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One of the worst events that can befall a band during a show, aside from actual injury to any of its members, is to have a screeching blast of feedback suddenly deafen the audience, sending the band's approval rating spiraling downward. If there is a sound man, he'll probably get the blame, although as we'll see, it might not be his fault!

Feedback can come through either the main speakers or the monitors, although in my experience it is more often the monitor speakers that feed back. (That way the band is briefly deafened too.) Usually, the microphones are behind the main speakers and the system isn't running at the very edge of feeding back, but if this is not the case, you may be able to coax out some feedback from the mains as well as the monitors!

Back to basics for a mo: what is feedback, anyway? As the name implies, it results from speaker sound "feeding back" into (i.e. being heard by) one or more microphones that are running through that speaker. Round and round goes the sound. Luckily, there are a number of steps that can be taken to minimize the possibility of this unfortunate menace. Here are five of them.

1. Minimize the number of microphones onstage.

If you can make a direct connection of some kind (DI box, amplifier output jack, etc.) to any or all of the instruments, do it. Each microphone you can eliminate from the stage is one fewer feedback source to plague you later.

2. Turn down any microphones that are not in use at the moment.

This is in keeping with the previous idea about minimizing the number of potential feedback-producing microphone-speaker loops. A microphone that is turned down all the way can't feed back!

The fact is, I'm going to recommend that you don't turn unused mikes all the way down - just halfway or so. This is so that when you forget that you are being careful about feedback and that you turned down the backup singer's mike ten minutes ago and she suddenly starts singing, you (and the audience) will at least hear her to some degree before you manage to restore the proper level.

3. Don't move the microphones around.

This is one of the main causes of monitor-speaker feedback. It's funny, but everything can be fine, then the lead singer moves his mike two feet to the left and shortly thereafter it starts feeding back. The pattern of locations that do and do not produce feedback onstage is very complex. Once you find a microphone placement that works, try not to let it change much.

If your singer is a flamboyant, Roger Daltrey type who needs some elbow room, be sure you have checked the setup out in advance (at full volume) to isolate any "hot spots" that may cause feedback. (You can mark these "no-go" areas with tape.)

4. Don't operate at the edge of feedback.

If you have the main or monitor speakers turned up so loud that the system is perpetually just about to feed back, there's something wrong with your setup. You can tell you're in this situation if there are tiny hints of tones or "notes" heard when someone talks through one of the mikes. Those tones are micro-bursts of feedback!

You don't have to be that close to the edge to have problems, though. Operating anywhere near the feedback point cuts your margin of error and makes it more likely that one of the factors mentioned earlier will kick in, and then there goes your reputation. Back off on the gain levels and sing louder or get closer to the mikes. Or, put up with sounding a bit quieter in the room. It's better than feedback!

5. Watch out for instrument feedback.

I've been talking about microphone feedback here, but electric guitars are notorious for "string feedback." This occurs when the amp speaker sets the guitar strings to vibrating, and then that note comes out of the amp speaker, which sets the strings to vibrating even more, which - well, round and round you go again.

If this happens while playing, the hand-on-the-strings method can be used (unless you want the feedback, Jimi). A more common problem is for the guitar player to put the guitar on a stand right in front of the amp, resulting in louder and louder string feedback. The best way to avoid this is to get in the habit of turning the volume knob(s) on the guitar all the way down before putting the guitar down. Just don't forget to turn it back up!

Mark Bendig is the Cheap Advice Guy, offering live sound tips as well as home recording and songwriting tips at http://cheapadviceonmusic.com. A series of eBooks on Live Sound, Home Recording, and Songwriting is available on the website.

Flyback Transformer-How to Locate the ABL Line

Whenever there is a complaint about monitor
contrast problem I will check on the flyback
transformer ABL (automatic blanking limiter)
circuit.
Sometimes a weak picture tube might caused
contrast problem. If the picture tube is
good, I will go straight to the contrast
circuit. The question is how do we find
where the contrast circuit located? Simple,
if you have a datasheet for the video pre-
amplifier ic, definitely you can find the
contrast pin. For LM1203N video pre-amp
ic the contrast pin is located at pin 12.
Replace this ic or trace from this circuit
to find the cause of the contrast problem.

From experienced, usually capacitors shorted
and resistor turned into high ohms were the
caused of the problem. A video pre-amplifier
can also be defective. To confirm whether is
circuit or ic problem, just solder out the
contrast pin and switch on the monitor. If
the monitor display normal picture (contrast ok)
then suspect the abl line circuit. If it remain
the same (dim contrast) replace the video
pre-amp will usually solve the problem.

The contrast signal came from one of the pin
(leg) of a flyback transformer which we called
it as abl pin. Many technicians have the
difficulty to trace the abl pin because
usually a flyback transformer consist of around
10 pins. Some pins are B+, GROUND, AFC, ABL,
X-RAY PROTECT, VCC, to HOT, HEATER, horizontal
centering and etc. But there is one and simple
way that i'm gone to show you how to locate the
abl pin. Use two analog multimeters set to x10kohm
range. Connect it in series (+ probe to - probe)
and measure the flyback transformer from anode
(the red cap) to all of the pins under the
flyback. You have to desolder all the pin or
just remove the flyback from the mainboard in
order to get an accurate reading.

If either one of the pin shows a diode
reading that is the abl pin. The reason we use
two multimeters is because the flyback circuit
internally have couple of high voltage diodes
connected in series. If you have a schematic
diagram or any monitor or television
troubleshooting book, you will see that the
internal flyback diode are connected in series.
By connecting two meters the ouput from the probe
is around 24 VOLTS which is enough power to check
the flyback internal diodes. After you have
determined the abl pin, you can proceed to follow
the line and find the bad components that cause
the contrast problem.

Television or tv flyback transformer generally
have the same design compare to monitor flyback.
Whether it is a montblanc flyback transformer,
rca, mitsubishi or sony flyback the checking
usually is the same.

About the Author

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Can I connect two amplifiers to one HOT line? One amp. is for my door speakers and the other amp. is for a 10?

I have a good inline fuse. I am not trying to shake the windows of the car next to me. I am trying to round out the sound.

ya, they make splitters for this...The main line would be a real heavy gauge, and the lines coming from the splitter will be a lighter gauge..

NASA Hubble Space Telescope Daily Report #5152
NASA Hubble Space Telescope Daily Report #5152

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