A way to reduce feedback?

[6267]

This has worked on some of my amps, and in some situations. Feedback is when a given frequency is produced or picked up, reinforced by the amplifier, and thus picked up and reinforced again etc. Often the particular frequency is dependent on the amp and its environs, but I had this idea: If the speaker is wired out of phase with the input, it is much harder to pick up the original frequency which causes the ringing, because the amplifier is actively cancelling it by projecting any sounds picked up in an inverted phase relationship.

I'm not too sure though, and so I wanted to open this up for an experiment!
For those who have amps which tend to feedback a bit with hotter mics etc., try switching which wire goes to which terminal on your speaker(s). PLEASE DO THIS ONLY WHILE THE AMP IS OFF. I found that I could sit in front of the amp and play harp into the microphone right in front of the speaker without feedback (whereas with it the other way I couldn't get within around a 5ft distance at that volume). For those who can try easily, I'd like to see if the problem improves (it didn't go away for me, but one polarity is definitely less feedback prone than the other).

If this is just snake oil, let the flamewar begin–I honestly see a pretty major difference though (almost as much as just turning down the treble )!

[d95err]

Another solution is to install a phase-shift switch for your guitar pickups. Amongst others, Brian May is known to use this to optimize for more/less feedback during live shows. Switching the pickup has the benefit of being able to switch while playing.

[Steve Conner]

Sure, it works. Many electroacoustic guitars, and amps designed for them, have a phase switch that lets you do just that. You try it both ways before the gig and use whichever way cancels feedback the best.

(It's not 100% effective because of the phase shift caused by the speed of sound in air.)

[d95err]

Oups, I just noticed this was in the "Harp Amps" section. No pickups then I guess. Oh well, I guess you could probably make a similar switch for a microphone too. Or you could build a small box with a switch to select phase reversal or not. A unity gain inverting opamp stage would do the trick nicely.

It is probably good to be able to switch on the fly during the performance, because as Steve points out the phase will change with the distance to the speaker, and feedback frequencies could be affected by the acoutstic environment on the stage as well.

[loudthud]

I think you will find this interesting:

http://www.harpamps.com/weber/Guitar51.htm

[JC@]

Originally Posted by loudthud

I think you will find this interesting:

http://www.harpamps.com/weber/Guitar51.htm

Good link. I will save that bookmark.

[kg]

you could also turn the amp around, or yourself. don't turn both though.

seriously though.. IF your amp uses a balanced input from a low z mic, i would consider a front panel switch to do it right at the input stage so that it could be done arbitrarily and without possible unloading of the amp (however momentary).

[arnenym]

Originally Posted by loudthud

I think you will find this interesting:

http://www.harpamps.com/weber/Guitar51.htm

It dont work so good as he says!
I've build a 5D3 and used his mods on it.
A ordinary feedback loop with a precense control works better for me.
Maybe a polarity switch works better. Yesterday I got more feedback when the guitarist turn up his amp!?

[kg]

Originally Posted by arnenym

Yesterday I got more feedback when the guitarist turn up his amp!?

the guitarist's amp's output excited the feedback frequencies which were then regenerated by your amp.

[MWJB]

I haven't found that reversing speaker plarity has a significant, or even a particularly noticable effect on acoustic feedback when playing harp (I always test mod jobs & builds forward & reverse polarity), it mainly affects harmonics generated.

Many folks focus too much on fighting feedback with harp...unfortunately it largely comes with the territory. Preamp voltage tweaking (say 150-200vdc on a 12AX plate), experimenting with switching in/out preamp and output cathode bypass caps, output tube type & biasing, preamp tube subs, careful speaker selection will all have a more tangible effect on feedback than speaker polarity. You have to approach feedback at points all the way through the circuit to get a substantial advantage.

I recently played a Kendrick harp amp, it's got a cool old school sound but, as Arnenym notes, it wasn't the "end of all feedback problems". Plenty of stock, big Fenders get louder, even if their volumes are barely off the stop.

At the end of the day if you really just need a more sensible sweep on the volume pot to make life easier run another, smaller resistor from the wiper of your 1meg vol pot to ground, to give a bigger sweet spot and a slower ramp up. Or, use a 250KA volume pot instead of 1Meg, fed with a 470K dropping resistor (the 250K pot on it's own will let more feedback inducing highs thru thr circuit due to the lack of series resistance).

I have an amp that has an almost stock 6G12 channel and a modded low gain configuration on the other channel (same preamp circuit but first stage is cathode follower but no recovery stage before PI). The stock 6G12 channel feeds back at "2.5" with most good output mics, the modded channel feeds back at "9" on the 1Meg volume control - but the amp makes identical power irrespective of channel used. The two channels do sound subtly different.

I try and concentrate more on getting good efficiency & power out, before feedback rears its head.

[kg]

have any of you harp guys ever experimented with putting a automatic feedback eliminator/notch filter somewhere in the signal path? they work pretty damned well with vocal monitors, etc.

ken

[MWJB]

John Kinder has his AFB+, outboard anti feedback box (the Harpking amp AFB kills feedback by circuit manipulation). One of the Rocktron Hush pedals has quite a following.

Trouble can be, after a point, you keep notching hi freq to the point that fidelity and cut are impaired (that's why I prefer few subtle tweaks throughout the amp circuit, rather than one big factor at a particular stage in the circuit). Sure the amp can get loud, but if you can't get decent note seperation and articulation it's not easy to play off such a sound (you can feel detached from the amp - this is one of my bugs with aggressive frequency notching)...of course, different players have very different thresholds on that front.

Again Kinder's Harpking amps have 2 controls to fight feedback, one of which also largely affects fidelity...rather like Weber's Kendrick PI mod. With these controls backed off the amps can feedback pretty early, but wind up the "special" controls and you get a lo fi sound as the trade off for volume. Some guys can live with the lo fi sound, others can't.

Scooter's Meteor amp uses conventional means to achieve good volume before feedback, with good fidelity (voltage/current tweaking, lower gain preamp tubes), as do many tweaked 4x10" Fender amps (Concerts, Super Reverbs & tweed bassmans).

Some sort of specific anti feedback maybe a useful thing to have, especially if you live out in the sticks and don't have access to an experienced harp amp tech, but most guys I know manage without one and don't have problems being heard, or with feedback on stage. Let's face it, on very large stages you're likely to be mic'd anyway. Could be a useful thing for pick up amps/unavoidable loud situations like jams, but I'd really expect an amp used for harp to work well enough straight off the bat.

Another problem is that generic feedback killing devices may also load down the mic signal, hurting punch & dynamics. A very hi Z input may be a good idea (op amp?). I think a lot of guys want as little in the signal chain prior to the amp input as possible, maybe just a delay? An exception might be Jason Ricci who has posted a couple of illuminating clips on youtube explaining his pedalboard used with his Harpgear HG50 amp.

[Bruce / Mission Amps]

Originally Posted by d95err

Oups, I just noticed this was in the "Harp Amps" section. No pickups then I guess. Oh well, I guess you could probably make a similar switch for a microphone too. Or you could build a small box with a switch to select phase reversal or not. A unity gain inverting opamp stage would do the trick nicely.

It is probably good to be able to switch on the fly during the performance, because as Steve points out the phase will change with the distance to the speaker, and feedback frequencies could be affected by the acoutstic environment on the stage as well.

Here a simple trick I've used when helping some harp players with their vintage amp gear.
I can't say I invented it as I can't even remember when I first ran into it's use but I've used it for at least 10 to 12 years.
You can not ground one of your OT leads without it going to the DPDT switch first... flipping the toggle one way or the other reverses the speaker phase... the down side is that some amps with NFB can really freak out so use it sparingly or with amps that have no NFB.
I guess you could implement this same idea on the front end of the preamp too.