Using antifeedback and phase on Fishman Loudbox Artist Pro 600

[BigsBbender]

Wanting to get back to playing some folk and return to roots I invested in a Fishman LBT 600 Artist Pro. I love the amp but the instructions don't really cover using the antifeedback knob and phase button very well. I've looked on Youtube but everything I've seen just has the presenter mentioning the ability of the amp to address feedback.

My main problem is I've got the guitar plugged in and sounding fine but when I plug in my Shure 58 dynamic mic I get a high pitched feedback. I've tried using phase to cut feedback on both channels but it doesn't work. I can't tell if its the guitar causing the feedback or the mic. I can use the Pad button to cut the mic signal by 10dB and that seems to help, but then I can't hear my vocals. Mic is placed parallel to floor pointing right at my mouth which is about 8" from it, just like the Shure directions advise.

My practice space is somewhat limited but not so much to be the problem as from some of the videos I've watched the players using both mic and guitar in smaller rooms don't seem to have this problem. My practice space is 10 feet wide by about 30' long, drywalled and heavy carpet for about 20 feet that the amp sits on.

If anyone has this amp I'd love if you could explain how to find the antifeedback sweet spot on both guitar and mic. Or if you know a video that covers this please share the url.

Thank you

[shufflebeat]

Because high frequencies tend to be more directional than low frequencies the cause is more likely to be that the speaker is pointing towards where the mic is picking up. The upside of this is that if you point the speaker towards the mic's null (deaf spot) then the chances of feedback should be minimised. This is usually much more effective than a hi-tech approach.

The sm58 is cardioid so the null is directly behind the most sensitive spot, i.e., if you hold the mic in your hand and point the XLR end towards the speaker (on the floor in front of you like a wedge monitor) you've pretty much got it.

Other mics have different polar patterns so the null is slightly off to the side.


That button you're pressing is not "phase" it is "polarity" which is connected but not the same thing [common misconception resulting in a recurring moan ].

[zeeway]

My experience with a Fishman Artist/Shure58 supports shufflebeat's comments. When I setup, my amp is to my left and slightly behind me, pointed straight ahead. My mike is on my front right side. Imaginary lines through the axis of the mike and the axis of the amp are at about 20 degrees to each other.

You also have to be careful that the mike is not "pointing at" a monitor speaker behind you. You also have to be careful that the front of the guitar is not pointing at the amp during setup or after (unless you are Jimi Hendrix).

With all that, sometimes I still get feedback when first turning on the mike. In that case, I turn off the mike, check my physical setup and get a little more angle separation, and then turn the mike back on.

With this arrangement, your body is between the amp and the mike, as well, providing another barrier to feedback.

[fazool]

Originally Posted by shufflebeat

Because high frequencies tend to be more directional than low frequencies the cause is more likely to be that the speaker is pointing towards where the mic is picking up. The upside of this is that if you point the speaker towards the mic's null (deaf spot) then the chances of feedback should be minimised. This is usually much more effective than a hi-tech approach.

The sm58 is cardioid so the null is directly behind the most sensitive spot, i.e., if you hold the mic in your hand and point the XLR end towards the speaker (on the floor in front of you like a wedge monitor) you've pretty much got it.

Other mics have different polar patterns so the null is slightly off to the side.


That button you're pressing is not "phase" it is "polarity" which is connected but not the same thing [common misconception resulting in a recurring moan ].

1) Low frequencies are where the resonance of an acoustic guitar reside - anti feedback functions limit those resonant frequencies.

2) phase is phase NOT polarity - phase functions invert the input signal (what you are calling polarity) which puts them 180 degrees out of phase with the output signal, "cancelling" thee feedback loop.

[shufflebeat]

Originally Posted by fazool

1) Low frequencies are where the resonance of an acoustic guitar reside - anti feedback functions limit those resonant frequencies.

It's the mic that's feeding back, not the guitar.

2) phase is phase NOT polarity - phase functions invert the input signal (what you are calling polarity) which puts them 180 degrees out of phase with the output signal, "cancelling" thee feedback loop.

How does the button affect the input signal? By flipping the polarity.

The result of this is to put the input signal and amplified output out of phase, nulling certain frequencies and solving feedback problems - unless of course they were already out of phase, in which case the button will align the phase of both waves, exacerbating all those issues.

If the signals were at 25% phase alignment (nominally) then flipping the polarity would have zero effect on phase of course, they would just swap places in which is 25% "ahead" of the other.

But what the button changes is polarity.

[Doug Young]

Originally Posted by fazool

1)
2) phase is phase NOT polarity - phase functions invert the input signal (what you are calling polarity) which puts them 180 degrees out of phase with the output signal, "cancelling" thee feedback loop.

It's true that phase and polarity are different things. Sadly, most guitar gear incorrectly labels a polarity inversion feature as "phase". What's in this amp is polarity switching, not phase. Here's a decent article explaining the difference:

https://www.justmastering.com/articl...d-polarity.php

or actually, this one may be better: https://audiouniversityonline.com/polarity-vs-phase/

Adjusting phase is more complex, and frequency-dependent. There are devices like the Little Labs IBP or Radial Phazor that do it, but these aren't typically used as a feedback tool, they are more for aligning different sound sources, like a mic and a pickup on the same instrument.

To the OP - generally that "phase" switch is useful for the guitar when plugged in with a pickup, less so for a mic. What it does is invert the polarity of the sound. If feedback is occurring because the sound is reaching the top of the guitar causing it re-enforce the vibration. By switching the polarity, the top will be encouraged to vibrate the opposite direction, thereby cancelling some of the sound instead of re-enforcing it. This effect is highly dependent on where you are standing and may or may not help. It will not affect what the audience hears (other than hopefully reducing feedback), only how the amp interacts with the guitar. There's no right setting - just try both positions and see which results in less feedback, which can range from a howling feedback to just a boominess of woofiness in the sound.

Anti-feedback controls are basically an EQ, a very sharp cut at a specific frequency. If you are getting feedback at a specific pitch, you can turn that knob until you find the frequency of the feedback, and hopefully it will go away. There are more sophisticated anti-feedback filters, some can automatically detect the feedback frequencies, some can handle more than one frequency and so on. I suspect the one in your amp is a simple one.

The ultimate solution to feedback is to lower your stage volume. Turn down, or position your amp of speakers so that you don't get feedback.

[shufflebeat]

Those graphs in the 2nd link are excellent. I'm gonna need another lockdown.

It's also true to say that these principles are reliable whether your system is 1 mic and 1 acoustic amp or a stage full of musicians and a multi-kilowatt sound system. I struggled for an hour in a venue which was notorious for it's horrific bass feedback and lack of clarity. I notched out so many rogue frequencies that I ended up pretty much flatlining the EQ again at -10dB, which is the equivalent of just turning the whole thing down to inaudible above crowd noise, which was of course amplified by the same acoustic environment.

In an attempt to separate the PA from the mics and the big wooden resonator they were all sitting on (the stage) I pulled the speakers forward as far as I could, it had no significant effect. I had no time to move them back but now the acoustic noise from the kit was out of time alignment with the mic'ed drums in the PA so I poked in a delay on the FoH to align them. Estimating what would be appropriate and overshooting by quite a lot I hit on the exact delay that nulled the resonance in the mic/stage/room/PA system and it was like switching off Hell. It changed throughout the night but I was able to keep on top of it with a combination of delay and EQ.

It turns out that these things are basic knowledge for people who know about these things but for me every day is a school day.

[BigsBbender]

Thank you all for the great "feedback". I tried moving things about, angled the amp pointing away 20 degrees, positioned mic's back to amp and moved my seat back 10' from amp. Result was amazing, no mic feedback without any pressing of phase (polarity) button and antifeedback knob is turned off completely. I realize this problem will probably reoccur with different venues but at least it's working for me at home now.

Thanks again gentlemen. Your input is invaluable and appreciated!

[Howard Emerson]

Originally Posted by BigsBbender

Wanting to get back to playing some folk and return to roots I invested in a Fishman LBT 600 Artist Pro. I love the amp but the instructions don't really cover using the antifeedback knob and phase button very well. I've looked on Youtube but everything I've seen just has the presenter mentioning the ability of the amp to address feedback.

My main problem is I've got the guitar plugged in and sounding fine but when I plug in my Shure 58 dynamic mic I get a high pitched feedback. I've tried using phase to cut feedback on both channels but it doesn't work. I can't tell if its the guitar causing the feedback or the mic. I can use the Pad button to cut the mic signal by 10dB and that seems to help, but then I can't hear my vocals. Mic is placed parallel to floor pointing right at my mouth which is about 8" from it, just like the Shure directions advise.

My practice space is somewhat limited but not so much to be the problem as from some of the videos I've watched the players using both mic and guitar in smaller rooms don't seem to have this problem. My practice space is 10 feet wide by about 30' long, drywalled and heavy carpet for about 20 feet that the amp sits on.

If anyone has this amp I'd love if you could explain how to find the antifeedback sweet spot on both guitar and mic. Or if you know a video that covers this please share the url.

Thank you

Page 15 in the instruction booklet covers it pretty well regarding the guitar.

Howard Emerson