Official Luthiers Forum!

I would like to throw out this topic for discussion as it pertains to an acoustic guitar with pickup being played thru an amp. I've heard of several ways of reducing/eliminating feedback such as covering the sound hole and some others that sound a little strange. Does anyone out there do this by somehow dynamically controlling the vibration of the plates?
Any "feedback" here is appreciated.
Thanks
Nelson

I had a friend who stuffed large chunk of soft foam into the soundhole and placed it under the bridge. It made contact with the top and back. It did work. if you played the guitar unpluged with the foam it sounded dead. But plug you could get your tone through the eq and amp. I personally never heard a acoustic pickup that I liked. I rather mic it.

Nelson -- are you playing AC/DC licks in front of your twin stack Marshall again....sheesh Anthony Z38866.8208564815

Anthony--Actually it's a "triple twin stack" of Marshalls.

Atta boy Nate .. nuthin like a MArshall stack or six to get some real gutiar tone goin on .... Do those ones go to 11 ??? Oh, and for feedback suppression, try Bondo

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Nelson -- good one! Nothin like seein George playing a Palen!

See now if you would have described the problem correctly in the first place you would have gotten some meaningful answers

[QUOTE=Tony Karol]Oh, and for feedback suppression, try Bondo [/QUOTE]

That's Funny! How's he supposed to use it?

I was going to suggest putting .015's on a Les Paul!

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

    Mark Farner, of "Grand Funk Railroad", used to put masking tape over the sound holes of his Messenger! Ah! You could try that!   

    What an odd visual, a Palen with duct tape over the sound holes! Don't think it's going to happen!

Billy Dean

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

Perhaps someone would care to explain the mechanics of feedback. I see it as the pickup driving the strings but I'm sure there's more to it.

Nelson, i'm not much help with that kind of feedback but i'm gonna give you guys some, it's just great to see you Archtop guys at it! Keep on Archtoppin'! Yeah!

where do you put the Bondo ???? Well to fill in them dang F holes where else ?? Thats where the problem is with them darn things. heck fill the whole thing up, its still an archtop !!!!! just like a Less Paul, only More

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Nelson,
As you stated it is due to the dynamics of the box and also the wind of the pickups. You may want to call Lindy Fralin about your problem (804-358-2699). He is very knowledgable and helpful. Also, and I know this will sound weird, BUT I recently had several ( yes, more than one) customer tell me that when they switched to Klotz cables some of their feedback/amp squeel problems WENT AWAY. They were playing Gibson 175's, 335's and 135's. I can't say why this is, but these guys swear to me that it's true. Maybe you could call Gerald Weber and ask him...again...another great guy...very knowledgable and helpful. Gerald's #512-932-3130.
I would be interested in your reslove. please keep us informed.

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Dave Bland

remember...

"If it doesn't play in tune...it's just pretty wood"

    As I understand it, the sound generated by the amp is resonated into the guitar, then back into the amp by the pick-ups, ad infinitum.

    This can be caused by two main reasons one especially from AT's, at high volume, is the resonance being picked up from the sound board itself, which, is what it is designed to do from a purely acoustic stand point. This same design point makes them prone to feedback.

   The other that I know of is from the windings of the pickups themselves receiving the energy from the amp. Thus potting the pickups was a big deal a while back. Potting being dipping the pickup windings into a wax to stop the tendency towards vibration.

    The humbucking pickups had windings going from two different directions thus canceling out the vibration induced by the amp.

    Microphones do the same thing easily because of the thin diaphrams easily resonating to the sound energy.

    At least this is how it was explained to me. I'm sure there is more to it, in detail.

There's also the string resonating sympathetically to the amplifier energy, A La Hendrix, the high pitched shreek is from the pickups themselves. Billy T38868.1950810185

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

Billy
Thanks for the explanation.
Nelson

[QUOTE]Thanks for the explanation.[/QUOTE]
Don't thank me, I'm just making this stuff up

    Actually, I forgot to mention if you touch the SB or the pickup as the guitar is feedbacking you can actually feel the vibration, reasonably intense.

   I've never heard of cords making a difference in feedback, but I've heard wilder things. I'll have to look into that!

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

Billy, i see that you're a fan of BobC also!

Billy--They put diodes in them cords so they don't feedback.

[QUOTE]Billy--They put diodes in them cords so they don't feedback. [/QUOTE]
Well that makes sense then. If I ever go eclectic again I'll have to use them! Thanks Nelson!

[QUOTE]Billy, i see that you're a fan of BobC also![/QUOTE]Never really had any dealings with Bob, directly, but all you guys seems to like him! I thought you guys would like the reference!

It was a toss up between Bob and you Serge, actually!
Billy T38869.6459027778

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

Seems to me that the cord effecting feedback is about as silly as wether you haul your equipment in an Escalade or an F-100. Correct me if I'm wrong but I just can't see that one.

AlA Peebels38869.7397106482

I can see it!

Guitar Pickup = long wire!
Guitar Cord = long wire!

The diode would keep current flowing in one direction as I understand it! What specific is involved, I don't know, but, it seems perfectly feasible that there could be an affect!

I'll have to look into it!

[QUOTE=Al]wether you haul your equipment in an Escalade or an F-100.[/QUOTE]

How about an 85 Ranger!
Billy T38870.2639699074

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

The effect of a diode in the guitar output circuit would be to eliminate half of the waveform,with no signal in between. Sort of a very crude fuzz tone. Try it. Take an old cord, open it up and solder in a diode. When you don't like the sound, you can remove the diode, and resolder the cord back the way it belongs. It will have no effect on feedback. However, an Op-amp with phase reversing will reduce feedback significantly. This little device simply inverts the output signal from the pickup. So when the top of the guitar is being driven by external sound pressure, the output from the amplifier is in the opposite part of the wave tending to cancel instead of reinforce the feedback.
Another effective way to reduce feedbacl is to use a notch filter. Basically this device removes a narrow band of the sound spectrum from the guitars output, and if you dial it to match the feedback frequency, once again feedback will be reduced.
Both of these fetures are incorporated into most high end preamplifiers. I know my Fishman has it, and it's not even their high end unit.

Al

Sorry guys, I was just kidding about the diode in the cable.   Guess I didn't put enough of these guys at the end of my tongue-in-cheek comment:

[QUOTE]Billy--They put diodes in them cords so they don't feedback. [/QUOTE]

Oh! Now I get it!
No Wait!
Yea! Er...I get it!
The blinky guy means a joke!! Oh that's funny!!!

Everything I know about electronics I know by touch!
Thanks guys!

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

Feedback is the regeneration of a particular frequency. i.e. The sound that comes out of your amp or PA makes it's way back to the system that created it. So to stop feedback you would
1)have to stop it from re-entering the amplification system
or
2)cut or narrow the frequency such as Al mentioned.

Covering the sound hole prevents it from entering the system through the original source, but if a stage mic was to pick up your guitar from a stage monitor you could still produce feedback, your just regenerating the sound through a different source. This is where the use an EQ, notch filter or phase reverse switch comes in handy.

I don't know if controlling the vibration of the plates would help in any way. Frequencies are effected by so many different sources that you would have to have a very flexible method of controlling them to be practical. Everything from the temperature to the amount of people in a room can effect the sound of an amplified instrument.

Dale
I would think that a semi-hollow or solid body guitar would be in effect "controlling the vibration of the plates".
In experimenting with an acoustic guitar on the bench, the volume can be turned up easily to "drive the guitar" as you probably know. This vicious cycle can be stopped by pressing on the plates. I realize that it ceases to be "an acoustic guitar" when this is done.
It would seem that the holy grail would be a guitar than can be played acoustically or amplified at higher volume without feedback.