Preamp or Piezo Pickup?

trekker · #1 05-30-2018, 06:35 PM

My acoustic bass guitar has a weird problem and I'm hoping that someone here my have run into something similar before. I'll try to describe it as best as I can.

It is a four-string (tuned E, A, D, G). Amplified, the A string is significantly louder than the other 3 strings. In fact, when plucked, the volume of the A string continually gets louder and louder, like acoustic feedback. It doesn't die away like the other 3 strings. In fact, the A string will continue to vibrate until I touch it. As soon as I touch it, of course, the sound goes away. Even if I'm playing the other strings, the A will start to vibrate and sound. Weird.

Now, the volume is reduced somewhat if I fret the A at the first fret. Not completely, but definitely reduced. And as I play the A string higher and higher up the neck, it sound like it should, it doesn't ring any more than it should. It seems to do this with just an open A.

This bass has a Fishman Classic 4 preamp in it with, of course, a copper ribbon piezo band. Do you think the problem is in the preamp? In the piezo pickup. Why in the world would the string continue to vibrate forever after a single pluck? Why doesn't it just fade away like the other strings do when plucked? Is it, perhaps, a saddle problem. I'm stumped. I've never seen a guitar or bass do this.

Any help would be appreciated.

mirwa · #2 05-31-2018, 02:49 AM

Fretting the string changes pressure on the saddle and leaning of the saddle.

I suspect maybe the saddle slot is a little wide or the base of the saddle slot is no longer flat. These are just my first points of reference when chasing volume issues.

Steve

jonfields45 · #3 05-31-2018, 05:41 AM

You can not get infinite sustain without feedback from something providing the energy. Are you amplifying the guitar when this happens? Likely your guitar has a resonant peak at the low A, 110 Hz. This is a little lower than most regular 6-string dreadnoughts. One test would be to fret the low E string at the 5th fret and see if you get similar behavior. Try reversing the phase of the amplified signal and see if that helps. Ultimately a notch filter, a sound hole cover or placing the guitar farther from the amp will likely be required. A 110 Hz wavelength is on the order of 10 feet and moving the guitar 5 feet further from the amp will reverse the phase of that feedback. Feedback needs to be in-phase to be regenerative.

trekker · #4 05-31-2018, 06:28 AM

Thanks, guys.

I'll check the feedback first. Yes, this happens amplified, not amplified. I'm pretty much in front of my 50W amp but I assumed that feedback is feedback for ALL strings, not just for one string at one frequency. I have a feedback buster soundhole plug that I'll try first as well as moving further from my amp. The Fishman preamp in this bass, unfortunately, does not have a phase switch. I hope to remedy this as the bass plays, looks, and sounds wonderful except for this darn A string problem. By the way, if I play the A string fretted further up the neck, the ringing goes away and the volume pretty well comes in line with the rest of the strings.

If these feedback-reducing steps don't work, I'll check the saddle. I just put new strings on it (it was doing this with the old strings also), and the saddle was very, very loose in the bridge slot. It is, perhaps, not the original saddle as this bass was made in 2004. I know that saddles are supposed to be snug in the slot (so that they don't move much) but loose enough to permit the transmission of the vibration of the strings to the piezo pickup.

Thanks again. I'll update you guys on what I discover.

trekker · #5 05-31-2018, 06:45 AM

Phase question. The piezo ribbon pickup on this bass connects to the input of the Fishman Class 4 preamp by 2 simple screw connector i.e. it is not soldered to the preamp. Can I change the phase by swapping these two wires around? Or is it a phase problem between the preamp and my bass amp? I'm not sure how I would remedy that?

ChalkLitIScream · #6 05-31-2018, 01:29 PM

I have the same problem. The open A gets louder gradually. The slightest movement will eventually become quite loud, and its fun seeing the string vibrate more and more all of a sudden. Its as if there is free energy being created (but of course only happens when amplififed)
I used my input on my Yamaha electric piano so the amp isnt the problem for us. 5th fret on the low E does not cause the same problem.

Using a LR Baggs Anthem with a UST a well. Saddle isnt loose-loose(doesnt wiggle around, but will fall out if the guitar is turned upside down)

jonfields45 · #7 05-31-2018, 02:42 PM

Quote:

Originally Posted by trekker

Phase question. The piezo ribbon pickup on this bass connects to the input of the Fishman Class 4 preamp by 2 simple screw connector i.e. it is not soldered to the preamp. Can I change the phase by swapping these two wires around? Or is it a phase problem between the preamp and my bass amp? I'm not sure how I would remedy that?

Probably one of the wires is a shield and is attached to the local ground. Phase reversal that way might make the guitar more prone to noise pickup. The soundhole cover sounds like an easier first step. Most acoustic guitar pedals have a notch filter and/or phase reversal.

The reason only one note, on one string, has this problem has to do with the entire guitar as system absorbing energy from your amp. It is not uniform across the audible frequency spectrum and there will be peaks where it is more susceptible to feedback. Most acoustic guitars have one of these peaks a bit higher in frequency than an open A string. It might not happen on the low E fretted at the 5th fret for multitude of small details that change the gain of that feedback loop.

trekker · #8 05-31-2018, 07:34 PM

Thanks, guys! Here's an update:

I tried moving my ABG about 15' from my amp and off to the side (that's the longest patch cord I have right now). Didn't help, the A still rang. Guitar volume at about 40%; amp volume at about 10 o'clock.

I tried fretting the E string at the fifth fret (an effect A) and it had the weirdest sound. It was the same pitch as the A (obviously), but the volume wavered, a wah-wah type of sound.

So in a last desperate attempt, a put a feedback buster in my sound hole. The ringing on the A stopped. Hallelu....crap...my E string now has a really low volume. So with the buster in the hole, the A no longer rings and has the same volume as the D and the G. But the E can hardly be heard, almost as if it isn't amplified. I tried turning the BASS EQ up, to get more out of the E string, but as one might suspect, this also amplified the A too much.

With the buster out, the A dominates everything. With the buster in, the A is normal, but the E can't be heard. Crapola.

Now I don't know what to do. I guess I'll call the store and see if I can return it.

It's too bad because I really like the looks and playability of this acoustic bass. But sound, in music, is everything.

Bruce Sexauer · #9 05-31-2018, 09:06 PM

Sounds like your instrument is resonating in sympathy to the amp, a typical feedback loop. All true acoustic instruments have this problem as volume increases. The normal solution is to get an electric bass.

trekker · #10 06-01-2018, 06:49 AM

That sounds about right, Bruce. Fortunately, I have a nice Ibanez SR3000 Prestige Soundgear.

But I also have a nice Ovation acoustic bass which doesn't give me any feedback whatsoever, no matter how close I sit to the amp.

Rodger Knox · #11 06-01-2018, 10:13 AM

Try covering part of the soundhole. That should reduce the resonance frequency and get it below the A note, which should get rid of the feedback without killing the lower frequencies. Or you could also shave the back braces.

John Arnold · #12 06-01-2018, 02:40 PM

Quote:

the A is normal, but the E can't be heard.

I suspect uneven pressure on the piezo pickup. Make sure that both the saddle and the bottom of the slot are perfectly flat.

martingitdave · #13 06-01-2018, 03:05 PM

Out of curiosity, do acoustic bass guitar string sets have relative equal tension across the strings like a guitar set does?

EDIT

No, as suspected, they don't. From D'Adarrio:

PBB045 G 0.0450 1.1400 47.500 21.540
PBB065 D 0.0650 1.6500 55.700 25.260
PBB080 A 0.0800 2.0300 47.400 21.500
PBB100 E 0.1000 2.5400 40.200 18.230

It may simply be the nature of your strings that the low E would be quieter than the rest, because the bearing pressure on the piezo is proportionally lower. Aside from that, I agree with Arnold that you need to check the saddle and slot. Sometimes when the strings are changed, things wiggle around in there, or it wasn't flat to begin with.

trekker · #14 06-01-2018, 07:06 PM

Hey, guys. Well, I very lightly sanded down the bottom of the saddle. I know this saddle is flat and there do not seem to be any humps in the saddle slot on the bridge. It still exhibits the same symptoms. So my problem may be the resonant feedback of the bass guitar combined with a weak piezo pickup.

Frankly, I'm getting tired of messing with it. I think I'll just stick with my electric bass and my Ovation bass. This one is more trouble and frustration than it is worth.

Thanks for the advice and opinions, though!