normal. The guitar's resonant 'note' is A or there abouts i guess, but its making the thing pretty unusable for him.
I've only known one other customer with this kind of problem, it was a beaut koa Martin. The customer contacted Martin to ask for help and Martin (quite rightly) said that they see it as their job to make guitars as loud and resonant as they could - in all the right places though - and that they could accept it as a fault. The customer is perfectly happy with that, and lives with it. It was the whole guitar, not just one string that time.
but...
this Gibson is SO prone to feedback, and its just the A string really. Even with that string fretted its a bit of a nightmare, it starts to rumble and builds into a loud feedback hum within seconds.

My problem is that i understand the customers point of view and I want to help him out - even though his guitar is most likely just out of warranty, but I also see that the guitar is not 'faulty' and I'd fully expect Gibson to not accept it. If i'd built it I'd feel the same.
It didnt cross my mind to suggest in ear monitoring, but maybe thats the way to go. I'm not opposed to finding a particular live spot on the top and brace it to try to quell the problem, but deadening someone guitar is not my ideal solution!

Has anyone else encountered (and solved) this kind of thing?
all the best,
steve

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Interesting thread title - I thought this was going to be a thread about cows producing methane......

A feedback plug?

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Reguards,

Ken H

If you are really certain the top main mode is A, what you can do is to deepen the brace scallops. Not too much sanding should bring the top plate mode down to G#, which is the best imo. This way both A and G open strings should sound ok.

One way you can test is to attach weights to the bridge. For a classical it doesn't take too much to push the modes down, but for a stiff SS..dunno...try 20grams, then 30 maybe even 40, see if the string sounds better. Might worth waiting a little for the guitar to adjust to the new "engine weight".

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Build log

tried it, it didnt really do it, plus he hated the muted tone.

He demonstrated the feedback to me and it was a real problem. The more i think about it the more i want to put a little brace of the main 'x'...it goes against the grain to do that to a guitar tho, so to speak!

Otherwise in-ear monitoring must surely be the only way forward....and a pretty easy one at that.


Hey Hesh, thought the thread might get lots of people looking....then going 'uh, BORING!'

steve

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I agree with you Ken. A Feedback Buster is the way to go.

Or a graphic EQ or notch filter to deal with the offending frequency.

Bob

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Bob Connor
Geelong, Australia

wow, thanks so much - yours is the kind of great luthier brain I need to learn from

the strange thing was, the box seemed to be F#, but I kind of assumed the whole instrument together must be A in order to rattle so readily when a minute amount of open string excitement is introduced.

if i can get the guy to bring it back in I'll try the weights, in the mean time maybe i'll try it with some acoustic's round here to get my brain round it.

I've screwed G-clamps to headstocks before to disrupt the overall resonance to check for this kind of thing, I didnt have time to try it this time - wish I had done, it might offer the route to a solution at least.

not sure he'd be cool with a 6inch G clamp hanging off the headstock for ever more tho....customers can be so picky!!
thanks again,
steve

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Hehe guys like Al Carruth and Greg Byers do the work and I (3 guitars noob) pick the credits, sweet!

Seriously now, I hope you can fix it. The box note and the top note are not necessarily the same. But you do want them to be at least 1 octave apart.
Atm I can't figure out the box note for my guitars, because I can't reliably sing in the hole and I was too lazy buying a tiny speaker that would have power enough and fit the hole. Picking the top plate freq is much easier with a software tongen and strong enough loudspeakers.

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Build log

This thread is interesting and confusing ,as I know I have read if the top mode matches a string mode ( if you have not coupled the back mode) then the top would vibrate more freely at that resonance ,absorb the vibrations . and basicly create a dead spot on the corresponding note ... Jody

On my last guitar the top mode (or at least what I believe to be the top mode) was between A 220Hz and A# 233, and the open A was poor. Loud, thud like, no sustain. In the following 2 months I made soem minor adjustments on the braces (thinned the ends through the soundhole) plus the natural opening up, and right now it is settled between G# (207Hz) and A, and the open string sounds OK.

Guess what, if I try a halfstep down tuning (or "vintage' 415 Hz tuning if you want) the A string (which would now be 415/2= 207 Hz) sounds like crap again. (( I tested this to see if I lose some volume as I have ordered gut strings which are very high tension an should not be tuned at over 430 Hz. ))

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Build log

Do you get the same response from the guitar when unplugged? i.e. does it really shout out acoustically when you're working the A string?

If not, I'd suspect something with the pickup. If you have an undersaddle piezo, perhaps there's a high spot on the underside of the saddle that causes it to press down on the piezo more than other strings.

This happens also when you fret B, or down-tune to G?

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The member formerly known as erikbojerik....

I have one other suggestion, which may lead you to the right spot on the top without causing any damage:

Get yourself a piece of sticky tack and roll up a piece about 1/4" round ad about 2" long. Gently place the sticky tack on the face of the guitar about 2" from the bottom of the guitar near the tail of the guitar and move it around until you find the right spot to deaden. This should give you a reasonable place to place a small brace inside the guitar. I wouldnt think that by deadening one particular spot would cause the loss of volume or tembre for the entire range of the guitar's voice. Otherwise, you are shooting in the dark.

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Reguards,

Ken H

acoustically it's pretty even, no complaints from him anyway. I wish I'd asked him to leave it with me.
If he does bring it in again I'll try the tape, i was pretty much going to just press the top lightly with my finger while it was feeding back to find the live spot but maybe tape is better.

Hi Jody, to me the problem is that its ringing in sympathy, and transferring the energy to the string. kind of like a standing wave, constructive interference and all that stuff.
we're talking about the same thing - but from a slightly different perspective i think.

fretting the note reduced the problem slightly - I'd obviously expect it too, but A notes around the neck didnt go madly into feedback, it was a strange one! possibly the fact that i was changing the resonance by fretting an A as opposed to letting it ring..but having said that, just playing the thing with an open A led instantly to feedback.

I would have re-levelled the underside of the saddle &/ or saddle slot if i'd had time....it'll be back i'm sure.

i think i've got enough ammo now to sort it out - thanks everyone, much appreciated!

I'll let you know how i fix it next time i get a crack at it...or go bald from all the head scratching!

cheers,
steve

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Sticky Tack is that stuff that's like modeling clay, that you knead in your fingers to soften up, and then you can use little pieces of it to stick stuff to a wall, like a map, or poster or something. It come in little blocks like clay. Pinch off a piece and knead it till it's soft, then stick the lump to the top. It has enough mass to deaden the spot.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Amplification exaggerates every anomaly in a guitar's frequency response. Dead notes are deader, lively wolf notes howl harder, and all goes to hell in a handbasket if the guitar is too lively. The best candidates for successful amplification are very even sounding guitars with fairly stiff tops. It's quite the paradoxical conundrum... It's why I build semi-hollow acoustic electrics. They can sound more acoustic plugged in than acoustic guitars do. Great acoustic guitars usually make for problematic amplified guitars.

Here's something to try...put in a sound post between the top and the back. Yes, it will suck acoustically, but it will probably make the amplified performance better.

It really would help to know what kind of pickup is in this thing - that's not an insignificant factor.

If he doesn't want to change the acoustic sound but wants to improve feedback resistance, you could easily end up chasing your tail. As Rick said, don't expect the best recording guitar to make the best live instrument, or vise versa.

Tell him to get a preamp with a notch filter. Cut out a narrow band around 110 and see how that helps.

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Eschew obfuscation, espouse elucidation.

If it's the open A string that's feeding back the problem is not with the 'main top' mode, but with the 'main air'. It's pretty common to have it that high in pitch on archtops. Loading the top won't help much, if at all. Try covering part of the soundhole with a piece of card stock, and see how little restriction you can get away with to control the problem.

very interesting, I've been mulling over different ways to alter the resonance, including a soundpost. I have some mini brace repair jacks, experimenting with that might produce positive results.

The plasticine brace is especially useful as the imprint of the corners that it fits into will help size the wooden version fairly accurately.

As far as I know its a stock under saddle (i'm expecting a Fishman) pick up, no mini mics or anything. The customer didnt like the feedback buster (plus it apparently didnt really work) but perhaps a partial cover is worth exploring.

Its a very good point to say that you want a less responsive acoustic guitar if its going to be amplified, I never really crystallized that idea in my mind before, but it does make a lot of sense...seems to go against everything i want from an acoustic, but if thats what it takes....

I'm so grateful to have so many greatly skilled people to pitch in ideas, many thanks to you all! I'm spending way more time on this than I should (especially as I'm not going to get paid for it!) but it's one of those interesting problems that yields that little bit more knowledge, and thats payment enough.

steve

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Steve,
This is great info.
Intuition tells me a fix could be something combining Al and Ricks ideas.
Experiment with a sound post that also holds a baffle that partly covers the sound hole.
He could install this when playing electric and take it out when playing acoustic.
Maybe?
I love thinking about these things..

Wade

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Wade
Nashua, NH
http://www.wadefx.com

me too, I'm up at 3am sometimes researching this kind of stuff - i'm such a nerd...

a sprung tool like Dan Erlewine's old brace repair tools ('love them old videos!) to be used when he's playing amplifies might be another answer.
...but to deaden a guitar does feel pretty wrong & the customer is also pretty much against that kind of idea... i think a 'stealth' brace is a favorite plan so far - if he doesnt know its there he wont object i guess....although I'm not 100% convinced it'll work as well as some of the other ideas.
He's kind of expecting the impossible perfect fix, which would be nice to be able to do, but I'm a realist and like David said - I'm likely to end up chasing my tail with this. He's likely to contact Gibson Europe, I'm working with them to get it sorted out.

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