Feedback by itself is not a bad thing. In fact, it's what
gives some guitar amps their singing sustain and can be a much sought after attribute of
the amp. Guitars, however, use pickups, not microphones as input devices and are therefore
not as sensitive to feedback loops as a microphone is.
Getting rid of feedback is a major problem
for a harmonica player and can be quite difficult to do, especially with a loud band.
Feedback is affected by many things and consequently there are no hard and fast rules for
eliminating it. The size and shape of the room, number of people in the room, amplifier
settings, equipment and microphone placements are only a few of the elements that can
allow feedback to occur. Knowing what these elements are can help us avoid certain
conditions or compensate for them. Here are a few things that may help out:
Some amps just feedback no
matter what you do. You can bring it to a technician and try to tweak the amp, but if the
amp is vintage then you probably don't want to wreck it's value. Even if you don't care
about the value, is it worth it to spend the money trying to get this type of amp to do
what you want or is it best to sell the amp and get a different one that will do what you
want.
Does your amp volume come
on "REAL FAST" when you turn the volume control up (is it loud by the time you
get to 2) or does it come up gradually (say by 6 or 7)? Older amps usually used what are
called linear tapered volume pots and tend to turn on real fast and can be very difficult
to fine tune or dial in your volume. While not causing feedback directly, this can
contribute to its appearance by not giving you enough control. You can change the pots to
audio taper ones, which will give you a smoother turn on or use an outboard mixer. I turn
my amp on 8 and use an external mixer to lower the volume. This lets the tubes sing and
sustain (they sound better when they are driven anyway) while giving me more control of
the volume.
Move the amp around on stage.
Don't stand too close to it or have it pointing straight at you. If I am in a small club
with a small amp, I sometimes face the amp towards the wall and angle it back. Try not to
put the amp on a chair (or anything else). Keep it on the floor. The amp will have more
bottom end and not feedback as fast. In a larger venue, I will put my amp against the back
wall, as far away from me as possible. Since I use an outboard mixer I can place the mixer
close to me up front. That way, once I set my tone controls, I can control my volume
through the mixer, eliminating the need to get to my amp, even though I still like to. I
don't like volume controls that are built into microphones. They are just a grounded out
volume pot and just steal away ALL of your tone.
Using pre-amp type effects, like
overdrive and distortion pedals, can be a big source of feedback. I avoid them like the
plague (I also think they sound too buzzy). Use the amps natural overdrive (by turning it
up as described above)
Use a secondary, unpowered,
speaker cabinet. Say you have one 12" 8 ohm speaker in the amp. Use another cabinet
with another 12" 8 ohm speaker run in parallel with the first one. (positive lead
goes to the + on both speakers and the negative lead goes to - on both speakers. Usually,
having a 100% mismatch on an 8 ohm speaker load won't blow your output transformer
(running in parallel will bring it down to a 4 ohm load). Don't do this if you are unsure
of what you are doing or are using an amp with 4 speakers and a 2 ohm load like an old
Bassman or Super Reverb. This would bring it down to a 1 ohm load (a dead short is 0
ohms...not safe for speakers or transformer). You could also try using two amps, one on
one side of the stage and one on the other. I have done this at large clubs. Most amps
have two inputs per channel. You could plug your mic into input #1 on channel one, then
take a patch cord from input #2 of channel one and plug it into the second amps input #1
on channel one. This is safe and easy but will reduce the gain on both amps because of the
built-in mixing resistor on the input connectors. Instead you could just get a 'Y'
connector and plug your mic into it then plug one branch of the 'Y' into one amp and the
other branch into the other amp. Safe, easy, no gain loss.
Putting a mic in front of an amp
that is feeding back will feedback through the PA since it is only increasing the volume
of what the amplifier is putting out. If you are going to mic it you MUST lower your amps
volume so that it doesn't feedback AT ANY TIME. Put the mic on a stand and point it
off-axis (not directly) towards one of the speakers. This will make the mic more
directional and reduce feedback from the PA. Adjust the PA's volume and tone controls (I
like to keep them pretty flat). Obviously, there is a point where turning up the volume on
the PA will cause the PA to feedback, not the amp. If this is the case, and you are still
not loud enough, try moving the amp and / or mic placement. If the amp still feeds
back...shoot it.
...or, if you are handy with a
soldering iron, you could add a simple line level output to the amp so you can plug your
amp directly into the PA. This would eliminate the mic to the PA (e-mail me and I'll send
you a schematic). Another variation of that is to use a line splitting direct box. With
this you plug your amp's main speaker out into the high impedance in of the box, take the
high impedance out of the box back into your amp's speaker and finally, take the low
impedance out of the box back to your PA. Try using a combination of mic and direct box.
Who knows? Never allow your harp microphone to get in front of the PA's speakers when
placing a microphone in front of your amp.
The mic you use makes a big
difference as does the way you hold it. If you use a Shure Green Bullet microphone, try to
find an old vintage one. The new ones are nothing like the old ones. If you are a serious
harp player, do yourself a favor and get an old Green Bullet or at least the cartridge
inside of it. If you can't find one yourself, call Tom Ellis of Tom's Mics at 214-328-3225
or go to his web site at http://www.island.net/~blues/tomsmics.html.
Hold the mic tightly to get a good, air-tight grip. You'll be surprised how much this cuts
down on feedback. Never, never let the microphone face towards the amplifier.
Mess around with those tone
controls. On most amps, the tone controls are "passive". That is, they don't add
any gain, they just short out or pass certain frequencies through capacitors and
resistors. This process always loses gain and tone. What that means is, if the tone
controls are turned all the way up, it is as through they weren't in the circuit at all.
You can then lower your volume and adjust your tone by backing off treble for more bass or
backing off the bass for more treble. Watch your midrange and treble as these give us the
most trouble.
There are several feedback
blockers on the market. The cheap ones don't work (I've tried several) and the expensive
ones are, well, expensive. A cheaper and simpler solution would be to...
...Turn down and get your band to
turn down. Most musicians who know what they are doing enjoy playing at a lower stage
volume. If you are too loud, you have no room for dynamics and are throwing away an entire
dimension of your sound. A good stage volume is vital to a good sounding band, regardless
of the instrument you play. It is called a band, right? Play together and it will all
sound better. Turn down and you won't feedback.
Feedback. The enemy of all harmonica players. We all know it. It's that piercing,
sustained high frequency squeal that our amplifiers pour out when they are in one of their
moods. Well, it's not our fault, right? Positive feedback occurs whenever a microphone is
placed in a position to "hear" the speaker of an amplifier. This signal is
"fed back" into the system and, if loud enough, will sustain itself, getting
ever louder and louder until the entire bar is cringing and saying "thank you"
(that's not really what they are saying). The "natural" frequency of the squeal
is determined by the time delay of getting the sound from the microphone to the speaker
through the air (feedback loop) back to the microphone.