Feedback Problems

[Greentone]

I'd be surprised if a set-in humbucker in the pickup mount on, say, the face of a Gibson archtop guitar is _that_ much different from a humbucker suspended from a neck bracket, a la the Gibson Howard Roberts model. In the HR, the pickup doesn't touch the top of the guitar. In, say, the ES-175, the pickup only touches two screws, that are suspended from the pickup mount held in place by four small screws.

I would be surprised if very much energy from the strings is transmitted through the pickup mounts to the pickups, but I could be wrong. I would think it would be on the order of approximately the energy that is transmitted through the neck mount to the pickup on the Howard Roberts guitar.

Does anybody have any insight into this?

[Runepune]

My assumption with a floater that doesn't rest on the top is that feedback occurs because the pickup structure is resonating. If so, the body or top construction would have no direct effect on feedback. I can imagine an indirect effect from the speaker vibrations moving the top and that the top vibrations create movement in the floater.

Guitar "feedback" isn't the same as microphone feedback (unless you get into extreme gain/loudness where it squeals with muted strings). Guitar "feedback" comes from string movement. Whether the pickup is floating or not is irrelevant, other than how it may affect the top/resonance/string movement.

[sgosnell]

The strings are the proximate cause of feedback. You can always stop feedback immediately by muting the strings. The vibration of the strings create a signal to the amp whether the string is plucked or excited another way. The loop is created by string vibration, from whatever source, and the amplifier amplifies it, as amplifiers are designed to do, and then the louder the sound from the amp speakers, the more the string is excited, the louder the sound from the speakers, until something stops the feedback loop. The strings can be excited by vibrations from the top, from the string being plucked or moved accidentally, or by something else, but it all begins with string vibration, which is then amplified. You can start this feedback loop with a solid-body guitar if you put it in front of an amp with the volume very high. It's certainly easier to start with a responsive hollow-body, because the top vibrates, which causes the strings to vibrate. Solid-bodies don't vibrate very much, obviously, but their strings do, even though it's somewhat more difficult to get them started.

[Marty Grass]

Why would lower pitched strings vibrate more than higher one? On some guitars I know which note on which string is most vulnerable.


The D# and E on the A string seem to like feeding back.

[Runepune]

Here's the response from a small body acoustic I have, when I hit the top of it. Notice where the energy is. Those peaks down there are the most easy to excite. The D#-E would be around 160Hz...not far from the second peak/resonance on my guitar.

[icr]

What exactly causes feedback?

Anything that vibrates and can alter the grid of V1.

[kevmoga]

Here's the response from a small body acoustic I have, when I hit the top of it. Notice where the energy is. Those peaks down there are the most easy to excite. The D#-E would be around 160Hz...not far from the second peak/resonance on my guitar.

Very cool. I have an old ES 150 that seems to have a resonance just up from that around G-Ab. A friend has a Heritage with the same tendency. I've always been very curious about this. How did you produce that graph? Just tapping on the top with your fingertips? What kind of software did you use? Is this a quantifiable documentation of "tap tuning"? Again, very cool, interested to learn more.

[Runepune]

Very cool. I have an old ES 150 that seems to have a resonance just up from that around G-Ab. A friend has a Heritage with the same tendency. I've always been very curious about this. How did you produce that graph? Just tapping on the top with your fingertips? What kind of software did you use? Is this a quantifiable documentation of "tap tuning"? Again, very cool, interested to learn more.

I muted the strings and hit the bridge/top with the side of my hand. You can obviously get a more precise FFT reading with a mic and computer, but I used the FFT analyser of an iPhone app, AudioTools. I often use it to check acoustic instruments when doing live sound. It can also display peak frequency and note in large letters, so it's nice to use when ringing out mics approaching feedback territory I see now that there's a free version, FFT Lite, and I'm sure there are others out there.

[kevmoga]

I muted the strings and hit the bridge/top with the side of my hand. You can obviously get a more precise FFT reading with a mic and computer, but I used the FFT analyser of an iPhone app, AudioTools. I often use it to check acoustic instruments when doing live sound. It can also display peak frequency and note in large letters, so it's nice to use when ringing out mics approaching feedback territory I see now that there's a free version, FFT Lite, and I'm sure there are others out there.

Thanks, I'll check out that app. I'm woefully ignorant of the details of tap tuning as an archtop (or any other carved instrument) is carved and have to admit that my perception is that there is an element of unquantifiable mystery to this process. I wonder if anyone has applied this type of technology to tap tuning to get at exactly what those who are skilled in that art are trying to achieve sonically?

[nopedals]

Reading all of this, one would think that putting a bit of padding under the tailpiece would kill feedback, but the standard remedy is to put tape on the F holes.

[TH]

That's not gonna break the feedback loop between strings, pickup and amp. The feedback comes from the strings getting excited. F holes and air resonance are only one type of feedback.
But maybe that's just my world. The problem can be different for different people at different volumes.
David

[KirkP]

I like thinking about this stuff since I've got both a love of music and an electrical engineering background. Here's a sketch of the primary paths in the feedback loop for an archtop guitar with a magnetic pickup at the onset of feedback. In some cases I've drawn two paths in parallel where I felt either one could dominate, depending on circumstances.
This is just a quick take. I might have botched something and won't be offended by thoughtful criticism. There are many other paths of course, but I feel these are the dominant ones.
Note that most paths work primarily in one direction but the mechanical are bidirectional. Many of the items in boxes have very complex transfer functions and relative phases, which can result in difficult to predict resonant frequencies for the complete system.
These feedback loops can dramatically affect tone long before the volume causes runaway feedback. I'm sure most of you have noticed your tone changing (for better or worse) as volume increases. This is usually attributed to pushing the amp and speaker into non-linearity, but I think these feedback loops are a factor too.
Edit: If the pickup is highly microphonic, add a mechanical path between the "Top" and "Pickup Windings" boxes. I think Roy Buchanan made some use of that path.

[KirkP]

...So when encountering a feedback issue on the bandstand, you can use this flowchart (or a mental image of it) to consider your options to deal with it. The options include reducing gains (e.g., turning down volume knobs or moving the guitar away from the speaker), muting a path (e.g., palm muting strings, covering soundholes), and changing transfer functions (e.g., adjusting tone knobs, moving speaker cabinet around in the room, choosing a different guitar or speaker cabinet, phase reversal). Feel free to add any options I missed.
Feedback is a messy affair and usually requires some trial and effort to resolve, but if you have a basic understanding of the primary feedback loop you can be more methodical about it.
End of my speech ... maybe!

[KirkP]

Strings initiate vibration. It's where the energy that moves the top comes from.

This may just be a nit-pick, but I think it's important to point out that feedback can be initiated not only by the stings, but by another loud sound generator in the room. For example, I might dial in large-bodied carved archtop so it sounds great with no hint of feedback, but then the bass player's speaker cabinet couples to my soundholes either generating a howl or at least screwing up my tone. If using a carved top, I try to keep both my speaker and the bassist's away from my soundhole. My laminated guitars seem to be more forgiving.

[max52]

Does a jazz amp such as Henriksen help with feedback issues?

[Jim Soloway]

A little maybe but in my experience feedback is mostly a function of the guitar, the volume level, your position relative to the amp, and the room in which you're playing.

[omphalopsychos]

Some acoustic amps have a notch filter with an adjustable frequency parameter. The henriksens do not have this, if that's what you mean.

[rio]

I don’t experience feedback much anymore due to being very careful about amp placement and when needed controlling it with my right hand, but when I was using the Bud I noticed no difference in terms of potential feedback vs. larger amps.


Sent from my iPhone using Tapatalk Pro

[Geechnyc]

I have a Henriksen Bud as well as a Jazzamp 110 and can attest that their EQ controls effectively serve as frequency volume controls that can be well used to help reduce or eliminate feedback. There are other factors involved in feedback and its management, however, as some have noted above.

[pauln]

My experience (using an arch top and tube amps) is that feedback comes from larger speaker paper surface area in too close proximity to the instrument. On a larger stage I can use two 12' of four 10" by setting the amp further away, but on a crowded stage even a single 12" a few feet away may feedback. However, the one 10" (Princeton) has worked on very small crowded stages before quite well, because in those cozy little places we play at a relatively soft level, anyway.

[jorgemg1984]

I have a Henriksen Bud as well as a Jazzamp 110 and can attest that their EQ controls effectively serve as frequency volume controls that can be well used to help reduce or eliminate feedback. There are other factors involved in feedback and its management, however, as some have noted above.

I've had a Henriksen and have (and had) lots of eq pedals. In order for you to kill feedback using the Henriksen's eq (or any other graphic eq) you need to be lucky the offending frequency is near the frequency centre. If so, you'll also kill a lot other desirable frequencies.

For the purpose intended, you'll ideally use a notch filter, so you can kill that specific frequency and nothing else. A parametric eq with an q control or a graphic eq with lots of bands, could also do that, although not as well.

[Geechnyc]

I've had a Henriksen and have (and had) lots of eq pedals. In order for you to kill feedback using the Henriksen's eq (or any other graphic eq) you need to be lucky the offending frequency is near the frequency centre. If so, you'll also kill a lot other desirable frequencies.

For the purpose intended, you'll ideally use a notch filter, so you can kill that specific frequency and nothing else. A parametric eq with an q control or a graphic eq with lots of bands, could also do that, although not as well.

That's your experience, then, not mine, which has not involved 'killing' any desirable frequencies: said without disputing the value of a notch filter as potentially an additional piece of equipment.

[jorgemg1984]

It's not really a matter of experience, a graphic eq has a large band. If you roll down 100hz it won't just kill the offending frequency, it will kill a lot more. It might not be sonically relevant, but it will happen.

[jschiele]

My experience differs from many of the above posters, I've found that feedback has as much to do with the amp as the guitar. My guitar (a Heritage 575 with a carved spruce top) might feed back on Bb with one amp but on C or D with a different amp which tells me that different amps have different resonant frequencies and interact with the guitar in different ways.

Decades ago I read an interview with Johnny Smith in which he claimed that feedback wasn't a problem of hollow body guitars, it was a problem caused by guitar amps which are primarily designed for solid body guitars and are made to boost certain frequencies that are lacking in the typical solid body. At the time I thought he was nuts! Turns out he knew what he was talking about and the best amp for archtop guitars is the one with the fewest resonate peaks (ie the flattest frequency response).

I've also found that a sealed cabinet amp like a Polytone feeds back less than an openback such as Fender and keeping the amp on your left away from the body of the guitar helps.

When I bought my Heritage in 2010 I already had two amps, a '76 Princeton Reverb and a '64 Princeton. They both fedback easily but on different notes! I soon moved to an old MiniBrute II which had far fewer feedback issues. After a few years I got a MegaBrute which I liked even better and now I have a Bud which I like best of all. The Polytones and the Bud are about the same for feedback and seldom a problem. My worst amp for feedback but best for tone (if I'm playing very quietly at home) is a 1952 Fender Pro.

Jonathan

[Alter]

Also phase reversal is a useful thing to have. Not as much as with acoustic guitars obviously, but still, with solid wood archtops reversing the phase can sometimes make a big difference.

I've had good luck with the henriksen regarding feedback. The eq, the small size that makes it easy to best position the amp, the 10' speaker. Generally the less bass you like to use the easier things will be.

I would also agree on the following..

I've also found that a sealed cabinet amp like a Polytone feeds back less than an openback such as Fender and keeping the amp on your left away from the body of the guitar helps.